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

[Rik van Riel <riel@redhat.com>]

On 07/19/2010 09:11 AM, Mel Gorman wrote:
> 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>