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Re: 3.14-rc2 XFS backtrace because irqs_disabled.

To: Dave Jones <davej@xxxxxxxxxx>, Al Viro <viro@xxxxxxxxxxxxxxxxxx>, Linus Torvalds <torvalds@xxxxxxxxxxxxxxxxxxxx>, Eric Sandeen <sandeen@xxxxxxxxxxx>, Linux Kernel <linux-kernel@xxxxxxxxxxxxxxx>, xfs@xxxxxxxxxxx
Subject: Re: 3.14-rc2 XFS backtrace because irqs_disabled.
From: Dave Chinner <david@xxxxxxxxxxxxx>
Date: Wed, 12 Feb 2014 17:31:50 +1100
Delivered-to: xfs@xxxxxxxxxxx
In-reply-to: <20140212061038.GC13997@dastard>
References: <20140211210841.GM13647@dastard> <52FA9ADA.9040803@xxxxxxxxxxx> <20140212004403.GA17129@xxxxxxxxxx> <20140212010941.GM18016@xxxxxxxxxxxxxxxxxx> <CA+55aFwoWT-0A_KTkXMkNqOy8hc=YmouTMBgWUD_z+8qYPphjA@xxxxxxxxxxxxxx> <20140212040358.GA25327@xxxxxxxxxx> <20140212042215.GN18016@xxxxxxxxxxxxxxxxxx> <20140212054043.GB13997@dastard> <20140212055027.GA28502@xxxxxxxxxx> <20140212061038.GC13997@dastard>
User-agent: Mutt/1.5.21 (2010-09-15)
On Wed, Feb 12, 2014 at 05:10:38PM +1100, Dave Chinner wrote:
> On Wed, Feb 12, 2014 at 12:50:27AM -0500, Dave Jones wrote:
> > On Wed, Feb 12, 2014 at 04:40:43PM +1100, Dave Chinner wrote:
> > 
> >  > None of the XFS code disables interrupts in that path, not does is
> >  > call outside XFS except to dispatch IO. The stack is pretty deep at
> >  > this point and I know that the standard (non stacked) IO stack can
> >  > consume >3kb of stack space when it gets down to having to do memory
> >  > reclaim during GFP_NOIO allocation at the lowest level of SCSI
> >  > drivers. Stack overruns typically show up with symptoms like we are
> >  > seeing.
> >  > ..
> >  > 
> >  > Dave, before chasing ghosts, can you (like Eric originally asked)
> >  > turn on stack overrun detection?
> > 
> > CONFIG_DEBUG_STACKOVERFLOW ? Already turned on.
> 
> That only checks stack usage when an interrupt is taken. If no
> interrupts are taken when stack usage is within 128 bytes of
> overflow, then it doesn't catch it.
> 
> I tend to use CONFIG_DEBUG_STACK_USAGE=y as it records the maximum
> stack usage of a process via canary overwrites and it records it in
> do_exit(). I also use the stack tracer to record the largest stack
> usage seen so I know exactly what code paths are approaching stack
> overruns...

FYI, just creating lots of files with open(O_CREAT):

[  348.718357] fs_mark (4828) used greatest stack depth: 2968 bytes left
[  348.769846] fs_mark (4814) used greatest stack depth: 2312 bytes left
[  349.777717] fs_mark (4826) used greatest stack depth: 2280 bytes left
[  418.139415] fs_mark (4928) used greatest stack depth: 1936 bytes left
[  460.492282] fs_mark (4993) used greatest stack depth: 1336 bytes left
[  544.825418] fs_mark (5104) used greatest stack depth: 1112 bytes left
[  689.503970] fs_mark (5265) used greatest stack depth: 1000 bytes left

We've got absolutely no spare stack space anymore in the IO path.
And the IO path can't get much simpler than filesystem -> virtio
block device.

Cheers,

Dave.
-- 
Dave Chinner
david@xxxxxxxxxxxxx

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