linux-kernel - Re: [PATCH 3/4] writeback: pay attention to wbc->nr_to_write in write_cache

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Date:	Thu, 29 Apr 2010 14:39:31 -0700
From:	Andrew Morton <akpm@...ux-foundation.org>
To:	Dave Chinner <david@...morbit.com>
Cc:	linux-fsdevel@...r.kernel.org, linux-kernel@...r.kernel.org,
	xfs@....sgi.com,
	"Aneesh Kumar K.V" <aneesh.kumar@...ux.vnet.ibm.com>,
	"Theodore Ts'o" <tytso@....edu>
Subject: Re: [PATCH 3/4] writeback: pay attention to wbc->nr_to_write in
 write_cache_pages

On Tue, 20 Apr 2010 12:41:53 +1000
Dave Chinner <david@...morbit.com> wrote:

> 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
> 

Bug.

AFAIT it's a regression introduced by

: commit 17bc6c30cf6bfffd816bdc53682dd46fc34a2cf4
: Author:     Aneesh Kumar K.V <aneesh.kumar@...ux.vnet.ibm.com>
: AuthorDate: Thu Oct 16 10:09:17 2008 -0400
: Commit:     Theodore Ts'o <tytso@....edu>
: CommitDate: Thu Oct 16 10:09:17 2008 -0400
: 
:     vfs: Add no_nrwrite_index_update writeback control flag

I suggest that what you do here is remove the local `nr_to_write' from
write_cache_pages() and go back to directly using wbc->nr_to_write
within the loop.

And thus we restore the convention that if the fs writes back more than
a single page, it subtracts (nr_written - 1) from wbc->nr_to_write.

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