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

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Date:	Tue, 08 Feb 2011 12:59:51 +0100
From:	Miklos Szeredi <miklos@...redi.hu>
To:	Gurudas Pai <gurudas.pai@...cle.com>
CC:	Trond.Myklebust@...app.com, miklos@...redi.hu, 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 Tue, 08 Feb 2011, Gurudas Pai wrote:
> > 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

Ahh, I was not aware of that.  No, in that case there's not much point
in trying this patch for you as it only fixes the issue in fuse. I
haven't looked at the NFS side of it yet.

Added Trond to the Cc.

Thanks,
Miklos


> > 
> > 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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