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-rw-r--r--mm/page-writeback.c45
1 files 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)
862{ 862{
863 struct address_space *mapping = page_mapping(page); 863 struct address_space *mapping = page_mapping(page);
864 864
865 if (!mapping) 865 if (mapping && mapping_cap_account_dirty(mapping)) {
866 return TestClearPageDirty(page); 866 /*
867 867 * Yes, Virginia, this is indeed insane.
868 if (TestClearPageDirty(page)) { 868 *
869 if (mapping_cap_account_dirty(mapping)) { 869 * We use this sequence to make sure that
870 page_mkclean(page); 870 * (a) we account for dirty stats properly
871 * (b) we tell the low-level filesystem to
872 * mark the whole page dirty if it was
873 * dirty in a pagetable. Only to then
874 * (c) clean the page again and return 1 to
875 * cause the writeback.
876 *
877 * This way we avoid all nasty races with the
878 * dirty bit in multiple places and clearing
879 * them concurrently from different threads.
880 *
881 * Note! Normally the "set_page_dirty(page)"
882 * has no effect on the actual dirty bit - since
883 * that will already usually be set. But we
884 * need the side effects, and it can help us
885 * avoid races.
886 *
887 * We basically use the page "master dirty bit"
888 * as a serialization point for all the different
889 * threads doing their things.
890 *
891 * FIXME! We still have a race here: if somebody
892 * adds the page back to the page tables in
893 * between the "page_mkclean()" and the "TestClearPageDirty()",
894 * we might have it mapped without the dirty bit set.
895 */
896 if (page_mkclean(page))
897 set_page_dirty(page);
898 if (TestClearPageDirty(page)) {
871 dec_zone_page_state(page, NR_FILE_DIRTY); 899 dec_zone_page_state(page, NR_FILE_DIRTY);
900 return 1;
872 } 901 }
873 return 1; 902 return 0;
874 } 903 }
875 return 0; 904 return TestClearPageDirty(page);
876} 905}
877EXPORT_SYMBOL(clear_page_dirty_for_io); 906EXPORT_SYMBOL(clear_page_dirty_for_io);
878 907