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

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Date:	Wed, 9 Jun 2010 14:09:42 -0700
From:	Andrew Morton <akpm@...ux-foundation.org>
To:	Dave Chinner <david@...morbit.com>
Cc:	torvalds@...ux-foundation.org, linux-fsdevel@...r.kernel.org,
	linux-kernel@...r.kernel.org, xfs@....sgi.com, stable@...nel.org
Subject: Re: [PATCH 1/3] writeback: pay attention to wbc->nr_to_write in
 write_cache_pages

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"??

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