Re: [PATCH 1/2] vmscan: Fix use of delta in zone_pagecache_reclaimable()

[KOSAKI Motohiro <kosaki.motohiro@jp.fujitsu.com>]

> zone_pagecache_reclaimable() works out how many pages are in a state
> that zone_reclaim() can reclaim based on the current zone_reclaim_mode.
> As part of this, it calculates a delta to the number of unmapped pages.
> The code was meant to check delta would not cause underflows and then apply
> it but it got accidentally removed.
> 
> This patch properly uses delta. It's excessively paranoid at the moment
> because it's impossible to underflow but the current form will make future
> patches to zone_pagecache_reclaimable() fixing any other scan-heuristic
> breakage easier to read and acts as self-documentation reminding authors
> of future patches to consider underflow.
> 
> This is a fix to patch
> vmscan-properly-account-for-the-number-of-page-cache-pages-zone_reclaim-can-reclaim.patch
> and they should be merged together.
> 
> Signed-off-by: Mel Gorman <mel@csn.ul.ie>
> ---
>  mm/vmscan.c |    6 +++++-
>  1 files changed, 5 insertions(+), 1 deletions(-)
> 
> diff --git a/mm/vmscan.c b/mm/vmscan.c
> index 026f452..bd8e3ed 100644
> --- a/mm/vmscan.c
> +++ b/mm/vmscan.c
> @@ -2398,7 +2398,11 @@ static long zone_pagecache_reclaimable(struct zone *zone)
>  	if (!(zone_reclaim_mode & RECLAIM_WRITE))
>  		delta += zone_page_state(zone, NR_FILE_DIRTY);
>  
> -	return nr_pagecache_reclaimable;
> +	/* Watch for any possible underflows due to delta */
> +	if (unlikely(delta > nr_pagecache_reclaimable))
> +		delta = nr_pagecache_reclaimable;
> +
> +	return nr_pagecache_reclaimable - delta;
>  }

looks good. thanks.



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