linux-kernel - Re: [PATCH v3 RFC 14/14] mm: speedup page alloc for MPOL_PREFERRED_MANY by adding a NO

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Date:   Wed, 10 Mar 2021 10:44:11 +0100
From:   Michal Hocko <mhocko@...e.com>
To:     Feng Tang <feng.tang@...el.com>
Cc:     Dave Hansen <dave.hansen@...el.com>,
        Ben Widawsky <ben.widawsky@...el.com>,
        "linux-mm@...ck.org" <linux-mm@...ck.org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Andrea Arcangeli <aarcange@...hat.com>,
        David Rientjes <rientjes@...gle.com>,
        Mel Gorman <mgorman@...hsingularity.net>,
        Mike Kravetz <mike.kravetz@...cle.com>,
        Randy Dunlap <rdunlap@...radead.org>,
        Vlastimil Babka <vbabka@...e.cz>,
        Andi Kleen <ak@...ux.intel.com>,
        "Williams, Dan J" <dan.j.williams@...el.com>
Subject: Re: [PATCH v3 RFC 14/14] mm: speedup page alloc for
 MPOL_PREFERRED_MANY by adding a NO_SLOWPATH gfp bit

On Wed 10-03-21 13:19:47, Feng Tang wrote:
[...]
> diff --git a/mm/mempolicy.c b/mm/mempolicy.c
> index d66c1c0..00b19f7 100644
> --- a/mm/mempolicy.c
> +++ b/mm/mempolicy.c
> @@ -2205,9 +2205,13 @@ static struct page *alloc_pages_policy(struct mempolicy *pol, gfp_t gfp,
>  	 * | MPOL_PREFERRED_MANY (round 2) | local         | NULL       |
>  	 * +-------------------------------+---------------+------------+
>  	 */
> -	if (pol->mode == MPOL_PREFERRED_MANY)
> +	if (pol->mode == MPOL_PREFERRED_MANY) {
>  		gfp_mask |= __GFP_RETRY_MAYFAIL | __GFP_NOWARN;
>  
> +		/* Skip direct reclaim, as there will be a second try */
> +		gfp_mask &= ~__GFP_DIRECT_RECLAIM;

__GFP_RETRY_MAYFAIL is a reclaim modifier which doesn't make any sense
without __GFP_DIRECT_RECLAIM. Also I think it would be better to have a
proper allocation flags in the initial patch which implements the
fallback.

> +	}
> +
>  	page = __alloc_pages_nodemask(gfp_mask, order,
>  				      policy_node(gfp, pol, preferred_nid),
>  				      policy_nodemask(gfp, pol));
> -- 
> 2.7.4
> 
> 

-- 
Michal Hocko
SUSE Labs