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mm/rmap: split try_to_munlock from try_to_unmap 

The behaviour of try_to_unmap_one() is difficult to follow because it
performs different operations based on a fairly large set of flags used in
different combinations.

TTU_MUNLOCK is one such flag.  However it is exclusively used by
try_to_munlock() which specifies no other flags.  Therefore rather than
overload try_to_unmap_one() with unrelated behaviour split this out into
it's own function and remove the flag.

Link: https://lkml.kernel.org/r/20210616105937.23201-4-apopple@nvidia.com
Signed-off-by: Alistair Popple <apopple@nvidia.com>
Reviewed-by: Ralph Campbell <rcampbell@nvidia.com>
Reviewed-by: Christoph Hellwig <hch@lst.de>
Cc: Ben Skeggs <bskeggs@redhat.com>
Cc: Hugh Dickins <hughd@google.com>
Cc: Jason Gunthorpe <jgg@nvidia.com>
Cc: John Hubbard <jhubbard@nvidia.com>
Cc: "Matthew Wilcox (Oracle)" <willy@infradead.org>
Cc: Peter Xu <peterx@redhat.com>
Cc: Shakeel Butt <shakeelb@google.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>
This commit is contained in:
Alistair Popple 2021-06-30 18:54:12 -07:00 committed by Linus Torvalds
parent 4dd845b5a3
commit cd62734ca6
4 changed files with 70 additions and 46 deletions
Show Stats Download Patch File Download Diff File Expand all files Collapse all files

33
Documentation/vm/unevictable-lru.rst
View File

@ -389,14 +389,14 @@ mlocked, munlock_vma_page() updates that zone statistics for the number of
mlocked pages. Note, however, that at this point we haven't checked whether
the page is mapped by other VM_LOCKED VMAs.
We can't call try_to_munlock(), the function that walks the reverse map to
We can't call page_mlock(), the function that walks the reverse map to
check for other VM_LOCKED VMAs, without first isolating the page from the LRU.
try_to_munlock() is a variant of try_to_unmap() and thus requires that the page
page_mlock() is a variant of try_to_unmap() and thus requires that the page
not be on an LRU list [more on these below]. However, the call to
isolate_lru_page() could fail, in which case we couldn't try_to_munlock(). So,
isolate_lru_page() could fail, in which case we can't call page_mlock(). So,
we go ahead and clear PG_mlocked up front, as this might be the only chance we
have. If we can successfully isolate the page, we go ahead and
try_to_munlock(), which will restore the PG_mlocked flag and update the zone
have. If we can successfully isolate the page, we go ahead and call
page_mlock(), which will restore the PG_mlocked flag and update the zone
page statistics if it finds another VMA holding the page mlocked. If we fail
to isolate the page, we'll have left a potentially mlocked page on the LRU.
This is fine, because we'll catch it later if and if vmscan tries to reclaim
@ -545,31 +545,24 @@ munlock or munmap system calls, mm teardown (munlock_vma_pages_all), reclaim,
holepunching, and truncation of file pages and their anonymous COWed pages.
try_to_munlock() Reverse Map Scan
page_mlock() Reverse Map Scan
---------------------------------
.. warning::
[!] TODO/FIXME: a better name might be page_mlocked() - analogous to the
page_referenced() reverse map walker.
When munlock_vma_page() [see section :ref:`munlock()/munlockall() System Call
Handling <munlock_munlockall_handling>` above] tries to munlock a
page, it needs to determine whether or not the page is mapped by any
VM_LOCKED VMA without actually attempting to unmap all PTEs from the
page. For this purpose, the unevictable/mlock infrastructure
introduced a variant of try_to_unmap() called try_to_munlock().
introduced a variant of try_to_unmap() called page_mlock().
try_to_munlock() calls the same functions as try_to_unmap() for anonymous and
mapped file and KSM pages with a flag argument specifying unlock versus unmap
processing. Again, these functions walk the respective reverse maps looking
for VM_LOCKED VMAs. When such a VMA is found, as in the try_to_unmap() case,
the functions mlock the page via mlock_vma_page() and return SWAP_MLOCK. This
undoes the pre-clearing of the page's PG_mlocked done by munlock_vma_page.
page_mlock() walks the respective reverse maps looking for VM_LOCKED VMAs. When
such a VMA is found the page is mlocked via mlock_vma_page(). This undoes the
pre-clearing of the page's PG_mlocked done by munlock_vma_page.
Note that try_to_munlock()'s reverse map walk must visit every VMA in a page's
Note that page_mlock()'s reverse map walk must visit every VMA in a page's
reverse map to determine that a page is NOT mapped into any VM_LOCKED VMA.
However, the scan can terminate when it encounters a VM_LOCKED VMA.
Although try_to_munlock() might be called a great many times when munlocking a
Although page_mlock() might be called a great many times when munlocking a
large region or tearing down a large address space that has been mlocked via
mlockall(), overall this is a fairly rare event.
@ -602,7 +595,7 @@ inactive lists to the appropriate node's unevictable list.
shrink_inactive_list() should only see SHM_LOCK'd pages that became SHM_LOCK'd
after shrink_active_list() had moved them to the inactive list, or pages mapped
into VM_LOCKED VMAs that munlock_vma_page() couldn't isolate from the LRU to
recheck via try_to_munlock(). shrink_inactive_list() won't notice the latter,
recheck via page_mlock(). shrink_inactive_list() won't notice the latter,
but will pass on to shrink_page_list().
shrink_page_list() again culls obviously unevictable pages that it could

