linux-kernel - [PATCH v4 05/11] mm/migrate: clear __GFP_RECLAIM for THP allocation for migration

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Date:   Tue,  7 Jul 2020 16:44:43 +0900
From:   js1304@...il.com
To:     Andrew Morton <akpm@...ux-foundation.org>
Cc:     linux-mm@...ck.org, linux-kernel@...r.kernel.org,
        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>,
        Joonsoo Kim <iamjoonsoo.kim@....com>
Subject: [PATCH v4 05/11] mm/migrate: clear __GFP_RECLAIM for THP allocation for migration

From: Joonsoo Kim <iamjoonsoo.kim@....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).

This patch fixes this situation by clearing __GFP_RECLAIM in provided
gfp_mask. Note that there are some other THP allocations for migration
and they just uses GFP_TRANSHUGE(_LIGHT) directly. This patch would make
all THP allocation for migration consistent.

Signed-off-by: Joonsoo Kim <iamjoonsoo.kim@....com>
---
 mm/migrate.c | 5 +++++
 1 file changed, 5 insertions(+)

diff --git a/mm/migrate.c b/mm/migrate.c
index 02b31fe..ecd7615 100644
--- a/mm/migrate.c
+++ b/mm/migrate.c
@@ -1547,6 +1547,11 @@ struct page *new_page_nodemask(struct page *page,
 	}

 	if (PageTransHuge(page)) {
+		/*
+		 * clear __GFP_RECALIM since GFP_TRANSHUGE is the gfp_mask
+		 * that chooses the reclaim masks deliberately.
+		 */
+		gfp_mask &= ~__GFP_RECLAIM;
 		gfp_mask |= GFP_TRANSHUGE;
 		order = HPAGE_PMD_ORDER;
 	}
-- 
2.7.4