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mm: munlock: fix potential race with THP page split

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Since commit ff6a6da60b ("mm: accelerate munlock() treatment of THP
pages") munlock skips tail pages of a munlocked THP page.  There is some
attempt to prevent bad consequences of racing with a THP page split, but
code inspection indicates that there are two problems that may lead to a
non-fatal, yet wrong outcome.

First, __split_huge_page_refcount() copies flags including PageMlocked
from the head page to the tail pages.  Clearing PageMlocked by
munlock_vma_page() in the middle of this operation might result in part
of tail pages left with PageMlocked flag.  As the head page still
appears to be a THP page until all tail pages are processed,
munlock_vma_page() might think it munlocked the whole THP page and skip
all the former tail pages.  Before ff6a6da60, those pages would be
cleared in further iterations of munlock_vma_pages_range(), but NR_MLOCK
would still become undercounted (related the next point).

Second, NR_MLOCK accounting is based on call to hpage_nr_pages() after
the PageMlocked is cleared.  The accounting might also become
inconsistent due to race with __split_huge_page_refcount()

- undercount when HUGE_PMD_NR is subtracted, but some tail pages are
  left with PageMlocked set and counted again (only possible before
  ff6a6da60)

- overcount when hpage_nr_pages() sees a normal page (split has already
  finished), but the parallel split has meanwhile cleared PageMlocked from
  additional tail pages

This patch prevents both problems via extending the scope of lru_lock in
munlock_vma_page().  This is convenient because:

- __split_huge_page_refcount() takes lru_lock for its whole operation

- munlock_vma_page() typically takes lru_lock anyway for page isolation

As this becomes a second function where page isolation is done with
lru_lock already held, factor this out to a new
__munlock_isolate_lru_page() function and clean up the code around.

[akpm@linux-foundation.org: avoid a coding-style ugly]
Signed-off-by: Vlastimil Babka <vbabka@suse.cz>
Cc: Sasha Levin <sasha.levin@oracle.com>
Cc: Michel Lespinasse <walken@google.com>
Cc: Andrea Arcangeli <aarcange@redhat.com>
Cc: Rik van Riel <riel@redhat.com>
Cc: Mel Gorman <mgorman@suse.de>
Cc: Hugh Dickins <hughd@google.com>
Cc: <stable@vger.kernel.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>
This commit is contained in:
Vlastimil Babka 2014-01-23 15:52:50 -08:00 • committed by Linus Torvalds
commit 01cc2e5869
1 changed files with 60 additions and 44 deletions
Download patch file Download diff file Expand all files Collapse all files

104
mm/mlock.c
View file

@ -90,6 +90,26 @@ void mlock_vma_page(struct page *page)
}
}
/*
* Isolate a page from LRU with optional get_page() pin.
* Assumes lru_lock already held and page already pinned.
*/
static bool __munlock_isolate_lru_page(struct page *page, bool getpage)
{
if (PageLRU(page)) {
struct lruvec *lruvec;
lruvec = mem_cgroup_page_lruvec(page, page_zone(page));
if (getpage)
get_page(page);
ClearPageLRU(page);
del_page_from_lru_list(page, lruvec, page_lru(page));
return true;
}
return false;
}
/*
* Finish munlock after successful page isolation
*
@ -126,9 +146,9 @@ static void __munlock_isolated_page(struct page *page)
static void __munlock_isolation_failed(struct page *page)
{
if (PageUnevictable(page))
count_vm_event(UNEVICTABLE_PGSTRANDED);
__count_vm_event(UNEVICTABLE_PGSTRANDED);
else
count_vm_event(UNEVICTABLE_PGMUNLOCKED);
__count_vm_event(UNEVICTABLE_PGMUNLOCKED);
}
/**
@ -152,28 +172,34 @@ static void __munlock_isolation_failed(struct page *page)
unsigned int munlock_vma_page(struct page *page)
{
unsigned int nr_pages;
struct zone *zone = page_zone(page);
BUG_ON(!PageLocked(page));
if (TestClearPageMlocked(page)) {
nr_pages = hpage_nr_pages(page);
mod_zone_page_state(page_zone(page), NR_MLOCK, -nr_pages);
if (!isolate_lru_page(page))
__munlock_isolated_page(page);
else
__munlock_isolation_failed(page);
} else {
nr_pages = hpage_nr_pages(page);
}
/*
* Regardless of the original PageMlocked flag, we determine nr_pages
* after touching the flag. This leaves a possible race with a THP page
* split, such that a whole THP page was munlocked, but nr_pages == 1.
* Returning a smaller mask due to that is OK, the worst that can
* happen is subsequent useless scanning of the former tail pages.
* The NR_MLOCK accounting can however become broken.
* Serialize with any parallel __split_huge_page_refcount() which
* might otherwise copy PageMlocked to part of the tail pages before
* we clear it in the head page. It also stabilizes hpage_nr_pages().
*/
spin_lock_irq(&zone->lru_lock);
nr_pages = hpage_nr_pages(page);
if (!TestClearPageMlocked(page))
goto unlock_out;
__mod_zone_page_state(zone, NR_MLOCK, -nr_pages);
if (__munlock_isolate_lru_page(page, true)) {
spin_unlock_irq(&zone->lru_lock);
__munlock_isolated_page(page);
goto out;
}
__munlock_isolation_failed(page);
unlock_out:
spin_unlock_irq(&zone->lru_lock);
out:
return nr_pages - 1;
}
@ -310,34 +336,24 @@ static void __munlock_pagevec(struct pagevec *pvec, struct zone *zone)
struct page *page = pvec->pages[i];
if (TestClearPageMlocked(page)) {
struct lruvec *lruvec;
int lru;
if (PageLRU(page)) {
lruvec = mem_cgroup_page_lruvec(page, zone);
lru = page_lru(page);
/*
* We already have pin from follow_page_mask()
* so we can spare the get_page() here.
*/
ClearPageLRU(page);
del_page_from_lru_list(page, lruvec, lru);
} else {
__munlock_isolation_failed(page);
goto skip_munlock;
}
} else {
skip_munlock:
/*
* We won't be munlocking this page in the next phase
* but we still need to release the follow_page_mask()
* pin. We cannot do it under lru_lock however. If it's
* the last pin, __page_cache_release would deadlock.
* We already have pin from follow_page_mask()
* so we can spare the get_page() here.
*/
pagevec_add(&pvec_putback, pvec->pages[i]);
pvec->pages[i] = NULL;
if (__munlock_isolate_lru_page(page, false))
continue;
else
__munlock_isolation_failed(page);
}
/*
* We won't be munlocking this page in the next phase
* but we still need to release the follow_page_mask()
* pin. We cannot do it under lru_lock however. If it's
* the last pin, __page_cache_release() would deadlock.
*/
pagevec_add(&pvec_putback, pvec->pages[i]);
pvec->pages[i] = NULL;
}
delta_munlocked = -nr + pagevec_count(&pvec_putback);
__mod_zone_page_state(zone, NR_MLOCK, delta_munlocked);