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![Oscar Salvador](/assets/img/avatar_default.png)
The premise of the refault distance is that it can be seen as a deficit of the inactive list space, so that if the inactive list would have had (R - E) more slots, the page would not have been evicted but promoted to the active list instead. However, the way the code is ordered right now set us to be off by one, so the real number of slots would be (R - E) + 1. I stumbled upon this when trying to understand the code and it puzzled me that the comments did not match what the code did. This it not an issue at all since evictions and refaults tend to happen in a number large enough that being off-by-one does not have any impact - and since the compiler and CPUs are free to rearrange the execution sequence anyway. But as Johannes says, it is better to re-arrange the code in the proper order since otherwise would be misleading to somebody who is actively reading and trying to understand the logic of the code - like it happened to me. Link: https://lkml.kernel.org/r/20210201060651.3781-1-osalvador@suse.de Signed-off-by: Oscar Salvador <osalvador@suse.de> Acked-by: Johannes Weiner <hannes@cmpxchg.org> Acked-by: Vlastimil Babka <vbabka@suse.cz> Signed-off-by: Andrew Morton <akpm@linux-foundation.org> Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>
626 lines
21 KiB
C
626 lines
21 KiB
C
// SPDX-License-Identifier: GPL-2.0
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/*
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* Workingset detection
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*
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* Copyright (C) 2013 Red Hat, Inc., Johannes Weiner
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*/
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#include <linux/memcontrol.h>
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#include <linux/mm_inline.h>
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#include <linux/writeback.h>
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#include <linux/shmem_fs.h>
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#include <linux/pagemap.h>
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#include <linux/atomic.h>
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#include <linux/module.h>
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#include <linux/swap.h>
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#include <linux/dax.h>
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#include <linux/fs.h>
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#include <linux/mm.h>
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/*
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* Double CLOCK lists
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*
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* Per node, two clock lists are maintained for file pages: the
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* inactive and the active list. Freshly faulted pages start out at
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* the head of the inactive list and page reclaim scans pages from the
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* tail. Pages that are accessed multiple times on the inactive list
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* are promoted to the active list, to protect them from reclaim,
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* whereas active pages are demoted to the inactive list when the
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* active list grows too big.
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*
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* fault ------------------------+
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* |
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* +--------------+ | +-------------+
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* reclaim <- | inactive | <-+-- demotion | active | <--+
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* +--------------+ +-------------+ |
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* | |
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* +-------------- promotion ------------------+
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*
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*
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* Access frequency and refault distance
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*
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* A workload is thrashing when its pages are frequently used but they
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* are evicted from the inactive list every time before another access
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* would have promoted them to the active list.
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*
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* In cases where the average access distance between thrashing pages
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* is bigger than the size of memory there is nothing that can be
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* done - the thrashing set could never fit into memory under any
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* circumstance.
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*
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* However, the average access distance could be bigger than the
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* inactive list, yet smaller than the size of memory. In this case,
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* the set could fit into memory if it weren't for the currently
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* active pages - which may be used more, hopefully less frequently:
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*
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* +-memory available to cache-+
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* | |
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* +-inactive------+-active----+
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* a b | c d e f g h i | J K L M N |
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* +---------------+-----------+
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*
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* It is prohibitively expensive to accurately track access frequency
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* of pages. But a reasonable approximation can be made to measure
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* thrashing on the inactive list, after which refaulting pages can be
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* activated optimistically to compete with the existing active pages.
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*
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* Approximating inactive page access frequency - Observations:
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*
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* 1. When a page is accessed for the first time, it is added to the
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* head of the inactive list, slides every existing inactive page
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* towards the tail by one slot, and pushes the current tail page
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* out of memory.
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*
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* 2. When a page is accessed for the second time, it is promoted to
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* the active list, shrinking the inactive list by one slot. This
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* also slides all inactive pages that were faulted into the cache
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* more recently than the activated page towards the tail of the
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* inactive list.
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*
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* Thus:
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*
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* 1. The sum of evictions and activations between any two points in
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* time indicate the minimum number of inactive pages accessed in
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* between.
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*
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* 2. Moving one inactive page N page slots towards the tail of the
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* list requires at least N inactive page accesses.
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*
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* Combining these:
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*
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* 1. When a page is finally evicted from memory, the number of
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* inactive pages accessed while the page was in cache is at least
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* the number of page slots on the inactive list.