3
include/linux/rmap.h
View File

@ -87,7 +87,6 @@ struct anon_vma_chain {
enum ttu_flags {
TTU_MIGRATION = 0x1, /* migration mode */
TTU_MUNLOCK = 0x2, /* munlock mode */
TTU_SPLIT_HUGE_PMD = 0x4, /* split huge PMD if any */
TTU_IGNORE_MLOCK = 0x8, /* ignore mlock */
@ -240,7 +239,7 @@ int page_mkclean(struct page *);
* called in munlock()/munmap() path to check for other vmas holding
* the page mlocked.
*/
void try_to_munlock(struct page *);
void page_mlock(struct page *page);
void remove_migration_ptes(struct page *old, struct page *new, bool locked);

12
mm/mlock.c
View File

@ -108,7 +108,7 @@ void mlock_vma_page(struct page *page)
/*
* Finish munlock after successful page isolation
*
* Page must be locked. This is a wrapper for try_to_munlock()
* Page must be locked. This is a wrapper for page_mlock()
* and putback_lru_page() with munlock accounting.
*/
static void __munlock_isolated_page(struct page *page)
@ -118,7 +118,7 @@ static void __munlock_isolated_page(struct page *page)
* and we don't need to check all the other vmas.
*/
if (page_mapcount(page) > 1)
try_to_munlock(page);
page_mlock(page);
/* Did try_to_unlock() succeed or punt? */
if (!PageMlocked(page))
@ -158,7 +158,7 @@ static void __munlock_isolation_failed(struct page *page)
* munlock()ed or munmap()ed, we want to check whether other vmas hold the
* page locked so that we can leave it on the unevictable lru list and not
* bother vmscan with it. However, to walk the page's rmap list in
* try_to_munlock() we must isolate the page from the LRU. If some other
* page_mlock() we must isolate the page from the LRU. If some other
* task has removed the page from the LRU, we won't be able to do that.
* So we clear the PageMlocked as we might not get another chance. If we
* can't isolate the page, we leave it for putback_lru_page() and vmscan
@ -168,7 +168,7 @@ unsigned int munlock_vma_page(struct page *page)
{
int nr_pages;
/* For try_to_munlock() and to serialize with page migration */
/* For page_mlock() and to serialize with page migration */
BUG_ON(!PageLocked(page));
VM_BUG_ON_PAGE(PageTail(page), page);
@ -205,7 +205,7 @@ static int __mlock_posix_error_return(long retval)
*
* The fast path is available only for evictable pages with single mapping.
* Then we can bypass the per-cpu pvec and get better performance.
* when mapcount > 1 we need try_to_munlock() which can fail.
* when mapcount > 1 we need page_mlock() which can fail.
* when !page_evictable(), we need the full redo logic of putback_lru_page to
* avoid leaving evictable page in unevictable list.
*
@ -414,7 +414,7 @@ static unsigned long __munlock_pagevec_fill(struct pagevec *pvec,
*
* We don't save and restore VM_LOCKED here because pages are
* still on lru. In unmap path, pages might be scanned by reclaim
* and re-mlocked by try_to_{munlock|unmap} before we unmap and
* and re-mlocked by page_mlock/try_to_unmap before we unmap and
* free them. This will result in freeing mlocked pages.
*/
void munlock_vma_pages_range(struct vm_area_struct *vma,

