linux-kernel - RE: [PATCH 1/4] mm/page_alloc: fix non cma alloc context

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Message-ID: <be55deff4ef1453983522d247bd36110@AcuMS.aculab.com>
Date:   Fri, 17 Jul 2020 08:32:07 +0000
From:   David Laight <David.Laight@...LAB.COM>
To:     "'js1304@...il.com'" <js1304@...il.com>,
        Andrew Morton <akpm@...ux-foundation.org>
CC:     "linux-mm@...ck.org" <linux-mm@...ck.org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        "kernel-team@....com" <kernel-team@....com>,
        Vlastimil Babka <vbabka@...e.cz>,
        Christoph Hellwig <hch@...radead.org>,
        Roman Gushchin <guro@...com>,
        Mike Kravetz <mike.kravetz@...cle.com>,
        "Naoya Horiguchi" <n-horiguchi@...jp.nec.com>,
        Michal Hocko <mhocko@...e.com>,
        "Aneesh Kumar K . V" <aneesh.kumar@...ux.ibm.com>,
        Joonsoo Kim <iamjoonsoo.kim@....com>,
        "stable@...r.kernel.org" <stable@...r.kernel.org>
Subject: RE: [PATCH 1/4] mm/page_alloc: fix non cma alloc context

From: js1304@...il.com
> Sent: 15 July 2020 06:05
> From: Joonsoo Kim <iamjoonsoo.kim@....com>
> 
> Currently, preventing cma area in page allocation is implemented by using
> current_gfp_context(). However, there are two problems of this
> implementation.
...
> diff --git a/mm/page_alloc.c b/mm/page_alloc.c
> index 6416d08..cd53894 100644
> --- a/mm/page_alloc.c
> +++ b/mm/page_alloc.c
...
> @@ -3693,6 +3691,16 @@ alloc_flags_nofragment(struct zone *zone, gfp_t gfp_mask)
>  	return alloc_flags;
>  }
> 
> +static inline void current_alloc_flags(gfp_t gfp_mask,
> +				unsigned int *alloc_flags)
> +{
> +	unsigned int pflags = READ_ONCE(current->flags);
> +
> +	if (!(pflags & PF_MEMALLOC_NOCMA) &&
> +		gfp_migratetype(gfp_mask) == MIGRATE_MOVABLE)
> +		*alloc_flags |= ALLOC_CMA;
> +}
> +

I'd guess this would be easier to understand and probably generate
better code if renamed and used as:
	alloc_flags |= can_alloc_cma(gpf_mask);

Given it is a 'static inline' the compiler might end up
generating the same code.
If you needed to clear a flag doing:
	alloc_flags = current_alloc_flags(gpf_mask, alloc_flags);
is much better for non-inlined function.

	David

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