linux-kernel - Re: [PATCH] mm: don't account shared file pages in user_reserve

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Message-ID: <20150129201147.GB9331@scruffy>
Date:	Thu, 29 Jan 2015 12:11:47 -0800
From:	Andrew Shewmaker <agshew@...il.com>
To:	Roman Gushchin <klamm@...dex-team.ru>
Cc:	linux-mm@...ck.org, linux-kernel@...r.kernel.org,
	Andrew Morton <akpm@...ux-foundation.org>,
	Rik van Riel <riel@...hat.com>,
	Konstantin Khlebnikov <khlebnikov@...dex-team.ru>
Subject: Re: [PATCH] mm: don't account shared file pages in user_reserve_pages

On Thu, Jan 29, 2015 at 02:51:27PM +0300, Roman Gushchin wrote:
> Shared file pages are never accounted in memory overcommit code,
> so it isn't reasonable to count them in a code that limits the
> maximal size of a process in OVERCOMMIT_NONE mode.
> 
> If a process has few large file mappings, the consequent attempts
> to allocate anonymous memory may unexpectedly fail with -ENOMEM,
> while there is free memory and overcommit limit if significantly
> larger than the committed amount (as displayed in /proc/meminfo).
> 
> The problem is significantly smoothed by commit c9b1d0981fcc
> ("mm: limit growth of 3% hardcoded other user reserve"),
> which limits the impact of this check with 128Mb (tunable via sysctl),
> but it can still be a problem on small machines.
> 
> Signed-off-by: Roman Gushchin <klamm@...dex-team.ru>
> Cc: Andrew Morton <akpm@...ux-foundation.org>
> Cc: Andrew Shewmaker <agshew@...il.com>
> Cc: Rik van Riel <riel@...hat.com>
> Cc: Konstantin Khlebnikov <khlebnikov@...dex-team.ru>
> ---
>  mm/mmap.c | 2 +-
>  1 file changed, 1 insertion(+), 1 deletion(-)
> 
> diff --git a/mm/mmap.c b/mm/mmap.c
> index 7f684d5..151fadf 100644
> --- a/mm/mmap.c
> +++ b/mm/mmap.c
> @@ -220,7 +220,7 @@ int __vm_enough_memory(struct mm_struct *mm, long pages, int cap_sys_admin)
>  	 */
>  	if (mm) {
>  		reserve = sysctl_user_reserve_kbytes >> (PAGE_SHIFT - 10);
> -		allowed -= min(mm->total_vm / 32, reserve);
> +		allowed -= min((mm->total_vm - mm->shared_vm) / 32, reserve);
>  	}
>  
>  	if (percpu_counter_read_positive(&vm_committed_as) < allowed)
> -- 
> 2.1.0

You're two patches conflict, don't they? Maybe you should resend
them as a patch series such that they can both be applied?

Does mm->shared_vm include memory that's mapped MAP_ANONYMOUS in
conjunction with MAP_SHARED? If so, then subtracting it could
overcommit the system OVERCOMMIT_NEVER mode.

-Andrew
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