Re: [PATCH 4/8] vmscan: Do not writeback filesystem pages in direct reclaim

Johannes Weiner
2010-07-21T14:28:55Z

On Wed, Jul 21, 2010 at 02:38:57PM +0100, Mel Gorman wrote:
> Here is an updated version. Thanks very much
>
> ==== CUT HERE ====
> vmscan: Do not writeback filesystem pages in direct reclaim
>
> When memory is under enough pressure, a process may enter direct
> reclaim to free pages in the same manner kswapd does. If a dirty page is
> encountered during the scan, this page is written to backing storage using
> mapping->writepage. This can result in very deep call stacks, particularly
> if the target storage or filesystem are complex. It has already been observed
> on XFS that the stack overflows but the problem is not XFS-specific.
>
> This patch prevents direct reclaim writing back filesystem pages by checking
> if current is kswapd or the page is anonymous before writing back. If the
> dirty pages cannot be written back, they are placed back on the LRU lists
> for either background writing by the BDI threads or kswapd. If in direct
> lumpy reclaim and dirty pages are encountered, the process will stall for
> the background flusher before trying to reclaim the pages again.
>
> As the call-chain for writing anonymous pages is not expected to be deep
> and they are not cleaned by flusher threads, anonymous pages are still
> written back in direct reclaim.
>
> Signed-off-by: Mel Gorman <mel@csn.ul.ie>
> Acked-by: Rik van Riel <riel@redhat.com>

Cool!

Except for one last tiny thing...

> @@ -858,7 +872,7 @@ keep:
>
> free_page_list(&free_pages);
>
> - list_splice(&ret_pages, page_list);

This will lose all retry pages forever, I think.

> + *nr_still_dirty = nr_dirty;
> count_vm_events(PGACTIVATE, pgactivate);
> return nr_reclaimed;
> }

Otherwise,
Reviewed-by: Johannes Weiner <hannes@cmpxchg.org>