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*
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* 2. In addition, measuring the sum of evictions and activations (E)
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* at the time of a page's eviction, and comparing it to another
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* reading (R) at the time the page faults back into memory tells
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* the minimum number of accesses while the page was not cached.
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* This is called the refault distance.
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*
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* Because the first access of the page was the fault and the second
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* access the refault, we combine the in-cache distance with the
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* out-of-cache distance to get the complete minimum access distance
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* of this page:
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*
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* NR_inactive + (R - E)
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*
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* And knowing the minimum access distance of a page, we can easily
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* tell if the page would be able to stay in cache assuming all page
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* slots in the cache were available:
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*
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* NR_inactive + (R - E) <= NR_inactive + NR_active
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*
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* which can be further simplified to
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*
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* (R - E) <= NR_active
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*
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* Put into words, the refault distance (out-of-cache) can be seen as
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* a deficit in inactive list space (in-cache). If the inactive list
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* had (R - E) more page slots, the page would not have been evicted
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* in between accesses, but activated instead. And on a full system,
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* the only thing eating into inactive list space is active pages.
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*
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*
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* Refaulting inactive pages
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*
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* All that is known about the active list is that the pages have been
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* accessed more than once in the past. This means that at any given
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* time there is actually a good chance that pages on the active list
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* are no longer in active use.
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*
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* So when a refault distance of (R - E) is observed and there are at
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* least (R - E) active pages, the refaulting page is activated
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* optimistically in the hope that (R - E) active pages are actually
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* used less frequently than the refaulting page - or even not used at
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* all anymore.
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*
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* That means if inactive cache is refaulting with a suitable refault
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* distance, we assume the cache workingset is transitioning and put
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* pressure on the current active list.
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*
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* If this is wrong and demotion kicks in, the pages which are truly
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* used more frequently will be reactivated while the less frequently
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* used once will be evicted from memory.
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*
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* But if this is right, the stale pages will be pushed out of memory
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* and the used pages get to stay in cache.
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*
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* Refaulting active pages
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*
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* If on the other hand the refaulting pages have recently been
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* deactivated, it means that the active list is no longer protecting
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* actively used cache from reclaim. The cache is NOT transitioning to
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* a different workingset; the existing workingset is thrashing in the
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* space allocated to the page cache.
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*
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*
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* Implementation
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*
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* For each node's LRU lists, a counter for inactive evictions and
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* activations is maintained (node->nonresident_age).
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*
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* On eviction, a snapshot of this counter (along with some bits to
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* identify the node) is stored in the now empty page cache
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* slot of the evicted page. This is called a shadow entry.
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*
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* On cache misses for which there are shadow entries, an eligible
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* refault distance will immediately activate the refaulting page.
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*/
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#define EVICTION_SHIFT ((BITS_PER_LONG - BITS_PER_XA_VALUE) + \
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1 + NODES_SHIFT + MEM_CGROUP_ID_SHIFT)
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#define EVICTION_MASK (~0UL >> EVICTION_SHIFT)
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/*
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* Eviction timestamps need to be able to cover the full range of
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* actionable refaults. However, bits are tight in the xarray
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* entry, and after storing the identifier for the lruvec there might
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* not be enough left to represent every single actionable refault. In
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* that case, we have to sacrifice granularity for distance, and group
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* evictions into coarser buckets by shaving off lower timestamp bits.
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*/
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static unsigned int bucket_order __read_mostly;
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static void *pack_shadow(int memcgid, pg_data_t *pgdat, unsigned long eviction,
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bool workingset)
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{
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eviction >>= bucket_order;
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eviction &= EVICTION_MASK;
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eviction = (eviction << MEM_CGROUP_ID_SHIFT) | memcgid;
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eviction = (eviction << NODES_SHIFT) | pgdat->node_id;
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eviction = (eviction << 1) | workingset;
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return xa_mk_value(eviction);
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}
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static void unpack_shadow(void *shadow, int *memcgidp, pg_data_t **pgdat,
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unsigned long *evictionp, bool *workingsetp)
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{
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unsigned long entry = xa_to_value(shadow);
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int memcgid, nid;
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bool workingset;
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workingset = entry & 1;
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entry >>= 1;
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nid = entry & ((1UL << NODES_SHIFT) - 1);
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entry >>= NODES_SHIFT;
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memcgid = entry & ((1UL << MEM_CGROUP_ID_SHIFT) - 1);
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entry >>= MEM_CGROUP_ID_SHIFT;
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*memcgidp = memcgid;
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*pgdat = NODE_DATA(nid);
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*evictionp = entry << bucket_order;
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*workingsetp = workingset;
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}
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/**
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* workingset_age_nonresident - age non-resident entries as LRU ages
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* @lruvec: the lruvec that was aged
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* @nr_pages: the number of pages to count
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*
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* As in-memory pages are aged, non-resident pages need to be aged as
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* well, in order for the refault distances later on to be comparable
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* to the in-memory dimensions. This function allows reclaim and LRU
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* operations to drive the non-resident aging along in parallel.
