Re: [BUGFIX re-send] [PATCH] write-back: fix nr_to_write counter

[Nick Piggin <nickpiggin@yahoo.com.au>]

On Tuesday 03 February 2009 20:13:56 Nick Piggin wrote:
> On Tuesday 03 February 2009 19:42:22 Artem Bityutskiy wrote:
> > Hi,
> >
> > commit 05fe478dd04e02fa230c305ab9b5616669821dd3
> > Author: Nick Piggin <npiggin@suse.de>
> > Date:   Tue Jan 6 14:39:08 2009 -0800
> >
> >     mm: write_cache_pages integrity fix
> >
> > broke wbc->nr_to_write handling. Here is the fix.
> >
> > I'm not 100% sure I got things right, because I am far not expert in the
> > area. Please, review it. The patch fixes my UBIFS issues, which are
> > caused by the fact that wbc->nr_to_write is not updated.
> > ======================================================================
> >
> > From: Artem Bityutskiy <Artem.Bityutskiy@nokia.com>
> > Date: Mon, 2 Feb 2009 18:33:49 +0200
> > Subject: [PATCH] write-back: fix nr_to_write counter
> >
> > Commit 05fe478dd04e02fa230c305ab9b5616669821dd3 broke @wbc->nr_to_write.
> > 'write_cache_pages()' changes it in the loop, but restores the original
> > value from @nr_to_write at the end, because of this code:
> >
> >        if (!wbc->no_nrwrite_index_update) {
> >                 if (wbc->range_cyclic || (range_whole && nr_to_write >
> > 0)) mapping->writeback_index = done_index;
> >                 wbc->nr_to_write = nr_to_write;
> >         }
>
> The commit you quote only moves nr_to_write to not take effect for
> WB_SYNC_ALL (ie. data integrity) writeout. And makes no other change
> to write_cache_pages.
>
> I thought your problem might have been that you were calling this
> with WB_SYNC_ALL and expecting it to heed nr_to_write, however...
>
> > Also, I think wbc->nr_to_write should be changed in all cases, not only
> > when wbc->sync_mode == WB_SYNC_NONE.
>
> ... you mention this here like it is an *additional* issue on top
> of your problem. So I fail to see how my commit could have caused
> this problem?
>
> > Well, in case of @wbc->no_nrwrite_index_update != 0, we do change
> > wbc->nr_to_write, while we should not. This patch fixes this behavior.
>
> And I don't know what you mean by this because the patch doesn't
> fix any problem there AFAIKS.
>
> Anyway, I did probably not pay enough attention to ubifs when making
> this change, and if it wants wbc->nr_to_write updated even for data
> integrity syncs, I don't see the harm in that. So I don't have any
> objection to your patch. Thanks.
>
> Can you cc stable@kernel.org when a final version gets merged upstream
> please?

What I mean to say is that while I think the patch is not a problem,
and actually should be merged if it is required for ubifs, I would
like to just see the changelog clarified...

Also if you add a nice comment there, maybe mention that nr_to_write
must reflect the number of pages written even for the WB_SYNC_ALL case
(arguably the data integrity issue which you comment on is more
obvious in comparison to the nr_to_write requirement of callers).