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Subject: Re: 2.6.18 mmap hangs unrelated apps
From: Trond Myklebust <trond.myklebust@fys.uio.no>
To: Andrew Morton <akpm@osdl.org>
Cc: Michal Sabala <lkml@saahbs.net>, linux-kernel@vger.kernel.org
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Date: Tue, 19 Dec 2006 19:17:43 -0500
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On Tue, 2006-12-19 at 16:03 -0800, Andrew Morton wrote:
> On Tue, 19 Dec 2006 18:19:38 -0500
> Trond Myklebust <trond.myklebust@fys.uio.no> wrote:
> 
> >     NFS: Fix race in nfs_release_page()
> >     
> >     invalidate_inode_pages2() may set the dirty bit on a page owing to the call
> >     to unmap_mapping_range() after the page was locked. In order to fix this,
> >     NFS has hooked the releasepage() method. This, however leads to deadlocks
> >     in other parts of the VM.
> 
> hmm, subtle.
> 
> >     Fix is to add a new callback: flushpage(), which will write out a dirty
> >     page that is under the page lock.
> >     
> 
> I guess this might permit us to clean up some of the nasties in
> invalidate_inode_pages2() - if the page comes dirty again, write it again. 
> But the requirement that the page remain locked makes it hard.  Need to
> think about it some more.

This was one of the reasons why I had to introduce
nfs_writepage_locked() for 2.6.20 (the other reason being readpage()).

The problem is that you can only protect against redirtying of the page
by holding the page lock across the call to unmap_mapping_range(), the
page writeout and the page removal.

> Are you sure this is the cause of the NFS problem?
> 
> >  	.prepare_write = nfs_prepare_write,
> >  	.commit_write = nfs_commit_write,
> >  	.invalidatepage = nfs_invalidate_page,
> > -	.releasepage = nfs_release_page,
> 
> A NULL ->releasepage means that try_to_release_page() will call
> try_to_free_buffers() if PagePrivate().  I suspect you'll need a stub to
> prevent this.

Ack, I'll add one in. If PagePrivate() is set during the call to
try_to_release_page(), then the page should never be freeable.

> (We were supposed to stop doing that about four years ago - change it so
> that all a_ops must implement ->releasepage, but nobody got around to it).

Would you still be interested in seeing this done?