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*/
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void workingset_age_nonresident(struct lruvec *lruvec, unsigned long nr_pages)
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{
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/*
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* Reclaiming a cgroup means reclaiming all its children in a
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* round-robin fashion. That means that each cgroup has an LRU
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* order that is composed of the LRU orders of its child
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* cgroups; and every page has an LRU position not just in the
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* cgroup that owns it, but in all of that group's ancestors.
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*
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* So when the physical inactive list of a leaf cgroup ages,
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* the virtual inactive lists of all its parents, including
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* the root cgroup's, age as well.
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*/
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do {
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atomic_long_add(nr_pages, &lruvec->nonresident_age);
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} while ((lruvec = parent_lruvec(lruvec)));
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}
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/**
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* workingset_eviction - note the eviction of a page from memory
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* @target_memcg: the cgroup that is causing the reclaim
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* @page: the page being evicted
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*
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* Returns a shadow entry to be stored in @page->mapping->i_pages in place
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* of the evicted @page so that a later refault can be detected.
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*/
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void *workingset_eviction(struct page *page, struct mem_cgroup *target_memcg)
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{
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struct pglist_data *pgdat = page_pgdat(page);
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unsigned long eviction;
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struct lruvec *lruvec;
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int memcgid;
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/* Page is fully exclusive and pins page's memory cgroup pointer */
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VM_BUG_ON_PAGE(PageLRU(page), page);
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VM_BUG_ON_PAGE(page_count(page), page);
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VM_BUG_ON_PAGE(!PageLocked(page), page);
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lruvec = mem_cgroup_lruvec(target_memcg, pgdat);
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/* XXX: target_memcg can be NULL, go through lruvec */
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memcgid = mem_cgroup_id(lruvec_memcg(lruvec));
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eviction = atomic_long_read(&lruvec->nonresident_age);
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workingset_age_nonresident(lruvec, thp_nr_pages(page));
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return pack_shadow(memcgid, pgdat, eviction, PageWorkingset(page));
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}
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/**
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* workingset_refault - evaluate the refault of a previously evicted page
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* @page: the freshly allocated replacement page
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* @shadow: shadow entry of the evicted page
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*
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* Calculates and evaluates the refault distance of the previously
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* evicted page in the context of the node and the memcg whose memory
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* pressure caused the eviction.
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*/
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void workingset_refault(struct page *page, void *shadow)
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{
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bool file = page_is_file_lru(page);
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struct mem_cgroup *eviction_memcg;
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struct lruvec *eviction_lruvec;
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unsigned long refault_distance;
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unsigned long workingset_size;
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struct pglist_data *pgdat;
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struct mem_cgroup *memcg;
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unsigned long eviction;
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struct lruvec *lruvec;
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unsigned long refault;
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bool workingset;
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int memcgid;
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unpack_shadow(shadow, &memcgid, &pgdat, &eviction, &workingset);
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rcu_read_lock();
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/*
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* Look up the memcg associated with the stored ID. It might
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* have been deleted since the page's eviction.
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*
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* Note that in rare events the ID could have been recycled
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* for a new cgroup that refaults a shared page. This is
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* impossible to tell from the available data. However, this
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* should be a rare and limited disturbance, and activations
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* are always speculative anyway. Ultimately, it's the aging
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* algorithm's job to shake out the minimum access frequency
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* for the active cache.
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*
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* XXX: On !CONFIG_MEMCG, this will always return NULL; it
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* would be better if the root_mem_cgroup existed in all
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* configurations instead.
