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From: Dave Chinner on 20 Apr 2010 19:40
On Wed, Apr 21, 2010 at 12:28:19AM +0100, Jamie Lokier wrote:
> Dave Chinner wrote:
> > sync can currently take a really long time if a concurrent writer is
> > extending a file. The problem is that the dirty pages on the address
> > space grow in the same direction as write_cache_pages scans, so if
> > the writer keeps ahead of writeback, the writeback will not
> > terminate until the writer stops adding dirty pages.
> >
> > For a data integrity sync, we only need to write the pages dirty at
> > the time we start the writeback, so we can stop scanning once we get
> > to the page that was at the end of the file at the time the scan
> > started.
> >
> > This will prevent operations like copying a large file preventing
> > sync from completing as it will not write back pages that were
> > dirtied after the sync was started. This does not impact the
> > existing integrity guarantees, as any dirty page (old or new)
> > within the EOF range at the start of the scan will still be
> > captured.
>
> I guess it can still get stuck if someone does ftruncate() first, then
> writes to the hole?

Yes, it would. It only deals with extending files because fixing the
problem w.r.t. writes into holes requires something much more
invasive like Jan's radix tree mark-and-sweep algorithm....

Cheers,

Dave.
--
Dave Chinner
david(a)fromorbit.com
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From: Jan Kara on 22 Apr 2010 15:20
On Tue 20-04-10 13:40:05, Dave Chinner wrote:
>
> sync can currently take a really long time if a concurrent writer is
> extending a file. The problem is that the dirty pages on the address
> space grow in the same direction as write_cache_pages scans, so if
> the writer keeps ahead of writeback, the writeback will not
> terminate until the writer stops adding dirty pages.
>
> For a data integrity sync, we only need to write the pages dirty at
> the time we start the writeback, so we can stop scanning once we get
> to the page that was at the end of the file at the time the scan
> started.
>
> This will prevent operations like copying a large file preventing
> sync from completing as it will not write back pages that were
> dirtied after the sync was started. This does not impact the
> existing integrity guarantees, as any dirty page (old or new)
> within the EOF range at the start of the scan will still be
> captured.
Looks good.

Acked-by: Jan Kara <jack(a)suse.cz>

> Signed-off-by: Dave Chinner <dchinner(a)redhat.com>
> ---
> mm/page-writeback.c | 15 +++++++++++++++
> 1 files changed, 15 insertions(+), 0 deletions(-)
>
> diff --git a/mm/page-writeback.c b/mm/page-writeback.c
> index e22af84..4ba2728 100644
> --- a/mm/page-writeback.c
> +++ b/mm/page-writeback.c
> @@ -852,7 +852,22 @@ int write_cache_pages(struct address_space *mapping,
> if (wbc->range_start == 0 && wbc->range_end == LLONG_MAX)
> range_whole = 1;
> cycled = 1; /* ignore range_cyclic tests */
> +
> + /*
> + * If this is a data integrity sync, cap the writeback to the
> + * current end of file. Any extension to the file that occurs
> + * after this is a new write and we don't need to write those
> + * pages out to fulfil our data integrity requirements. If we
> + * try to write them out, we can get stuck in this scan until
> + * the concurrent writer stops adding dirty pages and extending
> + * EOF.
> + */
> + if (wbc->sync_mode == WB_SYNC_ALL &&
> + wbc->range_end == LLONG_MAX) {
> + end = i_size_read(mapping->host) >> PAGE_CACHE_SHIFT;
> + }
> }
> +
> retry:
> done_index = index;
> while (!done && (index <= end)) {
> --
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--
Jan Kara <jack(a)suse.cz>
SUSE Labs, CR
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