Do dirty page accounting when removing a page from the page cache

Krzysztof Oledzki noticed a dirty page accounting leak on some of his
machines, causing the machine to eventually lock up when the kernel
decided that there was too much dirty data, but nobody could actually
write anything out to fix it.

The culprit turns out to be filesystems (cough ext3 with data=journal
cough) that re-dirty the page when the "->invalidatepage()" callback is
called.

Fix it up by doing a final dirty page accounting check when we actually
remove the page from the page cache.

This fixes bugzilla entry 9182:

http://bugzilla.kernel.org/show_bug.cgi?id=9182

Tested-by: Ingo Molnar <[e-mail]>
Tested-by: Krzysztof Oledzki <[e-mail]>
Cc: Andrew Morton <[e-mail]>
Cc: Nick Piggin <[e-mail]>
Cc: Peter Zijlstra <[e-mail]>
Signed-off-by: Linus Torvalds <[e-mail]>

mm/filemap.c

···
 	mapping->nrpages--;
 	__dec_zone_page_state(page, NR_FILE_PAGES);
 	BUG_ON(page_mapped(page));
+
+	/*
+	 * Some filesystems seem to re-dirty the page even after
+	 * the VM has canceled the dirty bit (eg ext3 journaling).
+	 *
+	 * Fix it up by doing a final dirty accounting check after
+	 * having removed the page entirely.
+	 */
+	if (PageDirty(page) && mapping_cap_account_dirty(mapping)) {
+		dec_zone_page_state(page, NR_FILE_DIRTY);
+		dec_bdi_stat(mapping->backing_dev_info, BDI_RECLAIMABLE);
+	}
 }
 
 void remove_from_page_cache(struct page *page)