Excerpts from Christoph Hellwig's message of 2010-11-15 14:12:04 -0500: > On Mon, Nov 15, 2010 at 07:46:57PM +0100, Andrea Arcangeli wrote: > > I've been reading the writeout() in mm/migrate.c and I wonder if maybe > > that should have been WB_SYNC_ALL or if we miss a > > wait_on_page_writeback in after ->writepage() returns? Can you have a > > look there? We check the PG_writeback bit when the page is not dirty > > (well before fallback_migrate_page is called), but after calling > > writeout() we don't return to wait on PG_writeback. We make sure to > > hold the page lock after ->writepage returns but that doesn't mean > > PG_writeback isn't still set. > > I didn't even notice that, but the WB_SYNC_NONE does indeed seem > buggy to me. If we set the sync_mode to WB_SYNC_NONE filesystem > can and frequently do trylock operations and might just skip to > write it out completely. > > So we defintively do need to change writeout to do a WB_SYNC_ALL > writeback. In addition to that we'll also need the > wait_on_page_writeback call to make sure we actually wait for I/O > to finish. > > Also what protects us from updating the page while we write it out? > PG_writeback on many filesystems doesn't protect writes from modifying > the in-flight buffer, and just locking the page after ->writepage > is racy without a check that nothing changed. > Oh, I should have thought of that. Btrfs (and most of the time xfs?) will wait on PageWriteback internally, but for the ext crowd we're in trouble. -chris -- To unsubscribe from this list: send the line "unsubscribe linux-fsdevel" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
