From 80a2c1763be3139b13f12374a293b04e94f139aa Mon Sep 17 00:00:00 2001 From: Vlastimil Babka Date: Fri, 3 Jan 2014 14:10:03 +1100 Subject: [PATCH] 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 Cc: Sasha Levin Cc: Michel Lespinasse Cc: Andrea Arcangeli Cc: Rik van Riel Cc: Mel Gorman Cc: Hugh Dickins Cc: Signed-off-by: Andrew Morton --- mm/mlock.c | 104 ++++++++++++++++++++++++++++++----------------------- 1 file changed, 60 insertions(+), 44 deletions(-) diff --git a/mm/mlock.c b/mm/mlock.c index 10819ed4df3e..69a0da93aeed 100644 --- a/mm/mlock.c +++ b/mm/mlock.c @@ -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 = 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); -- 2.39.5