mirror of
https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git
synced 2024-08-21 16:29:59 +00:00
![Mike Rapoport](/assets/img/avatar_default.png)
Now that the administrative information for transparent huge pages is nicely separated, move it to its own page under the admin guide. Signed-off-by: Mike Rapoport <rppt@linux.vnet.ibm.com> Signed-off-by: Jonathan Corbet <corbet@lwn.net>
197 lines
8.8 KiB
ReStructuredText
197 lines
8.8 KiB
ReStructuredText
.. _transhuge:
|
|
|
|
============================
|
|
Transparent Hugepage Support
|
|
============================
|
|
|
|
This document describes design principles Transparent Hugepage (THP)
|
|
Support and its interaction with other parts of the memory management.
|
|
|
|
Design principles
|
|
=================
|
|
|
|
- "graceful fallback": mm components which don't have transparent hugepage
|
|
knowledge fall back to breaking huge pmd mapping into table of ptes and,
|
|
if necessary, split a transparent hugepage. Therefore these components
|
|
can continue working on the regular pages or regular pte mappings.
|
|
|
|
- if a hugepage allocation fails because of memory fragmentation,
|
|
regular pages should be gracefully allocated instead and mixed in
|
|
the same vma without any failure or significant delay and without
|
|
userland noticing
|
|
|
|
- if some task quits and more hugepages become available (either
|
|
immediately in the buddy or through the VM), guest physical memory
|
|
backed by regular pages should be relocated on hugepages
|
|
automatically (with khugepaged)
|
|
|
|
- it doesn't require memory reservation and in turn it uses hugepages
|
|
whenever possible (the only possible reservation here is kernelcore=
|
|
to avoid unmovable pages to fragment all the memory but such a tweak
|
|
is not specific to transparent hugepage support and it's a generic
|
|
feature that applies to all dynamic high order allocations in the
|
|
kernel)
|
|
|
|
get_user_pages and follow_page
|
|
==============================
|
|
|
|
get_user_pages and follow_page if run on a hugepage, will return the
|
|
head or tail pages as usual (exactly as they would do on
|
|
hugetlbfs). Most gup users will only care about the actual physical
|
|
address of the page and its temporary pinning to release after the I/O
|
|
is complete, so they won't ever notice the fact the page is huge. But
|
|
if any driver is going to mangle over the page structure of the tail
|
|
page (like for checking page->mapping or other bits that are relevant
|
|
for the head page and not the tail page), it should be updated to jump
|
|
to check head page instead. Taking reference on any head/tail page would
|
|
prevent page from being split by anyone.
|
|
|
|
.. note::
|
|
these aren't new constraints to the GUP API, and they match the
|
|
same constrains that applies to hugetlbfs too, so any driver capable
|
|
of handling GUP on hugetlbfs will also work fine on transparent
|
|
hugepage backed mappings.
|
|
|
|
In case you can't handle compound pages if they're returned by
|
|
follow_page, the FOLL_SPLIT bit can be specified as parameter to
|
|
follow_page, so that it will split the hugepages before returning
|
|
them. Migration for example passes FOLL_SPLIT as parameter to
|
|
follow_page because it's not hugepage aware and in fact it can't work
|
|
at all on hugetlbfs (but it instead works fine on transparent
|
|
hugepages thanks to FOLL_SPLIT). migration simply can't deal with
|
|
hugepages being returned (as it's not only checking the pfn of the
|
|
page and pinning it during the copy but it pretends to migrate the
|
|
memory in regular page sizes and with regular pte/pmd mappings).
|
|
|
|
Graceful fallback
|
|
=================
|
|
|
|
Code walking pagetables but unaware about huge pmds can simply call
|
|
split_huge_pmd(vma, pmd, addr) where the pmd is the one returned by
|
|
pmd_offset. It's trivial to make the code transparent hugepage aware
|
|
by just grepping for "pmd_offset" and adding split_huge_pmd where
|
|
missing after pmd_offset returns the pmd. Thanks to the graceful
|
|
fallback design, with a one liner change, you can avoid to write
|
|
hundred if not thousand of lines of complex code to make your code
|
|
hugepage aware.
|
|
|
|
If you're not walking pagetables but you run into a physical hugepage
|
|
but you can't handle it natively in your code, you can split it by
|
|
calling split_huge_page(page). This is what the Linux VM does before
|
|
it tries to swapout the hugepage for example. split_huge_page() can fail
|
|
if the page is pinned and you must handle this correctly.
|
|
|
|
Example to make mremap.c transparent hugepage aware with a one liner
|
|
change::
|
|
|
|
diff --git a/mm/mremap.c b/mm/mremap.c
|
|
--- a/mm/mremap.c
|
|
+++ b/mm/mremap.c
|
|
@@ -41,6 +41,7 @@ static pmd_t *get_old_pmd(struct mm_stru
|
|
return NULL;
|
|
|
|
pmd = pmd_offset(pud, addr);
|
|
+ split_huge_pmd(vma, pmd, addr);
|
|
if (pmd_none_or_clear_bad(pmd))
|
|
return NULL;
|
|
|
|
Locking in hugepage aware code
|
|
==============================
|
|
|
|
We want as much code as possible hugepage aware, as calling
|
|
split_huge_page() or split_huge_pmd() has a cost.
