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Re: [PATCH 4/6] writeback: pay attention to wbc->nr_to_write in write_ca

To: Dave Chinner <david@xxxxxxxxxxxxx>
Subject: Re: [PATCH 4/6] writeback: pay attention to wbc->nr_to_write in write_cache_pages
From: Nick Piggin <npiggin@xxxxxxx>
Date: Tue, 8 Jun 2010 15:43:30 +1000
Cc: torvalds@xxxxxxxxxxxxxxxxxxxx, linux-fsdevel@xxxxxxxxxxxxxxx, linux-kernel@xxxxxxxxxxxxxxx, xfs@xxxxxxxxxxx, akpm@xxxxxxxxxxxxxxxxxxxx
In-reply-to: <1275957487-23633-5-git-send-email-david@xxxxxxxxxxxxx>
References: <1275957487-23633-1-git-send-email-david@xxxxxxxxxxxxx> <1275957487-23633-5-git-send-email-david@xxxxxxxxxxxxx>
User-agent: Mutt/1.5.20 (2009-06-14)
On Tue, Jun 08, 2010 at 10:38:05AM +1000, Dave Chinner 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.
> Signed-off-by: Dave Chinner <dchinner@xxxxxxxxxx>
> Reviewed-by: Christoph Hellwig <hch@xxxxxx>
Acked-by: Nick Piggin <npiggin@xxxxxxx>

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