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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: Jan Kara <jack@xxxxxxx>
Date: Thu, 22 Apr 2010 21:07:18 +0200
Cc: linux-fsdevel@xxxxxxxxxxxxxxx, linux-kernel@xxxxxxxxxxxxxxx, xfs@xxxxxxxxxxx
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>
User-agent: Mutt/1.5.20 (2009-06-14)
On Tue 20-04-10 12:41:53, 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. Make it observe the current
> value of wbc->nr_to_write and treat a value of <= 0 as though it is a either a
> termination condition or a trigger to reset to MAX_WRITEḆACK_PAGES for data
> integrity syncs.
> 
> Signed-off-by: Dave Chinner <dchinner@xxxxxxxxxx>
> ---
>  fs/fs-writeback.c                |    9 ---------
>  include/linux/writeback.h        |    9 +++++++++
>  include/trace/events/writeback.h |    1 +
>  mm/page-writeback.c              |   20 +++++++++++++++++++-
>  4 files changed, 29 insertions(+), 10 deletions(-)
> 
  <snip>

> diff --git a/mm/page-writeback.c b/mm/page-writeback.c
> index d45f59e..e22af84 100644
> --- a/mm/page-writeback.c
> +++ b/mm/page-writeback.c
> @@ -917,6 +917,7 @@ continue_unlock:
>                       if (!clear_page_dirty_for_io(page))
>                               goto continue_unlock;
>  
> +                     trace_wbc_writepage(wbc);
>                       ret = (*writepage)(page, wbc, data);
>                       if (unlikely(ret)) {
>                               if (ret == AOP_WRITEPAGE_ACTIVATE) {
> @@ -935,7 +936,7 @@ continue_unlock:
>                                       done = 1;
>                                       break;
>                               }
> -                     }
> +                     }
>  
>                       if (nr_to_write > 0) {
>                               nr_to_write--;
> @@ -955,6 +956,23 @@ continue_unlock:
>                                       break;
>                               }
>                       }
> +
> +                     /*
> +                      * Some filesystems will write multiple pages in
> +                      * ->writepage, so wbc->nr_to_write can change much,
> +                      * much faster than nr_to_write. Check this as an exit
> +                      * condition, or if we are doing a data integrity sync,
> +                      * reset the wbc to MAX_WRITEBACK_PAGES so that such
> +                      * filesystems can do optimal writeout here.
> +                      */
> +                     if (wbc->nr_to_write <= 0) {
> +                             if (wbc->sync_mode == WB_SYNC_NONE) {
> +                                     done = 1;
> +                                     nr_to_write = 0;
> +                                     break;
> +                             }
> +                             wbc->nr_to_write = MAX_WRITEBACK_PAGES;
> +                     }
  Honestly, this is an ugly hack. I'd rather work towards ignoring
nr_to_write completely in WB_SYNC_ALL mode since it doesn't really make
any sence to say "write me *safely* 5 pages".

                                                                Honza
-- 
Jan Kara <jack@xxxxxxx>
SUSE Labs, CR

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