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

To: Dave Chinner <david@xxxxxxxxxxxxx>
Subject: Re: [PATCH 1/3] writeback: pay attention to wbc->nr_to_write in write_cache_pages
From: Andrew Morton <akpm@xxxxxxxxxxxxxxxxxxxx>
Date: Wed, 9 Jun 2010 14:09:42 -0700
Cc: torvalds@xxxxxxxxxxxxxxxxxxxx, linux-fsdevel@xxxxxxxxxxxxxxx, linux-kernel@xxxxxxxxxxxxxxx, xfs@xxxxxxxxxxx, stable@xxxxxxxxxx
In-reply-to: <1276043840-1946-2-git-send-email-david@xxxxxxxxxxxxx>
References: <1276043840-1946-1-git-send-email-david@xxxxxxxxxxxxx> <1276043840-1946-2-git-send-email-david@xxxxxxxxxxxxx>
On Wed,  9 Jun 2010 10:37:18 +1000
Dave Chinner <david@xxxxxxxxxxxxx> wrote:

> From: Dave Chinner <dchinner@xxxxxxxxxx>
> 
> 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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