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*/
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eviction_memcg = mem_cgroup_from_id(memcgid);
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if (!mem_cgroup_disabled() && !eviction_memcg)
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goto out;
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eviction_lruvec = mem_cgroup_lruvec(eviction_memcg, pgdat);
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refault = atomic_long_read(&eviction_lruvec->nonresident_age);
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/*
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* Calculate the refault distance
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*
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* The unsigned subtraction here gives an accurate distance
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* across nonresident_age overflows in most cases. There is a
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* special case: usually, shadow entries have a short lifetime
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* and are either refaulted or reclaimed along with the inode
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* before they get too old. But it is not impossible for the
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* nonresident_age to lap a shadow entry in the field, which
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* can then result in a false small refault distance, leading
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* to a false activation should this old entry actually
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* refault again. However, earlier kernels used to deactivate
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* unconditionally with *every* reclaim invocation for the
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* longest time, so the occasional inappropriate activation
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* leading to pressure on the active list is not a problem.
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*/
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refault_distance = (refault - eviction) & EVICTION_MASK;
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/*
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* The activation decision for this page is made at the level
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* where the eviction occurred, as that is where the LRU order
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* during page reclaim is being determined.
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*
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* However, the cgroup that will own the page is the one that
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* is actually experiencing the refault event.
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*/
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memcg = page_memcg(page);
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lruvec = mem_cgroup_lruvec(memcg, pgdat);
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inc_lruvec_state(lruvec, WORKINGSET_REFAULT_BASE + file);
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/*
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* Compare the distance to the existing workingset size. We
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* don't activate pages that couldn't stay resident even if
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* all the memory was available to the workingset. Whether
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* workingset competition needs to consider anon or not depends
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* on having swap.
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*/
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workingset_size = lruvec_page_state(eviction_lruvec, NR_ACTIVE_FILE);
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if (!file) {
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workingset_size += lruvec_page_state(eviction_lruvec,
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NR_INACTIVE_FILE);
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}
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if (mem_cgroup_get_nr_swap_pages(memcg) > 0) {
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workingset_size += lruvec_page_state(eviction_lruvec,
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NR_ACTIVE_ANON);
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if (file) {
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workingset_size += lruvec_page_state(eviction_lruvec,
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NR_INACTIVE_ANON);
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}
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}
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if (refault_distance > workingset_size)
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goto out;
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SetPageActive(page);
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workingset_age_nonresident(lruvec, thp_nr_pages(page));
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inc_lruvec_state(lruvec, WORKINGSET_ACTIVATE_BASE + file);
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/* Page was active prior to eviction */
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if (workingset) {
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SetPageWorkingset(page);
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/* XXX: Move to lru_cache_add() when it supports new vs putback */
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lru_note_cost_page(page);
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inc_lruvec_state(lruvec, WORKINGSET_RESTORE_BASE + file);
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}
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out:
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rcu_read_unlock();
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}
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/**
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* workingset_activation - note a page activation
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* @page: page that is being activated
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*/
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void workingset_activation(struct page *page)
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{
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struct mem_cgroup *memcg;
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struct lruvec *lruvec;
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rcu_read_lock();
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/*
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* Filter non-memcg pages here, e.g. unmap can call
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* mark_page_accessed() on VDSO pages.
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*
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* XXX: See workingset_refault() - this should return
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* root_mem_cgroup even for !CONFIG_MEMCG.
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*/
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memcg = page_memcg_rcu(page);
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if (!mem_cgroup_disabled() && !memcg)
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goto out;
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lruvec = mem_cgroup_page_lruvec(page, page_pgdat(page));
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workingset_age_nonresident(lruvec, thp_nr_pages(page));
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out:
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rcu_read_unlock();
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}
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/*
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* Shadow entries reflect the share of the working set that does not
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* fit into memory, so their number depends on the access pattern of
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* the workload. In most cases, they will refault or get reclaimed
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* along with the inode, but a (malicious) workload that streams
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* through files with a total size several times that of available
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* memory, while preventing the inodes from being reclaimed, can
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* create excessive amounts of shadow nodes. To keep a lid on this,
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* track shadow nodes and reclaim them when they grow way past the
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* point where they would still be useful.
