From mboxrd@z Thu Jan 1 00:00:00 1970 From: Christoph Hellwig Subject: Re: Oops while rebalancing, now unmountable. Date: Mon, 15 Nov 2010 14:12:04 -0500 Message-ID: <20101115191204.GB11374@infradead.org> References: <1289236257.3611.3.camel@mars> <1289310046-sup-839@think> <1289326892.4231.2.camel@mars> <1289764507.4303.9.camel@mars> <20101114204206.GV6809@random.random> <20101114220018.GA4512@infradead.org> <20101114221222.GX6809@random.random> <20101115182314.GA2493@infradead.org> <20101115184657.GJ6809@random.random> Mime-Version: 1.0 Content-Type: text/plain; charset=us-ascii Cc: Christoph Hellwig , Shane Shrybman , linux-btrfs , Chris Mason , linux-mm@kvack.org, linux-fsdevel@vger.kernel.org To: Andrea Arcangeli Return-path: In-Reply-To: <20101115184657.GJ6809@random.random> List-ID: 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.