Subject: + mm-munlock-fix-potential-race-with-thp-page-split.patch added to -mm tree
To: vbabka@xxxxxxx,aarcange@xxxxxxxxxx,hughd@xxxxxxxxxx,mgorman@xxxxxxx,riel@xxxxxxxxxx,sasha.levin@xxxxxxxxxx,stable@xxxxxxxxxxxxxxx,walken@xxxxxxxxxx
From: akpm@xxxxxxxxxxxxxxxxxxxx
Date: Mon, 16 Dec 2013 16:34:39 -0800
The patch titled
Subject: mm: munlock: fix potential race with THP page split
has been added to the -mm tree. Its filename is
mm-munlock-fix-potential-race-with-thp-page-split.patch
This patch should soon appear at
http://ozlabs.org/~akpm/mmots/broken-out/mm-munlock-fix-potential-race-with-thp-page-split.patch
and later at
http://ozlabs.org/~akpm/mmotm/broken-out/mm-munlock-fix-potential-race-with-thp-page-split.patch
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------------------------------------------------------
From: Vlastimil Babka <vbabka@xxxxxxx>
Subject: mm: munlock: fix potential race with THP page split
Since commit ff6a6da60 ("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.
Signed-off-by: Vlastimil Babka <vbabka@xxxxxxx>
Cc: Sasha Levin <sasha.levin@xxxxxxxxxx>
Cc: Michel Lespinasse <walken@xxxxxxxxxx>
Cc: Andrea Arcangeli <aarcange@xxxxxxxxxx>
Cc: Rik van Riel <riel@xxxxxxxxxx>
Cc: Mel Gorman <mgorman@xxxxxxx>
Cc: Hugh Dickins <hughd@xxxxxxxxxx>
Cc: <stable@xxxxxxxxxxxxxxx>
Signed-off-by: Andrew Morton <akpm@xxxxxxxxxxxxxxxxxxxx>
---
mm/mlock.c | 104 +++++++++++++++++++++++++++++----------------------
1 file changed, 60 insertions(+), 44 deletions(-)
diff -puN mm/mlock.c~mm-munlock-fix-potential-race-with-thp-page-split mm/mlock.c
--- a/mm/mlock.c~mm-munlock-fix-potential-race-with-thp-page-split
+++ a/mm/mlock.c
@@ -91,6 +91,26 @@ 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 = 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
*
* Page must be locked. This is a wrapper for try_to_munlock()
@@ -126,9 +146,9 @@ static void __munlock_isolated_page(stru
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);
}
/**
@@ -149,28 +169,34 @@ static void __munlock_isolation_failed(s
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;
}
@@ -307,34 +333,24 @@ static void __munlock_pagevec(struct pag
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);
_
Patches currently in -mm which might be from vbabka@xxxxxxx are
mm-mempolicy-correct-putback-method-for-isolate-pages-if-failed.patch
mm-compaction-respect-ignore_skip_hint-in-update_pageblock_skip.patch
mm-mmapc-add-mlock_future_check-helper.patch
mm-mlock-prepare-params-outside-critical-region.patch
mm-compaction-trace-compaction-begin-and-end.patch
mm-compaction-encapsulate-defer-reset-logic.patch
mm-compaction-reset-cached-scanner-pfns-before-reading-them.patch
mm-compaction-detect-when-scanners-meet-in-isolate_freepages.patch
mm-compaction-do-not-mark-unmovable-pageblocks-as-skipped-in-async-compaction.patch
mm-compaction-reset-scanner-positions-immediately-when-they-meet.patch
mm-migrate-add-comment-about-permanent-failure-path.patch
mm-migrate-correct-failure-handling-if-hugepage_migration_support.patch
mm-migrate-remove-putback_lru_pages-fix-comment-on-putback_movable_pages.patch
mm-migrate-remove-unused-function-fail_migrate_page.patch
mm-documentation-remove-hopelessly-out-of-date-locking-doc.patch
mm-munlock-fix-a-bug-where-thp-tail-page-is-encountered.patch
mm-munlock-fix-deadlock-in-__munlock_pagevec.patch
mm-munlock-fix-deadlock-in-__munlock_pagevec-fix.patch
mm-munlock-fix-potential-race-with-thp-page-split.patch
mm-munlock-fix-potential-race-with-thp-page-split-fix.patch
linux-next.patch
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