linux-kernel - Re: [BUG?] possible recursive locking detected

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Message-ID: <44C884EF.6010705@yahoo.com.au>
Date:	Thu, 27 Jul 2006 19:18:39 +1000
From:	Nick Piggin <nickpiggin@...oo.com.au>
To:	Anton Altaparmakov <aia21@....ac.uk>
CC:	Andrew Morton <akpm@...l.org>, eike-kernel@...tec.de,
	linux-kernel@...r.kernel.org, aia21@...tab.net
Subject: Re: [BUG?] possible recursive locking detected

Anton Altaparmakov wrote:
> On Thu, 2006-07-27 at 00:38 -0700, Andrew Morton wrote:
> 
>>On Thu, 27 Jul 2006 08:15:27 +0100
>>Anton Altaparmakov <aia21@....ac.uk> wrote:
>>
>>
>>>>I'm surprised ext2 is allocating with __GFP_FS set, though. Would that
>>>>cause any problem?
>>>
>>>That is an ext2 bug IMO.
>>
>>There is no bug.
>>
>>What there is is an ill-defined set of rules.  If we want to tighten these
>>rules we have a choice between
> 
> 
> I beg to differ.  It is a bug.  You cannot reenter the file system when
> the file system is trying to allocate memory.  Otherwise you can never
> allocate memory with any locks held or you are bound to introduce an
> A->B B->A deadlock somewhere.

I don't think it is a bug in general. It really depends on the allocation:

- If it is a path that might be required in order to writeout a page, then
yes GFP_NOFS is going to help prevent deadlocks.

- If it is a path where you'll take the same locks as page reclaim requires,
then again GFP_NOFS is required.

For NTFS case, it seems like holding i_mutex on the write path falls foul
of the second problem. But I agree with Andrew that this is a critical case
where we do have to enter the fs. GFP_NOFS is too big a hammer to use.

I guess you'd have to change NTFS to do something sane privately, or come
up with a nice general solution that doesn't harm the common filesystems
that apparently don't have a problem here... can you just add GFP_NOFS to
NTFS's mapping_gfp_mask to start with?

-- 
SUSE Labs, Novell Inc.
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