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

To: Dave Chinner <david@xxxxxxxxxxxxx>
Subject: Re: [PATCH 3/4] writeback: pay attention to wbc->nr_to_write in write_cache_pages
From: Andrew Morton <akpm@xxxxxxxxxxxxxxxxxxxx>
Date: Thu, 29 Apr 2010 14:39:31 -0700
Cc: linux-fsdevel@xxxxxxxxxxxxxxx, linux-kernel@xxxxxxxxxxxxxxx, xfs@xxxxxxxxxxx, "Aneesh Kumar K.V" <aneesh.kumar@xxxxxxxxxxxxxxxxxx>, "Theodore Ts'o" <tytso@xxxxxxx>
In-reply-to: <1271731314-5893-4-git-send-email-david@xxxxxxxxxxxxx>
References: <1271731314-5893-1-git-send-email-david@xxxxxxxxxxxxx> <1271731314-5893-4-git-send-email-david@xxxxxxxxxxxxx>
On Tue, 20 Apr 2010 12:41:53 +1000
Dave Chinner <david@xxxxxxxxxxxxx> 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@xxxxxxxxxxxxxxxxxx>
: AuthorDate: Thu Oct 16 10:09:17 2008 -0400
: Commit:     Theodore Ts'o <tytso@xxxxxxx>
: 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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