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*/
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static struct list_lru shadow_nodes;
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void workingset_update_node(struct xa_node *node)
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{
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/*
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* Track non-empty nodes that contain only shadow entries;
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* unlink those that contain pages or are being freed.
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*
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* Avoid acquiring the list_lru lock when the nodes are
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* already where they should be. The list_empty() test is safe
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* as node->private_list is protected by the i_pages lock.
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*/
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VM_WARN_ON_ONCE(!irqs_disabled()); /* For __inc_lruvec_page_state */
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if (node->count && node->count == node->nr_values) {
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if (list_empty(&node->private_list)) {
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list_lru_add(&shadow_nodes, &node->private_list);
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__inc_lruvec_kmem_state(node, WORKINGSET_NODES);
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}
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} else {
|
|
if (!list_empty(&node->private_list)) {
|
|
list_lru_del(&shadow_nodes, &node->private_list);
|
|
__dec_lruvec_kmem_state(node, WORKINGSET_NODES);
|
|
}
|
|
}
|
|
}
|
|
|
|
static unsigned long count_shadow_nodes(struct shrinker *shrinker,
|
|
struct shrink_control *sc)
|
|
{
|
|
unsigned long max_nodes;
|
|
unsigned long nodes;
|
|
unsigned long pages;
|
|
|
|
nodes = list_lru_shrink_count(&shadow_nodes, sc);
|
|
if (!nodes)
|
|
return SHRINK_EMPTY;
|
|
|
|
/*
|
|
* Approximate a reasonable limit for the nodes
|
|
* containing shadow entries. We don't need to keep more
|
|
* shadow entries than possible pages on the active list,
|
|
* since refault distances bigger than that are dismissed.
|
|
*
|
|
* The size of the active list converges toward 100% of
|
|
* overall page cache as memory grows, with only a tiny
|
|
* inactive list. Assume the total cache size for that.
|
|
*
|
|
* Nodes might be sparsely populated, with only one shadow
|
|
* entry in the extreme case. Obviously, we cannot keep one
|
|
* node for every eligible shadow entry, so compromise on a
|
|
* worst-case density of 1/8th. Below that, not all eligible
|
|
* refaults can be detected anymore.
|
|
*
|
|
* On 64-bit with 7 xa_nodes per page and 64 slots
|
|
* each, this will reclaim shadow entries when they consume
|
|
* ~1.8% of available memory:
|
|
*
|
|
* PAGE_SIZE / xa_nodes / node_entries * 8 / PAGE_SIZE
|
|
*/
|
|
#ifdef CONFIG_MEMCG
|
|
if (sc->memcg) {
|
|
struct lruvec *lruvec;
|
|
int i;
|
|
|
|
lruvec = mem_cgroup_lruvec(sc->memcg, NODE_DATA(sc->nid));
|
|
for (pages = 0, i = 0; i < NR_LRU_LISTS; i++)
|
|
pages += lruvec_page_state_local(lruvec,
|
|
NR_LRU_BASE + i);
|
|
pages += lruvec_page_state_local(
|
|
lruvec, NR_SLAB_RECLAIMABLE_B) >> PAGE_SHIFT;
|
|
pages += lruvec_page_state_local(
|
|
lruvec, NR_SLAB_UNRECLAIMABLE_B) >> PAGE_SHIFT;
|
|
} else
|
|
#endif
|
|
pages = node_present_pages(sc->nid);
|
|
|
|
max_nodes = pages >> (XA_CHUNK_SHIFT - 3);
|
|
|
|
if (nodes <= max_nodes)
|
|
return 0;
|
|
return nodes - max_nodes;
|
|
}
|
|
|
|
static enum lru_status shadow_lru_isolate(struct list_head *item,
|
|
struct list_lru_one *lru,
|
|
spinlock_t *lru_lock,
|
|
void *arg) __must_hold(lru_lock)
|
|
{
|
|
struct xa_node *node = container_of(item, struct xa_node, private_list);
|
|
struct address_space *mapping;
|
|
int ret;
|
|
|
|
/*
|
|
* Page cache insertions and deletions synchronously maintain
|
|
* the shadow node LRU under the i_pages lock and the
|
|
* lru_lock. Because the page cache tree is emptied before
|
|
* the inode can be destroyed, holding the lru_lock pins any
|
|
* address_space that has nodes on the LRU.
