linux-kernel - Re: [PATCH] mm/vmpressure.c: Include GFP

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Message-ID: <20200309115818.GK8447@dhcp22.suse.cz>
Date:   Mon, 9 Mar 2020 12:58:18 +0100
From:   Michal Hocko <mhocko@...nel.org>
To:     Shaju Abraham <shajunutanix@...il.com>
Cc:     akpm@...ux-foundation.org, linux-mm@...ck.org,
        linux-kernel@...r.kernel.org,
        Shaju Abraham <shaju.abraham@...anix.com>
Subject: Re: [PATCH] mm/vmpressure.c: Include GFP_KERNEL flag to vmpressure

On Mon 09-03-20 11:31:41, Shaju Abraham wrote:
> The VM pressure notification flags have excluded GFP_KERNEL with the
> reasoning that user land will not be able to take any action in case of
> kernel memory being low. This is not true always. Consider the case of
> a user land program managing all the huge memory pages. By including
> GFP_KERNEL flag whenever the kernel memory is low, pressure notification
> can be send, and the manager process can split huge pages to satisfy kernel
> memory requirement.

Are you sure about this reasoning? GFP_KERNEL = __GFP_FS | __GFP_IO | __GFP_RECLAIM
Two of the flags mentioned there are already listed so we are talking
about __GFP_RECLAIM here. Including it here would be a more appropriate
change than GFP_KERNEL btw.

But still I do not really understand what is the actual problem and how
is this patch meant to fix it. vmpressure is triggered only from the
reclaim path which inherently requires to have __GFP_RECLAIM present
so I fail to see how this can make any change at all. How have you
tested it?

> This is a common scanario in cloud. Most of the host memory is reserved
> as hugepages and can be broken down to small pages on demand. This is
> done to minimise fragmentation so that Virtual Machine power on will be
> successful always.
> 
> Signed-off-by: Shaju Abraham <shaju.abraham@...anix.com>
> ---
>  mm/vmpressure.c | 3 ++-
>  1 file changed, 2 insertions(+), 1 deletion(-)
> 
> diff --git a/mm/vmpressure.c b/mm/vmpressure.c
> index 4bac22fe1aa2..7ccfb3dd8173 100644
> --- a/mm/vmpressure.c
> +++ b/mm/vmpressure.c
> @@ -253,7 +253,8 @@ void vmpressure(gfp_t gfp, struct mem_cgroup *memcg, bool tree,
>  	 * Indirect reclaim (kswapd) sets sc->gfp_mask to GFP_KERNEL, so
>  	 * we account it too.
>  	 */
> -	if (!(gfp & (__GFP_HIGHMEM | __GFP_MOVABLE | __GFP_IO | __GFP_FS)))
> +	if (!(gfp & (__GFP_HIGHMEM | __GFP_MOVABLE | __GFP_IO |
> +		     __GFP_FS | GFP_KERNEL)))
>  		return;
>  
>  	/*
> -- 
> 2.20.1
> 

-- 
Michal Hocko
SUSE Labs