From mboxrd@z Thu Jan 1 00:00:00 1970 Authentication-Results: smtp.subspace.kernel.org; arc=none smtp.remote-ip=100.103.45.18 ARC-Seal: i=1; d=subspace.kernel.org; s=arc20261015; a=rsa-sha256; cv=none; t=1794703256; b=fEZsAiFUz+rYZe8fIsT1J/OzEcZggIy1U2ce8JTYwNDyHti/GkrIJkPgML1Ke8l+aeBa rL8qaI1a6DqPOEVIOAYmtatiKR5eHc8tzvGHHTl+6M9oaulDJE8z+H7hp7HmkpByCx0wU sPtbh8pjYaAGDxRn24jlQjT/WffuqUyguBarR95N2bMHTHoBUOuKMpkQc7ioHCiq55lJm 8PKLA2TJJkdywcjcYjaPKFd1IoEkp8vLRP8atu276spB/R8mpElDPBY+yytdsXLv0iWke 4ZMDhM9BhQJAnl4QuKEoHBGxH+1iPHj1MPUNJo/EIYibdDgeF7Z3OZ8bsF4+VNgBpeA== ARC-Message-Signature: i=1; d=subspace.kernel.org; s=arc20261015; a=rsa-sha256; c=relaxed/relaxed; t=1794703256; h=Received:DKIM-Signature:From:To:Cc:Subject:Date:Message-ID:X-Mailer: In-Reply-To:References:MIME-Version:Content-Transfer-Encoding; bh=og7Y+0LVZku2LSNXuavngxyZPpoFiT1xhPDH5XrmFDg=; b=foVaSqQiD/41F0aCph685pa3zl0OHHaBX4yiY9Exn2MgexlPckX6cA6mepl2RuLrcf3g Qkirn3rdf+TrtCbqhZ70coLOzqSu6pHJC31P2DS3Ri5bI96CPwiKRF6lvdrdZ2A2zUXnT sKhMurEd2hC3Y1a9HMcjm/lfBPL6W2G5Ni32GEDfbFzUg5HlcdCYqkWlUK7f7QmkgwT5B b0kmbj5/pANatfiwLtPT5fZLyzLronCFplPKHpyVMkrCe617Z6gfMo3Ld5lhsBLQzGE2y HMJGWnhLMFgHeLCQyfaaC5MCDR6HiZhsBrKwNI+DToxZP7je8SV3G1cvr4E1rUOwU1g== ARC-Authentication-Results: i=1; smtp.subspace.kernel.org; dkim=pass header.d=kernel.org header.i=@kernel.org header.a=rsa-sha256 header.s=k20260515 header.b=TSYGk17c; arc=none smtp.remote-ip=100.103.45.18 Received: from smtp.kernel.org (quasi.space.kernel.org [100.103.45.18]) (using TLSv1.3 with cipher TLS_AES_256_GCM_SHA384 (256/256 bits) key-exchange X25519 server-signature ECDSA (prime256v1)) (No client certificate requested) by smtp.subspace.kernel.org (Postfix) with ESMTPS id 25012606A323 for ; Sun, 15 Nov 2026 00:40:55 +0000 (UTC) Authentication-Results: smtp.subspace.kernel.org; dkim=pass (2048-bit key) header.d=kernel.org header.i=@kernel.org header.a=rsa-sha256 header.s=k20260515 header.b=TSYGk17c Received: by smtp.kernel.org (Postfix) with ESMTPSA id 20EBA1F000C4; Sun, 15 Nov 2026 00:40:54 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=kernel.org; s=k20260515; t=1794703255; bh=og7Y+0LVZku2LSNXuavngxyZPpoFiT1xhPDH5XrmFDg=; h=From:To:Cc:Subject:Date:In-Reply-To:References; b=TSYGk17c7+We5dUmCRhREAz+7iNRUnTrKwenRIiv6FKwXFO5VpI2HfJSXoFVJZNov YDA39GH5IURoCF29em5BOpToZgyG/ciK/ogOUuVeBUpcM5wfWtpJ7fc3bq0Z17a5KP IR45sgMwTsoNei+Z+nyTguFqoJAnp5MQctoIozmxBA4/0FWRUZVWqkBa7St0DMHOUC GHhD/XKa+z0bOO0C9dZBfYrkw6hbqvC8Onncw5X21inRewRf8I7LMIo8WT9NQLq00u m4XFTcY/PXf0G0FvCS4a+VXFT2sxuf3eN8Qugekn9F+iQ8rBTR9QMsVuav0yUEvjmO LdiFGpiVSPsTA== From: Sasha Levin To: stable@vger.kernel.org Cc: Johannes Weiner , Andrew Morton , Balbir Singh , "Vlastimil Babka (SUSE)" , "Matthew Wilcox (Oracle)" , Salvatore Dipietro , Christoph Hellwig , Dave Chinner , Matt Fleming , Mel Gorman , Michal Hocko , Suren Baghdasaryan , Brendan Jackman , Zi Yan , Sasha Levin Subject: [PATCH 6.12.y] mm: page_alloc: remove ALLOC_NON_BLOCK from ALLOC_RESERVES Date: Sat, 14 Nov 2026 19:40:52 -0500 Message-ID: <20261115002107.535582-1-sashal@kernel.org> X-Mailer: git-send-email 2.53.0 In-Reply-To: <20261109200815-failed-6.12-fe21f930d1e6@kernel.org> References: <20261109200815-failed-6.12-fe21f930d1e6@kernel.org> X-Mailing-List: stable@vger.kernel.org List-Id: List-Post: List-Help: List-Subscribe: List-Unsubscribe: List-Owner: Precedence: list MIME-Version: 1.0 Content-Transfer-Encoding: 8bit From: Johannes Weiner [ Upstream commit fe21f930d1e64d121191673076fc4ebe79fc82d5 ] 1ebbb21811b7 ("mm/page_alloc: explicitly define how __GFP_HIGH non-blocking allocations accesses reserves") stopped handing out reserve access for ALLOC_NON_BLOCK on its own: the extra 25% below the min watermark is now only granted on top of ALLOC_MIN_RESERVE. But the flag was left in ALLOC_RESERVES, which produces something odd: With the ALLOC_RESERVES match, __zone_watermark_unusable_free() doesn't subtract the free highatomic pages for them. So in the slowpath, they get to consume regular blocks below the min watermark by the number of free highatomic pages. The highatomic reserve is