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

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Date:	Wed, 21 Jul 2010 16:28:44 +0200
From:	Johannes Weiner <hannes@...xchg.org>
To:	Mel Gorman <mel@....ul.ie>
Cc:	linux-kernel@...r.kernel.org, linux-fsdevel@...r.kernel.org,
	linux-mm@...ck.org, Dave Chinner <david@...morbit.com>,
	Chris Mason <chris.mason@...cle.com>,
	Nick Piggin <npiggin@...e.de>, Rik van Riel <riel@...hat.com>,
	Christoph Hellwig <hch@...radead.org>,
	Wu Fengguang <fengguang.wu@...el.com>,
	KAMEZAWA Hiroyuki <kamezawa.hiroyu@...fujitsu.com>,
	KOSAKI Motohiro <kosaki.motohiro@...fujitsu.com>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Andrea Arcangeli <aarcange@...hat.com>
Subject: Re: [PATCH 4/8] vmscan: Do not writeback filesystem pages in direct
 reclaim

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@....ul.ie>
> Acked-by: Rik van Riel <riel@...hat.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@...xchg.org>
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