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Date:   Wed, 8 Jul 2020 09:48:20 +0200
From:   Michal Hocko <mhocko@kernel.org>
To:     Joonsoo Kim <js1304@gmail.com>
Cc:     Andrew Morton <akpm@linux-foundation.org>, linux-mm@kvack.org,
        linux-kernel@vger.kernel.org, kernel-team@lge.com,
        Vlastimil Babka <vbabka@suse.cz>,
        Christoph Hellwig <hch@infradead.org>,
        Roman Gushchin <guro@fb.com>,
        Mike Kravetz <mike.kravetz@oracle.com>,
        Naoya Horiguchi <n-horiguchi@ah.jp.nec.com>
Subject: Re: [PATCH v4 05/11] mm/migrate: clear __GFP_RECLAIM for THP
 allocation for migration
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On Wed 08-07-20 16:19:17, Joonsoo Kim wrote:
> On Tue, Jul 07, 2020 at 01:40:19PM +0200, Michal Hocko wrote:
[...]
> Subject: [PATCH] mm/migrate: clear __GFP_RECLAIM for THP allocation for
>  migration
> 
> In migration target allocation functions, THP allocations uses different
> gfp_mask, especially, in regard to the reclaim gfp_mask. There is no
> reason to use different reclaim gfp_mask for each cases and it is
> an obstacle to make a common function in order to clean-up migration
> target allocation functions. This patch fixes this situation by using
> common reclaim gfp_mask for THP allocation.

I would find the following more understandable, feel free to reuse parts
that you like:
"
new_page_nodemask is a migration callback and it tries to use a common
gfp flags for the target page allocation whether it is a base page or a
THP. The later only adds GFP_TRANSHUGE to the given mask. This results
in the allocation being slightly more aggressive than necessary because
the resulting gfp mask will contain also __GFP_RECLAIM_KSWAPD. THP
allocations usually exclude this flag to reduce over eager background
reclaim during a high THP allocation load which has been seen during
large mmaps initialization. There is no indication that this is a
problem for migration as well but theoretically the same might happen
when migrating large mappings to a different node. Make the migration
callback consistent with regular THP allocations.
"

-- 
Michal Hocko
SUSE Labs