Re: [PATCH 6/6] writeback: limit write_cache_pages integrityscanning to current EOF

[Nick Piggin]

On Tue, Jun 08, 2010 at 10:38:07AM +1000, Dave Chinner wrote:
> From: Dave Chinner <dchinner@xxxxxxxxxx>
> 
> 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.
> 
> This patch will not prevent sync from blocking on large writes into
> holes.

The writes don't have to be into holes to cause this starvation
problem, do they?


> That requires more complex intervention while this patch only
> addresses the common append-case of this sync holdoff.

Jan's tagging patch looks pretty good to me and isn't so complex.
I think we should just take that.

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