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Date:   Wed, 8 Jul 2020 16:19:17 +0900
From:   Joonsoo Kim <js1304@gmail.com>
To:     Michal Hocko <mhocko@kernel.org>
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
Message-ID: <20200708071916.GD16543@js1304-desktop>
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On Tue, Jul 07, 2020 at 01:40:19PM +0200, Michal Hocko wrote:
> On Tue 07-07-20 16:44:43, Joonsoo Kim wrote:
> > From: Joonsoo Kim <iamjoonsoo.kim@lge.com>
> > 
> > In mm/migrate.c, THP allocation for migration is called with the provided
> > gfp_mask | GFP_TRANSHUGE. This gfp_mask contains __GFP_RECLAIM and it
> > would be conflict with the intention of the GFP_TRANSHUGE.
> > 
> > GFP_TRANSHUGE/GFP_TRANSHUGE_LIGHT is introduced to control the reclaim
> > behaviour by well defined manner since overhead of THP allocation is
> > quite large and the whole system could suffer from it. So, they deals
> > with __GFP_RECLAIM mask deliberately. If gfp_mask contains __GFP_RECLAIM
> > and uses gfp_mask | GFP_TRANSHUGE(_LIGHT) for THP allocation, it means
> > that it breaks the purpose of the GFP_TRANSHUGE(_LIGHT).
> 
> GFP_TRANSHUGE* is not a carved in stone design. Their primary reason to
> exist is to control how hard to try for different allocation
> paths/configurations because their latency expectations might be
> largerly different. It is mostly the #PF path which aims to be as
> lightweight as possible I believe nobody simply considered migration to be
> very significant to even care. And I am still not sure it matters but
> I would tend to agree that a consistency here is probably a very minor
> plus.
> 
> Your changelog is slightly misleading in that regard because it suggests
> that this is a real problem while it doesn't present any actual data.
> It would be really nice to make the effective change really stand out.
> We are only talking about __GFP_RECLAIM_KSWAPD here. So the only
> difference is that the migration won't wake up kswapd now.
> 
> All that being said the changelog should be probably more explicit about
> the fact that this is solely done for consistency and be honest that the
> runtime effect is not really clear. This would help people reading it in
> future.

Okay. How about following changelog?

Thanks.

----------->8--------------------
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.