linux-kernel - Re: [PATCH v6 2/6] mm/memplicy: add page allocation function for MPOL_PREFERRED

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Date:   Wed, 28 Jul 2021 14:42:26 +0200
From:   Michal Hocko <mhocko@...e.com>
To:     Feng Tang <feng.tang@...el.com>
Cc:     linux-mm@...ck.org, Andrew Morton <akpm@...ux-foundation.org>,
        David Rientjes <rientjes@...gle.com>,
        Dave Hansen <dave.hansen@...el.com>,
        Ben Widawsky <ben.widawsky@...el.com>,
        linux-kernel@...r.kernel.org, linux-api@...r.kernel.org,
        Andrea Arcangeli <aarcange@...hat.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>,
        Dan Williams <dan.j.williams@...el.com>, ying.huang@...el.com
Subject: Re: [PATCH v6 2/6] mm/memplicy: add page allocation function for
 MPOL_PREFERRED_MANY policy

On Mon 12-07-21 16:09:30, Feng Tang wrote:
> The semantics of MPOL_PREFERRED_MANY is similar to MPOL_PREFERRED,
> that it will first try to allocate memory from the preferred node(s),
> and fallback to all nodes in system when first try fails.
> 
> Add a dedicated function for it just like 'interleave' policy.
> 
> Link: https://lore.kernel.org/r/20200630212517.308045-9-ben.widawsky@intel.com
> Suggested-by: Michal Hocko <mhocko@...e.com>
> Co-developed-by: Ben Widawsky <ben.widawsky@...el.com>
> Signed-off-by: Ben Widawsky <ben.widawsky@...el.com>
> Signed-off-by: Feng Tang <feng.tang@...el.com>

It would be better to squash this together with the actual user of the
function added by the next patch.

> ---
>  mm/mempolicy.c | 19 +++++++++++++++++++
>  1 file changed, 19 insertions(+)
> 
> diff --git a/mm/mempolicy.c b/mm/mempolicy.c
> index 17b5800b7dcc..d17bf018efcc 100644
> --- a/mm/mempolicy.c
> +++ b/mm/mempolicy.c
> @@ -2153,6 +2153,25 @@ static struct page *alloc_page_interleave(gfp_t gfp, unsigned order,
>  	return page;
>  }
>  
> +static struct page *alloc_page_preferred_many(gfp_t gfp, unsigned int order,
> +						struct mempolicy *pol)

We likely want a node parameter to know which one we want to start with
for locality. Callers should use policy_node for that.

> +{
> +	struct page *page;
> +
> +	/*
> +	 * This is a two pass approach. The first pass will only try the
> +	 * preferred nodes but skip the direct reclaim and allow the
> +	 * allocation to fail, while the second pass will try all the
> +	 * nodes in system.
> +	 */
> +	page = __alloc_pages(((gfp | __GFP_NOWARN) & ~__GFP_DIRECT_RECLAIM),
> +				order, first_node(pol->nodes), &pol->nodes);

Although most users will likely have some form of GFP_*USER* here and
clearing __GFP_DIRECT_RECLAIM will put all other reclaim modifiers out
of game I think it would be better to explicitly disable some of them to
prevent from surprises. E.g. any potential __GFP_NOFAIL would be more
than surprising here. We do not have any (hopefully) but this should be
pretty cheap to exclude as we already have to modify already.

	preferred_gfp = gfp | __GFP_NOWARN;
	preferred_gfp &= ~(__GFP_DIRECT_RECLAIM | __GFP_NOFAIL)


> +	if (!page)
> +		page = __alloc_pages(gfp, order, numa_node_id(), NULL);
> +
> +	return page;
> +}
> +
>  /**
>   * alloc_pages_vma - Allocate a page for a VMA.
>   * @gfp: GFP flags.
> -- 
> 2.7.4

-- 
Michal Hocko
SUSE Labs