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Message-Id: <1261065506.4882.47.camel@castor>
Date: Thu, 17 Dec 2009 15:58:26 +0000
From: Richard Kennedy <richard@....demon.co.uk>
To: Eric Sandeen <sandeen@...hat.com>
Cc: tytso@....edu, adilger@....com, linux-ext4@...r.kernel.org,
lkml <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH] ext4: return correct wbc.nr_to_write in
ext4_da_writepages
On Thu, 2009-12-17 at 09:40 -0600, Eric Sandeen wrote:
> Richard Kennedy wrote:
> > ext4: always re-base nr_to_write in ext4_da_writepages
> >
> > When ext4_da_writepages increases the nr_to_write in writeback_control
> > then it must always re-base the return value.
> >
> > Without this change, when wb_writeback calculates how many pages were
> > actually written it can get a negative value and loop more times than
> > necessary. In tests I have seen nearly all the dirty pages pushed out to
> > writeback due to this issue.
> >
> > Signed-off-by: Richard Kennedy <richard@....demon.co.uk>
> >
> > ----
> >
> > patch against 2.6.32
> > tested on x86_64
> >
> > wb_writeback calculates (MAX_WRITE_PAGES - nr_to_write) & cannot know
> > that the value got changed.
> >
> > I'm not sure what the test I removed was for.
> > Perhaps
> > if (nr_to_writebump)
> > wbc->nr_to_write -= nr_to_writebump;
> > was intended?
>
> Ted's commit 55138e0b added it (just part of the commit):
>
> @@ -2914,7 +2994,8 @@ retry:
> out_writepages:
> if (!no_nrwrite_index_update)
> wbc->no_nrwrite_index_update = 0;
> - wbc->nr_to_write -= nr_to_writebump;
> + if (wbc->nr_to_write > nr_to_writebump)
> + wbc->nr_to_write -= nr_to_writebump;
> wbc->range_start = range_start;
> trace_ext4_da_writepages_result(inode, wbc, ret, pages_written);
> return ret;
>
> so it looks like the intent there was to stop ->nr_to_write from
> going negative ...
>
>
wb_writeback is OK with negative, it just needs to know how many pages
were written. Then it can decide if it's done the work it was asked to
do. balance_dirty_pages uses this throttle a device by asking for
writeback on a small number of pages.
regards
Richard
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