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

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Message-Id: <E1PhSO8-0005yN-Dp@pomaz-ex.szeredi.hu>
Date:	Mon, 24 Jan 2011 20:47:44 +0100
From:	Miklos Szeredi <miklos@...redi.hu>
To:	Hugh Dickins <hughd@...gle.com>
CC:	miklos@...redi.hu, 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 Fri, 21 Jan 2011, Hugh Dickins wrote:
> 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.

lookup_one_len
lookup_hash
  __lookup_hash
    do_revalidate
      d_revalidate

I don't see an easy way to get rid of i_mutex for lookup_one_len() and
lookup_hash().

> 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.

Yeah, fuse could use i_alloc_sem or a private mutex, but that would
leave the other uses of unmap_mapping_range() to sort this out for
themsevels.

Thanks,
Miklos


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