68
mm/rmap.c
View File

@ -1411,10 +1411,6 @@ static bool try_to_unmap_one(struct page *page, struct vm_area_struct *vma,
if (flags & TTU_SYNC)
pvmw.flags = PVMW_SYNC;
/* munlock has nothing to gain from examining un-locked vmas */
if ((flags & TTU_MUNLOCK) && !(vma->vm_flags & VM_LOCKED))
return true;
if (IS_ENABLED(CONFIG_MIGRATION) && (flags & TTU_MIGRATION) &&
is_zone_device_page(page) && !is_device_private_page(page))
return true;
@ -1476,8 +1472,6 @@ static bool try_to_unmap_one(struct page *page, struct vm_area_struct *vma,
page_vma_mapped_walk_done(&pvmw);
break;
}
if (flags & TTU_MUNLOCK)
continue;
}
/* Unexpected PMD-mapped THP? */
@ -1790,20 +1784,58 @@ void try_to_unmap(struct page *page, enum ttu_flags flags)
rmap_walk(page, &rwc);
}
/**
* try_to_munlock - try to munlock a page
* @page: the page to be munlocked
*
* Called from munlock code. Checks all of the VMAs mapping the page
* to make sure nobody else has this page mlocked. The page will be
* returned with PG_mlocked cleared if no other vmas have it mlocked.
/*
* Walks the vma's mapping a page and mlocks the page if any locked vma's are
* found. Once one is found the page is locked and the scan can be terminated.
*/
static bool page_mlock_one(struct page *page, struct vm_area_struct *vma,
unsigned long address, void *unused)
{
struct page_vma_mapped_walk pvmw = {
.page = page,
.vma = vma,
.address = address,
};
void try_to_munlock(struct page *page)
/* An un-locked vma doesn't have any pages to lock, continue the scan */
if (!(vma->vm_flags & VM_LOCKED))
return true;
while (page_vma_mapped_walk(&pvmw)) {
/*
* Need to recheck under the ptl to serialise with
* __munlock_pagevec_fill() after VM_LOCKED is cleared in
* munlock_vma_pages_range().
*/
if (vma->vm_flags & VM_LOCKED) {
/* PTE-mapped THP are never mlocked */
if (!PageTransCompound(page))
mlock_vma_page(page);
page_vma_mapped_walk_done(&pvmw);
}
/*
* no need to continue scanning other vma's if the page has
* been locked.
*/
return false;
}
return true;
}
/**
* page_mlock - try to mlock a page
* @page: the page to be mlocked
*
* Called from munlock code. Checks all of the VMAs mapping the page and mlocks
* the page if any are found. The page will be returned with PG_mlocked cleared
* if it is not mapped by any locked vmas.
*/
void page_mlock(struct page *page)
{
struct rmap_walk_control rwc = {
.rmap_one = try_to_unmap_one,
.arg = (void *)TTU_MUNLOCK,
.rmap_one = page_mlock_one,
.done = page_not_mapped,
.anon_lock = page_lock_anon_vma_read,
@ -1855,7 +1887,7 @@ static struct anon_vma *rmap_walk_anon_lock(struct page *page,
* Find all the mappings of a page using the mapping pointer and the vma chains
* contained in the anon_vma struct it points to.
*
* When called from try_to_munlock(), the mmap_lock of the mm containing the vma
* When called from page_mlock(), the mmap_lock of the mm containing the vma
* where the page was found will be held for write. So, we won't recheck
* vm_flags for that VMA. That should be OK, because that vma shouldn't be
* LOCKED.
@ -1908,7 +1940,7 @@ static void rmap_walk_anon(struct page *page, struct rmap_walk_control *rwc,
* Find all the mappings of a page using the mapping pointer and the vma chains
* contained in the address_space struct it points to.
*
* When called from try_to_munlock(), the mmap_lock of the mm containing the vma
* When called from page_mlock(), the mmap_lock of the mm containing the vma
* where the page was found will be held for write. So, we won't recheck
* vm_flags for that VMA. That should be OK, because that vma shouldn't be
* LOCKED.