linux-kernel - Re: [PATCH] mm: prevent concurrent unmap_mapping

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Message-ID: <4D512E63.1040202@oracle.com>
Date:	Tue, 08 Feb 2011 17:22:03 +0530
From:	Gurudas Pai <gurudas.pai@...cle.com>
To:	Miklos Szeredi <miklos@...redi.hu>
CC:	Hugh Dickins <hughd@...gle.com>, hch@...radead.org,
	akpm@...ux-foundation.org, lkml20101129@...ton.leun.net,
	linux-kernel@...r.kernel.org, linux-mm@...ck.org
Subject: Re: [PATCH] mm: prevent concurrent unmap_mapping_range() on the same
 inode

> On Wed, 26 Jan 2011, Hugh Dickins wrote:
>> I had wanted to propose that for now you modify just fuse to use
>> i_alloc_sem for serialization there, and I provide a patch to
>> unmap_mapping_range() to give safety to whatever other cases there are
>> (I'm now sure there are other cases, but also sure that I cannot
>> safely identify them all and fix them correctly at source myself -
>> even if I found time to do the patches, they'd need at least a release
>> cycle to bed in with BUG_ONs).
> 
> Since fuse is the only one where the BUG has actually been triggered,
> and since there are problems with all the proposed generic approaches,
> I concur.  I didn't want to use i_alloc_sem here as it's more
> confusing than a new mutex.
> 
> Gurudas, could you please give this patch a go in your testcase?
I found this BUG with nfs, so trying with current patch may not help.
https://lkml.org/lkml/2010/12/29/9

Let me know if I have to run this
> 
> From: Miklos Szeredi <mszeredi@...e.cz>
> Subject: fuse: prevent concurrent unmap on the same inode
> 
> Running a fuse filesystem with multiple open()'s in parallel can
> trigger a "kernel BUG at mm/truncate.c:475"
> 
> The reason is, unmap_mapping_range() is not prepared for more than
> one concurrent invocation per inode.
> 
> Truncate and hole punching already serialize with i_mutex.  Other
> callers of unmap_mapping_range() do not, and it's difficult to get
> i_mutex protection for all callers.  In particular ->d_revalidate(),
> which calls invalidate_inode_pages2_range() in fuse, may be called
> with or without i_mutex.
> 
> This patch adds a new mutex to fuse_inode to prevent running multiple
> concurrent unmap_mapping_range() on the same mapping.

Thanks,
-Guru



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