Re: [PATCH 5/4] writeback: limit write_cache_pages integrity scanning to current EOF

[Jan Kara <jack@suse.cz>]

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@suse.cz>

> Signed-off-by: Dave Chinner <dchinner@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@suse.cz>
SUSE Labs, CR

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