Fix memory ordering bug in page reclaim (3d80636a) · Commits · 戴 / test

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 <torvalds@osdl.org>

mm/vmscan.c

+9 −4

Original line number	Diff line number	Diff line
		@@ -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++;

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