From 7658cc289288b8ae7dd2c2224549a048431222b3 Mon Sep 17 00:00:00 2001 From: Linus Torvalds Date: Fri, 29 Dec 2006 10:00:58 -0800 Subject: [PATCH] VM: Fix nasty and subtle race in shared mmap'ed page writeback The VM layer (on the face of it, fairly reasonably) expected that when it does a ->writepage() call to the filesystem, it would write out the full page at that point in time. Especially since it had earlier marked the whole page dirty with "set_page_dirty()". But that isn't actually the case: ->writepage() does not actually write a page, it writes the parts of the page that have been explicitly marked dirty before, *and* that had not got written out for other reasons since the last time we told it they were dirty. That last caveat is the important one. Which _most_ of the time ends up being the whole page (since we had called "set_page_dirty()" on the page earlier), but if the filesystem had done any dirty flushing of its own (for example, to honor some internal write ordering guarantees), it might end up doing only a partial page IO (or none at all) when ->writepage() is actually called. That is the correct thing in general (since we actually often _want_ only the known-dirty parts of the page to be written out), but the shared dirty page handling had implicitly forgotten about these details, and had a number of cases where it was doing just the "->writepage()" part, without telling the low-level filesystem that the whole page might have been re-dirtied as part of being mapped writably into user space. Since most of the time the FS did actually write out the full page, we didn't notice this for a loong time, and this needed some really odd patterns to trigger. But it caused occasional corruption with rtorrent and with the Debian "apt" database, because both use shared mmaps to update the end result. This fixes it. Finally. After way too much hair-pulling. Acked-by: Nick Piggin Acked-by: Martin J. Bligh Acked-by: Martin Michlmayr Acked-by: Martin Johansson Acked-by: Ingo Molnar Acked-by: Andrei Popa Cc: High Dickins Cc: Andrew Morton , Cc: Peter Zijlstra Cc: Segher Boessenkool Cc: David Miller Cc: Arjan van de Ven Cc: Gordon Farquharson Cc: Guillaume Chazarain Cc: Theodore Tso Cc: Kenneth Cheng Cc: Tobias Diedrich Signed-off-by: Linus Torvalds --- mm/page-writeback.c | 45 +++++++++++++++++++++++++++++++++++++-------- 1 file changed, 37 insertions(+), 8 deletions(-) diff --git a/mm/page-writeback.c b/mm/page-writeback.c index b3a198c9248d..1d2fc89ca56d 100644 --- a/mm/page-writeback.c +++ b/mm/page-writeback.c @@ -862,17 +862,46 @@ int clear_page_dirty_for_io(struct page *page) { struct address_space *mapping = page_mapping(page); - if (!mapping) - return TestClearPageDirty(page); - - if (TestClearPageDirty(page)) { - if (mapping_cap_account_dirty(mapping)) { - page_mkclean(page); + if (mapping && mapping_cap_account_dirty(mapping)) { + /* + * Yes, Virginia, this is indeed insane. + * + * We use this sequence to make sure that + * (a) we account for dirty stats properly + * (b) we tell the low-level filesystem to + * mark the whole page dirty if it was + * dirty in a pagetable. Only to then + * (c) clean the page again and return 1 to + * cause the writeback. + * + * This way we avoid all nasty races with the + * dirty bit in multiple places and clearing + * them concurrently from different threads. + * + * Note! Normally the "set_page_dirty(page)" + * has no effect on the actual dirty bit - since + * that will already usually be set. But we + * need the side effects, and it can help us + * avoid races. + * + * We basically use the page "master dirty bit" + * as a serialization point for all the different + * threads doing their things. + * + * FIXME! We still have a race here: if somebody + * adds the page back to the page tables in + * between the "page_mkclean()" and the "TestClearPageDirty()", + * we might have it mapped without the dirty bit set. + */ + if (page_mkclean(page)) + set_page_dirty(page); + if (TestClearPageDirty(page)) { dec_zone_page_state(page, NR_FILE_DIRTY); + return 1; } - return 1; + return 0; } - return 0; + return TestClearPageDirty(page); } EXPORT_SYMBOL(clear_page_dirty_for_io);