linux-kernel - Re: [PATCH] swap: do not skip lowest_bit in scan_swap

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Date:	Tue, 21 Jan 2014 23:00:44 -0800 (PST)
From:	Hugh Dickins <hughd@...gle.com>
To:	Jamie Liu <jamieliu@...gle.com>
cc:	Andrew Morton <akpm@...ux-foundation.org>,
	Shaohua Li <shli@...ionio.com>,
	Minchan Kim <minchan@...nel.org>,
	Akinobu Mita <akinobu.mita@...il.com>, linux-mm@...ck.org,
	linux-kernel@...r.kernel.org
Subject: Re: [PATCH] swap: do not skip lowest_bit in scan_swap_map() scan
 loop

On Tue, 21 Jan 2014, Jamie Liu wrote:

> In the second half of scan_swap_map()'s scan loop, offset is set to
> si->lowest_bit and then incremented before entering the loop for the
> first time, causing si->swap_map[si->lowest_bit] to be skipped.
> 
> Signed-off-by: Jamie Liu <jamieliu@...gle.com>

Acked-by: Hugh Dickins <hughd@...gle.com>

Good catch.  At first I was puzzled that this off-by-one could have
gone unnoticed for so long (ever since 2.6.29); but now I think that
almost always we have a good amount of slack, in those pages duplicated
between swap and swapcache, which can be reclaimed at the vm_swap_full()
check, and so conceal this loss of a single slot.

> ---
>  mm/swapfile.c | 3 ++-
>  1 file changed, 2 insertions(+), 1 deletion(-)
> 
> diff --git a/mm/swapfile.c b/mm/swapfile.c
> index 612a7c9..6635081 100644
> --- a/mm/swapfile.c
> +++ b/mm/swapfile.c
> @@ -616,7 +616,7 @@ scan:
>  		}
>  	}
>  	offset = si->lowest_bit;
> -	while (++offset < scan_base) {
> +	while (offset < scan_base) {
>  		if (!si->swap_map[offset]) {
>  			spin_lock(&si->lock);
>  			goto checks;
> @@ -629,6 +629,7 @@ scan:
>  			cond_resched();
>  			latency_ration = LATENCY_LIMIT;
>  		}
> +		offset++;
>  	}
>  	spin_lock(&si->lock);
>  
> -- 
> 1.8.5.3
> 
> 
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