From mboxrd@z Thu Jan 1 00:00:00 1970 Date: Mon, 27 Oct 2008 16:30:04 +1100 From: Dave Chinner Subject: Re: deadlock with latest xfs Message-ID: <20081027053004.GF11948@disturbed> References: <4900412A.2050802@sgi.com> <20081023205727.GA28490@infradead.org> <49013C47.4090601@sgi.com> <20081024052418.GO25906@disturbed> <20081024064804.GQ25906@disturbed> <20081026005351.GK18495@disturbed> <20081026025013.GL18495@disturbed> <49051C71.9040404@sgi.com> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <49051C71.9040404@sgi.com> Sender: owner-linux-mm@kvack.org Return-Path: To: Lachlan McIlroy Cc: Christoph Hellwig , xfs-oss , linux-mm@kvack.org List-ID: On Mon, Oct 27, 2008 at 12:42:09PM +1100, Lachlan McIlroy wrote: > Dave Chinner wrote: >> On Sun, Oct 26, 2008 at 11:53:51AM +1100, Dave Chinner wrote: >>> On Fri, Oct 24, 2008 at 05:48:04PM +1100, Dave Chinner wrote: >>>> OK, I just hung a single-threaded rm -rf after this completed: >>>> >>>> # fsstress -p 1024 -n 100 -d /mnt/xfs2/fsstress >>>> >>>> It has hung with this trace: .... >> Got it now. I can reproduce this in a couple of minutes now that both >> the test fs and the fs hosting the UML fs images are using lazy-count=1 >> (and the frequent 10s long host system freezes have gone away, too). >> >> Looks like *another* new memory allocation problem [1]: ..... >> We've entered memory reclaim inside the xfsdatad while trying to do >> unwritten extent completion during I/O completion, and that memory >> reclaim is now blocked waiting for I/o completion that cannot make >> progress. >> >> Nasty. >> >> My initial though is to make _xfs_trans_alloc() able to take a KM_NOFS argument >> so we don't re-enter the FS here. If we get an ENOMEM in this case, we should >> then re-queue the I/O completion at the back of the workqueue and let other >> I/o completions progress before retrying this one. That way the I/O that >> is simply cleaning memory will make progress, hence allowing memory >> allocation to occur successfully when we retry this I/O completion... > It could work - unless it's a synchronous I/O in which case the I/O is not > complete until the extent conversion takes place. Right. Pushing unwritten extent conversion onto a different workqueue is probably the only way to handle this easily. That's the same solution Irix has been using for a long time (the xfsc thread).... > Could we allocate the memory up front before the I/O is issued? Possibly, but that will create more memory pressure than allocation in I/O completion because now we could need to hold thousands of allocations across an I/O - think of the case where we are running low on memory and have a disk subsystem capable of a few hundred thousand I/Os per second. the allocation failing would prevent the I/os from being issued, and if this is buffered writes into unwritten extents we'd be preventing dirty pages from being cleaned.... Cheers, Dave. -- Dave Chinner david@fromorbit.com -- To unsubscribe, send a message with 'unsubscribe linux-mm' in the body to majordomo@kvack.org. For more info on Linux MM, see: http://www.linux-mm.org/ . Don't email: email@kvack.org