Date: Fri, 3 Oct 2008 09:50:17 -0400 (EDT)
From: Mikulas Patocka <mpatocka@redhat.com>
To: Nick Piggin <nickpiggin@yahoo.com.au>
cc: Andrew Morton <akpm@linux-foundation.org>, linux-kernel@vger.kernel.org,
       agk@redhat.com, mbroz@redhat.com, chris@arachsys.com
Subject: Re: [PATCH] Memory management livelock
In-Reply-To: <200810032231.03985.nickpiggin@yahoo.com.au>
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 <200810031254.29121.nickpiggin@yahoo.com.au>
 <Pine.LNX.4.64.0810030716310.4875@hs20-bc2-1.build.redhat.com>
 <200810032231.03985.nickpiggin@yahoo.com.au>
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> > LVM does (that is where the bug was discovered). Basically, it scans all
> > the block devices with direct IO and if someone else does buffered IO on
> > any device simultaneously, it locks up.
> 
> Scans all block devices with direct IO? Hmm, why, I wonder? Should
> really consider using buffered (posix_fadvise to readahead/dropbehind).

LVM must not allocate any memory when doing IO because it suspends the 
block device and memory allocation could trigger writeback on the 
suspended device and deadlock.

So it preallocates heap and stack, mlockall()s itself and does direct IO.

Well, there are still two allocations on direct IO path --- one in 
__blockdev_direct_IO and the other in dio_bio_alloc. If you have an idea 
how to get rid of them (especially that kzalloc(sizeof(*dio), GFP_KERNEL), 
that bio allocation would be quite easy to avoid), it would be benefical.

Mikulas
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