From 3d80636a0d5f056ffc26472d05b6027a7a9f6e1c Mon Sep 17 00:00:00 2001 From: Linus Torvalds Date: Sun, 16 Oct 2005 17:36:06 -0700 Subject: [PATCH] Fix memory ordering bug in page reclaim As noticed by Nick Piggin, we need to make sure that we check the page count before we check for PageDirty, since the dirty check is only valid if the count implies that we're the only possible ones holding the page. We always did do this, but the code needs a read-memory-barrier to make sure that the orderign is also honored by the CPU. (The writer side is ordered due to the atomic decrement and test on the page count, see the discussion on linux-kernel) Signed-off-by: Linus Torvalds --- mm/vmscan.c | 13 +++++++++---- 1 file changed, 9 insertions(+), 4 deletions(-) diff --git a/mm/vmscan.c b/mm/vmscan.c index 0ea71e887bb..64f9570cff5 100644 --- a/mm/vmscan.c +++ b/mm/vmscan.c @@ -511,10 +511,11 @@ static int shrink_list(struct list_head *page_list, struct scan_control *sc) * PageDirty _after_ making sure that the page is freeable and * not in use by anybody. (pagecache + us == 2) */ - if (page_count(page) != 2 || PageDirty(page)) { - write_unlock_irq(&mapping->tree_lock); - goto keep_locked; - } + if (unlikely(page_count(page) != 2)) + goto cannot_free; + smp_rmb(); + if (unlikely(PageDirty(page))) + goto cannot_free; #ifdef CONFIG_SWAP if (PageSwapCache(page)) { @@ -538,6 +539,10 @@ free_it: __pagevec_release_nonlru(&freed_pvec); continue; +cannot_free: + write_unlock_irq(&mapping->tree_lock); + goto keep_locked; + activate_locked: SetPageActive(page); pgactivate++; -- 2.20.1