Re: [PATCH] ext4: return correct wbc.nr_to_write in ext4_da_writepages

Aneesh Kumar K.V
2009-12-17T17:32:41Z

On Thu, Dec 17, 2009 at 09:40:25AM -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@rsk.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 ...

I guess writeback code can handle nr_to_write going negative. If we are
not updating wbc->nr_to_write then i guess writeback code will get a
wrong value for number of pages written and can end up doing wrong things
We had it that way as a part of 22208dedbd7626e5fc4339c417f8d24cc21f79d7
and i guess we didn't had any problems with that

So for the patch

Acked-by: Aneesh Kumar K.V <aneesh.kumar@linux.vnet.ibm.com>

-aneesh