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SubjectRe: sync-Regression in 2.6.28.2?
Nick Piggin wrote:
> [...]
> Thanks, could you reply-to-all when replying to retain ccs please?
>
> Common theme is ext4, which uses no_nrwrite_index_update, and I introduced
> a bug in there which could possibly cause ext4 to go into a loop...
>
> Would it be possible if you can test the following patch?
>

I'll test it as soon as I get home.

Meanwhile, I think the new patch may be slightly wrong. If I understand
correctly PageWriteback(page) is called before nr_to_write is tested for
being > 0 and then decremented if true, but "done" is not set to 1
until the next iteration. So another call to PageWriteback(page) while
take place and then "done" will be set to true (if wbc->sync_mode ==
WB_SYNC_NONE).

If nr_to_write == 1 at the beginning of the loop then two pages will be
written.

I think the test condition should something like:

if (--nr_to_write <= 0 && wbc->sync_mode == WB_SYNC_NONE) {
done = 1;
break;
}

.
.

> Signed-off-by: Artem Bityutskiy <Artem.Bityutskiy@nokia.com>
> Acked-by: Nick Piggin <npiggin@suse.de>
> Cc: Andrew Morton <akpm@linux-foundation.org>
> ---
> mm/page-writeback.c | 21 +++++++++++++++------
> 1 files changed, 15 insertions(+), 6 deletions(-)
>
> diff --git a/mm/page-writeback.c b/mm/page-writeback.c
> index b493db7..dc32dae 100644
> --- a/mm/page-writeback.c
> +++ b/mm/page-writeback.c
> @@ -1051,13 +1051,22 @@ continue_unlock:
> }
> }
>
> - if (wbc->sync_mode == WB_SYNC_NONE) {
> - wbc->nr_to_write--;
> - if (wbc->nr_to_write <= 0) {
> - done = 1;
> - break;
> - }
> + if (nr_to_write > 0)
> + nr_to_write--;
> + else if (wbc->sync_mode == WB_SYNC_NONE) {
> + /*
> + * We stop writing back only if we are not
> + * doing integrity sync. In case of integrity
> + * sync we have to keep going because someone
> + * may be concurrently dirtying pages, and we
> + * might have synced a lot of newly appeared
> + * dirty pages, but have not synced all of the
> + * old dirty pages.
> + */
> + done = 1;
> + break;
> }
> +
> if (wbc->nonblocking && bdi_write_congested(bdi)) {
> wbc->encountered_congestion = 1;
> done = 1;
>



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