|
|
*
|
|
* We can then safely transition to the i_pages lock to
|
|
* pin only the address_space of the particular node we want
|
|
* to reclaim, take the node off-LRU, and drop the lru_lock.
|
|
*/
|
|
|
|
mapping = container_of(node->array, struct address_space, i_pages);
|
|
|
|
/* Coming from the list, invert the lock order */
|
|
if (!xa_trylock(&mapping->i_pages)) {
|
|
spin_unlock_irq(lru_lock);
|
|
ret = LRU_RETRY;
|
|
goto out;
|
|
}
|
|
|
|
list_lru_isolate(lru, item);
|
|
__dec_lruvec_kmem_state(node, WORKINGSET_NODES);
|
|
|
|
spin_unlock(lru_lock);
|
|
|
|
/*
|
|
* The nodes should only contain one or more shadow entries,
|
|
* no pages, so we expect to be able to remove them all and
|
|
* delete and free the empty node afterwards.
|
|
*/
|
|
if (WARN_ON_ONCE(!node->nr_values))
|
|
goto out_invalid;
|
|
if (WARN_ON_ONCE(node->count != node->nr_values))
|
|
goto out_invalid;
|
|
mapping->nrexceptional -= node->nr_values;
|
|
xa_delete_node(node, workingset_update_node);
|
|
__inc_lruvec_kmem_state(node, WORKINGSET_NODERECLAIM);
|
|
|
|
out_invalid:
|
|
xa_unlock_irq(&mapping->i_pages);
|
|
ret = LRU_REMOVED_RETRY;
|
|
out:
|
|
cond_resched();
|
|
spin_lock_irq(lru_lock);
|
|
return ret;
|
|
}
|
|
|
|
static unsigned long scan_shadow_nodes(struct shrinker *shrinker,
|
|
struct shrink_control *sc)
|
|
{
|
|
/* list_lru lock nests inside the IRQ-safe i_pages lock */
|
|
return list_lru_shrink_walk_irq(&shadow_nodes, sc, shadow_lru_isolate,
|
|
NULL);
|
|
}
|
|
|
|
static struct shrinker workingset_shadow_shrinker = {
|
|
.count_objects = count_shadow_nodes,
|
|
.scan_objects = scan_shadow_nodes,
|
|
.seeks = 0, /* ->count reports only fully expendable nodes */
|
|
.flags = SHRINKER_NUMA_AWARE | SHRINKER_MEMCG_AWARE,
|
|
};
|
|
|
|
/*
|
|
* Our list_lru->lock is IRQ-safe as it nests inside the IRQ-safe
|
|
* i_pages lock.
|
|
*/
|
|
static struct lock_class_key shadow_nodes_key;
|
|
|
|
static int __init workingset_init(void)
|
|
{
|
|
unsigned int timestamp_bits;
|
|
unsigned int max_order;
|
|
int ret;
|
|
|
|
BUILD_BUG_ON(BITS_PER_LONG < EVICTION_SHIFT);
|
|
/*
|
|
* Calculate the eviction bucket size to cover the longest
|
|
* actionable refault distance, which is currently half of
|
|
* memory (totalram_pages/2). However, memory hotplug may add
|
|
* some more pages at runtime, so keep working with up to
|
|
* double the initial memory by using totalram_pages as-is.
|
|
*/
|
|
timestamp_bits = BITS_PER_LONG - EVICTION_SHIFT;
|
|
max_order = fls_long(totalram_pages() - 1);
|
|
if (max_order > timestamp_bits)
|
|
bucket_order = max_order - timestamp_bits;
|
|
pr_info("workingset: timestamp_bits=%d max_order=%d bucket_order=%u\n",
|
|
timestamp_bits, max_order, bucket_order);
|
|
|
|
ret = prealloc_shrinker(&workingset_shadow_shrinker);
|
|
if (ret)
|
|
goto err;
|
|
ret = __list_lru_init(&shadow_nodes, true, &shadow_nodes_key,
|
|
&workingset_shadow_shrinker);
|
|
if (ret)
|
|
goto err_list_lru;
|
|
register_shrinker_prepared(&workingset_shadow_shrinker);
|
|
return 0;
|
|
err_list_lru:
|
|
free_prealloced_shrinker(&workingset_shadow_shrinker);
|
|
err:
|
|
return ret;
|
|
}
|
|
module_init(workingset_init);
|