Re: [PATCH 4/5] mm: vmscan: Immediately reclaim end-of-LRU dirty pages when writeback completes

[Johannes Weiner <jweiner@redhat.com>]

On Wed, Jul 13, 2011 at 03:31:26PM +0100, Mel Gorman wrote:
> When direct reclaim encounters a dirty page, it gets recycled around
> the LRU for another cycle. This patch marks the page PageReclaim using
> deactivate_page() so that the page gets reclaimed almost immediately
> after the page gets cleaned. This is to avoid reclaiming clean pages
> that are younger than a dirty page encountered at the end of the LRU
> that might have been something like a use-once page.
> 
> Signed-off-by: Mel Gorman <mgorman@suse.de>
> ---
>  include/linux/mmzone.h |    2 +-
>  mm/vmscan.c            |   10 ++++++++--
>  mm/vmstat.c            |    2 +-
>  3 files changed, 10 insertions(+), 4 deletions(-)
> 
> diff --git a/include/linux/mmzone.h b/include/linux/mmzone.h
> index c4508a2..bea7858 100644
> --- a/include/linux/mmzone.h
> +++ b/include/linux/mmzone.h
> @@ -100,7 +100,7 @@ enum zone_stat_item {
>  	NR_UNSTABLE_NFS,	/* NFS unstable pages */
>  	NR_BOUNCE,
>  	NR_VMSCAN_WRITE,
> -	NR_VMSCAN_WRITE_SKIP,
> +	NR_VMSCAN_INVALIDATE,
>  	NR_VMSCAN_THROTTLED,
>  	NR_WRITEBACK_TEMP,	/* Writeback using temporary buffers */
>  	NR_ISOLATED_ANON,	/* Temporary isolated pages from anon lru */
> diff --git a/mm/vmscan.c b/mm/vmscan.c
> index 9826086..8e00aee 100644
> --- a/mm/vmscan.c
> +++ b/mm/vmscan.c
> @@ -834,8 +834,13 @@ static unsigned long shrink_page_list(struct list_head *page_list,
>  			 */
>  			if (page_is_file_cache(page) &&
>  					(!current_is_kswapd() || priority >= DEF_PRIORITY - 2)) {
> -				inc_zone_page_state(page, NR_VMSCAN_WRITE_SKIP);
> -				goto keep_locked;
> +				inc_zone_page_state(page, NR_VMSCAN_INVALIDATE);
> +
> +				/* Immediately reclaim when written back */
> +				unlock_page(page);
> +				deactivate_page(page);
> +
> +				goto keep_dirty;
>  			}
>  
>  			if (references == PAGEREF_RECLAIM_CLEAN)
> @@ -956,6 +961,7 @@ keep:
>  		reset_reclaim_mode(sc);
>  keep_lumpy:
>  		list_add(&page->lru, &ret_pages);
> +keep_dirty:
>  		VM_BUG_ON(PageLRU(page) || PageUnevictable(page));
>  	}

I really like the idea behind this patch, but I think all those pages
are lost as PageLRU is cleared on isolation and lru_deactivate_fn
bails on them in turn.

If I'm not mistaken, the reference from the isolation is also leaked.

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