linux-kernel - Re: [PATCH 1/2]: Fix BUG in cancel_dirty

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Date:	Thu, 25 Jan 2007 11:05:15 +1100
From:	Nick Piggin <nickpiggin@...oo.com.au>
To:	Peter Zijlstra <a.p.zijlstra@...llo.nl>
CC:	David Chinner <dgc@....com>, linux-kernel@...r.kernel.org,
	xfs@....sgi.com, akpm@...l.org
Subject: Re: [PATCH 1/2]: Fix BUG in cancel_dirty_pages on XFS

Peter Zijlstra wrote:
> On Thu, 2007-01-25 at 00:43 +1100, Nick Piggin wrote:
> 
> 
>>>Have you seen the new launder_page() a_op? called from
>>>invalidate_inode_pages2_range()
>>
>>It would have been nice to make that one into a more potentially
>>useful generic callback.
> 
> 
> That can still be done when the need arises, right?

Yeah I guess so.

>>But why was it introduced, exactly? I can't tell from the code or
>>the discussion why NFS couldn't start the IO, and signal the caller
>>to wait_on_page_writeback and retry? That seemed to me like the
>>convetional fix.
> 
> 
> to quote a bit:
> 
> On Tue, 19 Dec 2006 18:19:38 -0500
> Trond Myklebust <trond.myklebust@....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.
> 
> 
> and:
> 
> 
>>>Now, arguably the VM shouldn't be calling try_to_release_page() with
>>>__GFP_FS when it's holding a lock on a page.
>>>
>>>But otoh, NFS should never be running lock_page() within nfs_release_page()
>>>against the page which was passed into nfs_release_page().  It'll deadlock
>>>for sure.
>>
>>The reason why it is happening is that the last dirty page from that
>>inode gets cleaned, resulting in a call to dput().

OK but what's the problem with just failing to release the page if it
is dirty, I wonder? In the worst case, page reclaim will just end up
doing a writeout to clean it.

-- 
SUSE Labs, Novell Inc.
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