linux-kernel - Re: [PATCH 2/2] proc: meminfo: estimate available memory more conservatively

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Message-ID: <20151203164609.GC9271@dhcp22.suse.cz>
Date:	Thu, 3 Dec 2015 17:46:09 +0100
From:	Michal Hocko <mhocko@...nel.org>
To:	Johannes Weiner <hannes@...xchg.org>
Cc:	Andrew Morton <akpm@...ux-foundation.org>,
	Rik van Riel <riel@...hat.com>, Mel Gorman <mgorman@...e.de>,
	linux-mm@...ck.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH 2/2] proc: meminfo: estimate available memory more
 conservatively

On Mon 30-11-15 15:00:22, Johannes Weiner wrote:
> The MemAvailable item in /proc/meminfo is to give users a hint of how
> much memory is allocatable without causing swapping, so it excludes
> the zones' low watermarks as unavailable to userspace.
> 
> However, for a userspace allocation, kswapd will actually reclaim
> until the free pages hit a combination of the high watermark and the
> page allocator's lowmem protection that keeps a certain amount of DMA
> and DMA32 memory from userspace as well.
> 
> Subtract the full amount we know to be unavailable to userspace from
> the number of free pages when calculating MemAvailable.

I am not sure this will make a big or even noticeable difference in the
real life but it makes sense.

> 
> Signed-off-by: Johannes Weiner <hannes@...xchg.org>

Acked-by: Michal Hocko <mhocko@...e.com>

> ---
>  fs/proc/meminfo.c | 5 +----
>  1 file changed, 1 insertion(+), 4 deletions(-)
> 
> diff --git a/fs/proc/meminfo.c b/fs/proc/meminfo.c
> index 9155a5a..df4661a 100644
> --- a/fs/proc/meminfo.c
> +++ b/fs/proc/meminfo.c
> @@ -57,11 +57,8 @@ static int meminfo_proc_show(struct seq_file *m, void *v)
>  	/*
>  	 * Estimate the amount of memory available for userspace allocations,
>  	 * without causing swapping.
> -	 *
> -	 * Free memory cannot be taken below the low watermark, before the
> -	 * system starts swapping.
>  	 */
> -	available = i.freeram - wmark_low;
> +	available = i.freeram - totalreserve_pages;
>  
>  	/*
>  	 * Not all the page cache can be freed, otherwise the system will
> -- 
> 2.6.2
> 
> --
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-- 
Michal Hocko
SUSE Labs
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