linux-kernel - Re: [stable] [PATCH 1/3] writeback: pay attention to wbc->nr_to_write in write_cache

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Date:	Wed, 23 Jun 2010 13:36:53 -0700
From:	Greg KH <greg@...ah.com>
To:	Dave Chinner <david@...morbit.com>
Cc:	Andrew Morton <akpm@...ux-foundation.org>,
	linux-fsdevel@...r.kernel.org, stable@...nel.org,
	torvalds@...ux-foundation.org, linux-kernel@...r.kernel.org,
	xfs@....sgi.com
Subject: Re: [stable] [PATCH 1/3] writeback: pay attention to
 wbc->nr_to_write in write_cache_pages

On Thu, Jun 10, 2010 at 10:08:11AM +1000, Dave Chinner wrote:
> On Thu, Jun 10, 2010 at 08:58:04AM +1000, Dave Chinner wrote:
> > On Wed, Jun 09, 2010 at 02:09:42PM -0700, Andrew Morton wrote:
> > > On Wed,  9 Jun 2010 10:37:18 +1000
> > > Dave Chinner <david@...morbit.com> wrote:
> > > 
> > > > From: Dave Chinner <dchinner@...hat.com>
> > > > 
> > > > If a filesystem writes more than one page in ->writepage, write_cache_pages
> > > > fails to notice this and continues to attempt writeback when wbc->nr_to_write
> > > > has gone negative - this trace was captured from XFS:
> > > > 
> > > > 
> > > >     wbc_writeback_start: towrt=1024
> > > >     wbc_writepage: towrt=1024
> > > >     wbc_writepage: towrt=0
> > > >     wbc_writepage: towrt=-1
> > > >     wbc_writepage: towrt=-5
> > > >     wbc_writepage: towrt=-21
> > > >     wbc_writepage: towrt=-85
> > > > 
> > > > This has adverse effects on filesystem writeback behaviour. write_cache_pages()
> > > > needs to terminate after a certain number of pages are written, not after a
> > > > certain number of calls to ->writepage are made.  This is a regression
> > > > introduced by 17bc6c30cf6bfffd816bdc53682dd46fc34a2cf4 ("vfs: Add
> > > > no_nrwrite_index_update writeback control flag"), but cannot be reverted
> > > > directly due to subsequent bug fixes that have gone in on top of it.
> > > 
> > > Might be needed in -stable.  Unfortunately the most important piece of
> > > information which is needed to make that decision was cunningly hidden
> > > from us behind the vague-to-the-point-of-uselessness term "adverse
> > > effects".
> > > 
> > > _what_ "adverse effects"??
> > 
> > Depends on how the specific filesystem handles a negative
> > nr_to_write, doesn't it? I can't speak for the exact effect on
> > anything other than XFS except to say that most ->write_page
> > implemetnations don't handle the wbc->nr_to_write < 0 specifically...
> > 
> > For XFS, it results in increased CPU usage because it triggers
> > page-at-a-time allocation (i.e no clustering), which increases
> > overhead in the elveator due to merging requirements of single page
> > bios and increased fragmentation due to small interleaved
> > allocations on concurrent writeback workloads. Effectively it causes
> > accelerated aging of XFS filesystems...
> 
> Sorry, forgot to address the -stable part of the question.
> 
> This series is dependent on the ext4 change to use it's own
> writepage going into -stable first.  (i.e.
> 8e48dcfbd7c0892b4cfd064d682cc4c95a29df32 "ext4: Use our own
> write_cache_pages()").
> 
> I'd suggest that all 4 patches (the ext4 patch and the three in this
> series) should go back to 2.6.34-stable due to the long term affect
> this writeback bug could have on XFS filesystems, and the sync
> taking too long problem has been fairly widely reported since at
> least .32...

Ok, can someone please tell me the git commit ids that need to be
applied to the -stable trees?

thanks,

greg k-h
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