Re: Ok, explained.. (was Re: [PATCH] mm: fix page_mkclean_one)

[Martin Michlmayr]

* Stephen Clark <Stephen.Clark@xxxxxxxxxx> [2006-12-29 10:17]:
> >It works for me now, both your testcase as well as an installation of
> >Debian on this ARM device.  I manually applied the patch to 2.6.19.
> 
> Can you post a diff against 2.6.19?

--- a/mm/page-writeback.c	2006-11-29 21:57:37.000000000 +0000
+++ b/mm/page-writeback.c	2006-12-29 11:02:55.555147896 +0000
@@ -893,16 +893,45 @@
 {
 	struct address_space *mapping = page_mapping(page);
 
-	if (mapping) {
+	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
+		 * threds 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)) {
-			if (mapping_cap_account_dirty(mapping)) {
-				page_mkclean(page);
-				dec_zone_page_state(page, NR_FILE_DIRTY);
-			}
+			dec_zone_page_state(page, NR_FILE_DIRTY);
 			return 1;
 		}
 		return 0;
-	}
+ 	}
 	return TestClearPageDirty(page);
 }
 EXPORT_SYMBOL(clear_page_dirty_for_io);

-- 
Martin Michlmayr
http://www.cyrius.com/
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