capped at 1% of the zone, which on any decently sized machine is a multiple of the min watermark: GFP_NOWAIT can drain regular memory to zero. The user-visible result is brutal hiccups during bursts of GFP_NOWAIT allocations under memory pressure. This was found in review and reproduced with a synthetic burst, not observed in production, but the sequence of events is a realistic one: On a 32G box with an anonymous working set, swap, and a filled 290M highatomic reserve, a GFP_NOWAIT burst drove regular free memory in the 28G Normal zone (min=60M) to 28M, 0.8M and 0.6M in three runs. Swapout failed to allocate its swap table, reclaim scanned 13M pages to reclaim 200k, page faults stalled for tens to hundreds of milliseconds. The machine survives it, but not by design: direct reclaimers eventually fail and start unreserving highatomic blocks, until the allocation succeeds or the reserve is gone and the OOM killer runs. That reserve exists for high-order atomic allocations; here it is destroyed to bail out a GFP_NOWAIT consumer that was never entitled to the memory. Remove ALLOC_NON_BLOCK from ALLOC_RESERVES. With that, the GFP_NOWAIT burst is stopped short at the min watermark. No direct reclaim, no stalls, no failed allocations, and the highatomic reserve stays intact for the requests it exists for. __zone_watermark_ok() is unaffected, since everything it keys on ALLOC_NON_BLOCK is already nested under ALLOC_MIN_RESERVE. Update the flag comments accordingly: ALLOC_NON_BLOCK just means the caller can't block; the reserve math belongs with ALLOC_MIN_RESERVE. GFP_ATOMIC order-0 requests have a milder version of the same problem. Through ALLOC_MIN_RESERVE they also count the free highatomic pages in their watermark check, but rmqueue_buddy() serves them regular pages first and only falls back to the reserve once those are gone. They can still push regular free memory to 37.5% of min minus the free highatomic pages before the reserve is touched. Unlike GFP_NOWAIT they are entitled to the reserve, so this needs the watermark check and the freelist order reworked together, and is left to a separate patch. Link: https://lore.kernel.org/20261028171719.2043421-3-hannes@cmpxchg.org Fixes: 1ebbb21811b7 ("mm/page_alloc: explicitly define how __GFP_HIGH non-blocking allocations accesses reserves") Signed-off-by: Johannes Weiner Signed-off-by: Andrew Morton Acked-by: Balbir Singh Reviewed-by: Vlastimil Babka (SUSE) Cc: Matthew Wilcox (Oracle) Cc: Salvatore Dipietro Cc: Christoph Hellwig Cc: Dave Chinner Cc: Matt Fleming Cc: Mel Gorman Cc: Michal Hocko Cc: Suren Baghdasaryan Cc: Brendan Jackman Cc: Zi Yan Cc: [ sashal: adjusted context for allocation flag definitions in the internal header ] Signed-off-by: Sasha Levin --- mm/internal.h | 11 +++++------ 1 file changed, 5 insertions(+), 6 deletions(-) diff --git a/mm/internal.h b/mm/internal.h index 9fbed032a9c1a..a13a4799bfe87 100644 --- a/mm/internal.h +++ b/mm/internal.h @@ -1210,12 +1210,11 @@ unsigned int reclaim_clean_pages_from_list(struct zone *zone, #define ALLOC_OOM ALLOC_NO_WATERMARKS #endif -#define ALLOC_NON_BLOCK 0x10 /* Caller cannot block. Allow access - * to 25% of the min watermark or - * 62.5% if __GFP_HIGH is set. - */ +#define ALLOC_NON_BLOCK 0x10 /* Caller cannot block. */ #define ALLOC_MIN_RESERVE 0x20 /* __GFP_HIGH set. Allow access to 50% - * of the min watermark. + * of the min watermark, or 62.5% if + * the caller cannot block either + * (ALLOC_NON_BLOCK). */ #define ALLOC_CPUSET 0x40 /* check for correct cpuset */ #define ALLOC_CMA 0x80 /* allow allocations from CMA areas */ @@ -1228,7 +1227,7 @@ unsigned int reclaim_clean_pages_from_list(struct zone *zone, #define ALLOC_KSWAPD 0x800 /* allow waking of kswapd, __GFP_KSWAPD_RECLAIM set */ /* Flags that allow allocations below the min watermark. */ -#define ALLOC_RESERVES (ALLOC_NON_BLOCK|ALLOC_MIN_RESERVE|ALLOC_HIGHATOMIC|ALLOC_OOM) +#define ALLOC_RESERVES (ALLOC_MIN_RESERVE|ALLOC_HIGHATOMIC|ALLOC_OOM) /* Flags that mean GFP_ATOMIC */ #define ALLOC_MASK_ATOMIC (ALLOC_NON_BLOCK|ALLOC_MIN_RESERVE) -- 2.53.0