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. > Compaction practically only happens in the context of the task > allocating memory (in my tree it is also used by kswapd). Not > immediate to ask a separate daemon to invoke it. Not sure why this > should screw delalloc. Compaction isn't freeing any memory at all, > it's not reclaim. It just defragments and moves stuff around and it > may have to write dirty pages to do so. kswapd is fine. Other task allocation memory are direct reclaimers. Direct reclaim through the filesystem delalloc conversion and the I/O stack guarantees you stack overflows, that's why filesystems refuse to do anything in ->writepage for this case. btrfs and XFS have explicit checks for PF_MEMALLOC (with a carve out for kswapd in XFS), and ext4 only writes already allocated blocks in ->writepage but never does delalloc conversions. -- 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
