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

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Date:	Thu, 20 Jan 2011 07:40:43 -0500
From:	Christoph Hellwig <hch@...radead.org>
To:	Miklos Szeredi <miklos@...redi.hu>
Cc:	akpm@...ux-foundation.org, hughd@...gle.com,
	gurudas.pai@...cle.com, 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 Thu, Jan 20, 2011 at 01:30:58PM +0100, Miklos Szeredi wrote:
> From: Miklos Szeredi <mszeredi@...e.cz>
> 
> 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.  For example:
> 
>   thread1: going through a big range, stops in the middle of a vma and
>      stores the restart address in vm_truncate_count.
> 
>   thread2: comes in with a small (e.g. single page) unmap request on
>      the same vma, somewhere before restart_address, finds that the
>      vma was already unmapped up to the restart address and happily
>      returns without doing anything.
> 
> Another scenario would be two big unmap requests, both having to
> restart the unmapping and each one setting vm_truncate_count to its
> own value.  This could go on forever without any of them being able to
> finish.
> 
> 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.


Which I think is mostly a fuse problem.  I really hate bloating the
generic inode (into which the address_space is embedded) with another
mutex for deficits in rather special case filesystems. 

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