diff options
| -rw-r--r-- | mm/page-writeback.c | 45 |
1 files changed, 37 insertions, 8 deletions
diff --git a/mm/page-writeback.c b/mm/page-writeback.c index b3a198c9248..1d2fc89ca56 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); |