linux-kernel - [PATCH] mm/page_alloc: only set ALLOC_HIGHATOMIC for __GPF

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Message-ID: <20250814172245.1259625-1-cascardo@igalia.com>
Date: Thu, 14 Aug 2025 14:22:45 -0300
From: Thadeu Lima de Souza Cascardo <cascardo@...lia.com>
To: linux-kernel@...r.kernel.org
Cc: linux-mm@...ck.org,
	Andrew Morton <akpm@...ux-foundation.org>,
	Vlastimil Babka <vbabka@...e.cz>,
	Suren Baghdasaryan <surenb@...gle.com>,
	Michal Hocko <mhocko@...e.com>,
	Brendan Jackman <jackmanb@...gle.com>,
	Johannes Weiner <hannes@...xchg.org>,
	Zi Yan <ziy@...dia.com>,
	Mel Gorman <mgorman@...hsingularity.net>,
	kernel-dev@...lia.com,
	Thadeu Lima de Souza Cascardo <cascardo@...lia.com>,
	Helen Koike <koike@...lia.com>,
	Matthew Wilcox <willy@...radead.org>,
	NeilBrown <neilb@...e.de>,
	Thierry Reding <thierry.reding@...il.com>
Subject: [PATCH] mm/page_alloc: only set ALLOC_HIGHATOMIC for __GPF_HIGH allocations

Commit 524c48072e56 ("mm/page_alloc: rename ALLOC_HIGH to
ALLOC_MIN_RESERVE") is the start of a series that explains how __GFP_HIGH,
which implies ALLOC_MIN_RESERVE, is going to be used instead of
__GFP_ATOMIC for high atomic reserves.

Commit eb2e2b425c69 ("mm/page_alloc: explicitly record high-order atomic
allocations in alloc_flags") introduced ALLOC_HIGHATOMIC for such
allocations of order higher than 0. It still used __GFP_ATOMIC, though.

Then, commit 1ebbb21811b7 ("mm/page_alloc: explicitly define how __GFP_HIGH
non-blocking allocations accesses reserves") just turned that check for
!__GFP_DIRECT_RECLAIM, ignoring that high atomic reserves were expected to
test for __GFP_HIGH.

This leads to high atomic reserves being added for high-order GFP_NOWAIT
allocations and others that clear __GFP_DIRECT_RECLAIM, which is
unexpected. Later, those reserves lead to 0-order allocations going to the
slow path and starting reclaim.

>From /proc/pagetypeinfo, without the patch:

Node    0, zone      DMA, type   HighAtomic      0      0      0      0      0      0      0      0      0      0      0
Node    0, zone    DMA32, type   HighAtomic      1      8     10      9      7      3      0      0      0      0      0
Node    0, zone   Normal, type   HighAtomic     64     20     12      5      0      0      0      0      0      0      0

With the patch:

Node    0, zone      DMA, type   HighAtomic      0      0      0      0      0      0      0      0      0      0      0
Node    0, zone    DMA32, type   HighAtomic      0      0      0      0      0      0      0      0      0      0      0
Node    0, zone   Normal, type   HighAtomic      0      0      0      0      0      0      0      0      0      0      0

Fixes: 1ebbb21811b7 ("mm/page_alloc: explicitly define how __GFP_HIGH non-blocking allocations accesses reserves")
Signed-off-by: Thadeu Lima de Souza Cascardo <cascardo@...lia.com>
Tested-by: Helen Koike <koike@...lia.com>
Cc: Mel Gorman <mgorman@...hsingularity.net>
Cc: Michal Hocko <mhocko@...e.com>
Cc: Matthew Wilcox <willy@...radead.org>
Cc: NeilBrown <neilb@...e.de>
Cc: Thierry Reding <thierry.reding@...il.com>
Cc: Vlastimil Babka <vbabka@...e.cz>
---
 mm/page_alloc.c | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/mm/page_alloc.c b/mm/page_alloc.c
index 2ef3c07266b3..bf52e3bef626 100644
--- a/mm/page_alloc.c
+++ b/mm/page_alloc.c
@@ -4219,7 +4219,7 @@ gfp_to_alloc_flags(gfp_t gfp_mask, unsigned int order)
 		if (!(gfp_mask & __GFP_NOMEMALLOC)) {
 			alloc_flags |= ALLOC_NON_BLOCK;
 
-			if (order > 0)
+			if (order > 0 && (alloc_flags & ALLOC_MIN_RESERVE))
 				alloc_flags |= ALLOC_HIGHATOMIC;
 		}
 
-- 
2.47.2