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

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Date:	Fri, 21 Jan 2011 20:46:00 -0800 (PST)
From:	Hugh Dickins <hughd@...gle.com>
To:	Miklos Szeredi <miklos@...redi.hu>
cc:	Christoph Hellwig <hch@...radead.org>, akpm@...ux-foundation.org,
	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, 20 Jan 2011, Miklos Szeredi wrote:
> On Thu, 20 Jan 2011, Christoph Hellwig wrote:
> > On Thu, Jan 20, 2011 at 01:30:58PM +0100, Miklos Szeredi wrote:
> > > 
> > > 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. 
> 
> As Hugh pointed out unmap_mapping_range() has grown a varied set of
> callers, which are difficult to fix up wrt i_mutex.  Fuse was just an
> example.
> 
> I don't like the bloat either, but this is the best I could come up
> with for fixing this problem generally.  If you have a better idea,
> please share it.

If we start from the point that this is mostly a fuse problem (I expect
that a thorough audit will show up a few other filesystems too, but
let's start from this point): you cite ->d_revalidate as a particular
problem, but can we fix up its call sites so that it is always called
either with, or much preferably without, i_mutex held?  Though actually
I couldn't find where ->d_revalidate() is called while holding i_mutex.

Failing that, can fuse down_write i_alloc_sem before calling
invalidate_inode_pages2(_range), to achieve the same exclusion?
The setattr truncation path takes i_alloc_sem as well as i_mutex,
though I'm not certain of its full coverage.

I did already consider holding and dropping i_alloc_sem inside
invalidate_inode_pages2_range(); but direct-io.c very much wants
to take mmap_sem (when get_user_pages_fast goes slow) after taking
i_alloc_sem, whereas fuse_direct_mmap() very much wants to call
invalidate_inode_pages2() while mmap_sem is held.

Hugh
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