|
|
|
|
To make pagetable walks huge pmd aware, all you need to do is to call
|
|
pmd_trans_huge() on the pmd returned by pmd_offset. You must hold the
|
|
mmap_sem in read (or write) mode to be sure an huge pmd cannot be
|
|
created from under you by khugepaged (khugepaged collapse_huge_page
|
|
takes the mmap_sem in write mode in addition to the anon_vma lock). If
|
|
pmd_trans_huge returns false, you just fallback in the old code
|
|
paths. If instead pmd_trans_huge returns true, you have to take the
|
|
page table lock (pmd_lock()) and re-run pmd_trans_huge. Taking the
|
|
page table lock will prevent the huge pmd to be converted into a
|
|
regular pmd from under you (split_huge_pmd can run in parallel to the
|
|
pagetable walk). If the second pmd_trans_huge returns false, you
|
|
should just drop the page table lock and fallback to the old code as
|
|
before. Otherwise you can proceed to process the huge pmd and the
|
|
hugepage natively. Once finished you can drop the page table lock.
|
|
|
|
Refcounts and transparent huge pages
|
|
====================================
|
|
|
|
Refcounting on THP is mostly consistent with refcounting on other compound
|
|
pages:
|
|
|
|
- get_page()/put_page() and GUP operate in head page's ->_refcount.
|
|
|
|
- ->_refcount in tail pages is always zero: get_page_unless_zero() never
|
|
succeed on tail pages.
|
|
|
|
- map/unmap of the pages with PTE entry increment/decrement ->_mapcount
|
|
on relevant sub-page of the compound page.
|
|
|
|
- map/unmap of the whole compound page accounted in compound_mapcount
|
|
(stored in first tail page). For file huge pages, we also increment
|
|
->_mapcount of all sub-pages in order to have race-free detection of
|
|
last unmap of subpages.
|
|
|
|
PageDoubleMap() indicates that the page is *possibly* mapped with PTEs.
|
|
|
|
For anonymous pages PageDoubleMap() also indicates ->_mapcount in all
|
|
subpages is offset up by one. This additional reference is required to
|
|
get race-free detection of unmap of subpages when we have them mapped with
|
|
both PMDs and PTEs.
|
|
|
|
This is optimization required to lower overhead of per-subpage mapcount
|
|
tracking. The alternative is alter ->_mapcount in all subpages on each
|
|
map/unmap of the whole compound page.
|
|
|
|
For anonymous pages, we set PG_double_map when a PMD of the page got split
|
|
for the first time, but still have PMD mapping. The additional references
|
|
go away with last compound_mapcount.
|
|
|
|
File pages get PG_double_map set on first map of the page with PTE and
|
|
goes away when the page gets evicted from page cache.
|
|
|
|
split_huge_page internally has to distribute the refcounts in the head
|
|
page to the tail pages before clearing all PG_head/tail bits from the page
|
|
structures. It can be done easily for refcounts taken by page table
|
|
entries. But we don't have enough information on how to distribute any
|
|
additional pins (i.e. from get_user_pages). split_huge_page() fails any
|
|
requests to split pinned huge page: it expects page count to be equal to
|
|
sum of mapcount of all sub-pages plus one (split_huge_page caller must
|
|
have reference for head page).
|
|
|
|
split_huge_page uses migration entries to stabilize page->_refcount and
|
|
page->_mapcount of anonymous pages. File pages just got unmapped.
|
|
|
|
We safe against physical memory scanners too: the only legitimate way
|
|
scanner can get reference to a page is get_page_unless_zero().
|
|
|
|
All tail pages have zero ->_refcount until atomic_add(). This prevents the
|
|
scanner from getting a reference to the tail page up to that point. After the
|
|
atomic_add() we don't care about the ->_refcount value. We already known how
|
|
many references should be uncharged from the head page.
|
|
|
|
For head page get_page_unless_zero() will succeed and we don't mind. It's
|
|
clear where reference should go after split: it will stay on head page.
|
|
|
|
Note that split_huge_pmd() doesn't have any limitation on refcounting:
|
|
pmd can be split at any point and never fails.
|
|
|
|
Partial unmap and deferred_split_huge_page()
|
|
============================================
|
|
|
|
Unmapping part of THP (with munmap() or other way) is not going to free
|
|
memory immediately. Instead, we detect that a subpage of THP is not in use
|
|
in page_remove_rmap() and queue the THP for splitting if memory pressure
|
|
comes. Splitting will free up unused subpages.
|
|
|
|
Splitting the page right away is not an option due to locking context in
|
|
the place where we can detect partial unmap. It's also might be
|
|
counterproductive since in many cases partial unmap happens during exit(2) if
|
|
a THP crosses a VMA boundary.
|
|
|
|
Function deferred_split_huge_page() is used to queue page for splitting.
|
|
The splitting itself will happen when we get memory pressure via shrinker
|
|
interface.
|