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=1794537868; b=tIYgbtLQZC1L2EaTXHOb/PdbfXanYVDpKe/P6+QytPQE6jtycR5UDbiPRayV4iguGL1Z ofnGVDgueJX3LsjFPSppsWavABkMQV2vqcp8lIdnbx+Xj7oKh6K0QsnpDi2+VNro60GDN n4llgcDo3aBT+lY5L3nU3zGY98kdrahI2SiCqLoeBck68S8v1mFkg7WtusuL0vf5jUzZT gUVQqn2SSovSEM3LUCVulQqxdn9Bb0dV1YG/Kv5arTDuVjcMPPCrcOB3iM/5GrfHhExuf CPmJFvcTgAgFQ302zLbIdPgYLbhSbqwmcOkIy1rJtaO8K0ulgsImCE3XN5ULqqBxccA== ARC-Message-Signature: i=1; d=subspace.kernel.org; s=arc20261015; a=rsa-sha256; c=relaxed/relaxed; t=1794537868; h=Received:DKIM-Signature:From:To:Cc:Subject:Date:Message-ID:X-Mailer: In-Reply-To:References:MIME-Version:Content-Transfer-Encoding; bh=yKLpzjQ6J8yCja5oBDK9zH5qDXnAqD8YWwpc7xYunGw=; b=Qu0oe7TTh/RuE/QYtnhHOL59ZX+xxc++L2Hkg5TobASbGjuXGH8v/s/lnZPkmpqGKiZh e4IWW3slGh1pF+9zbJBMGN8o3fj0qS39CxgwWRsJSdRXLgv9pBIzs4aXDddzqmuJd1zLc 1DK64W0HKTVHM0DIJ+TTmnjSF5t8ky7OFUIc1Rt5rR4Rb7J0M96UFvdnI7gyc6RSO0xsF 3zVEfh5rukapCEAWDge2y+J+P95uXbqDiP0jVzqtrpl6GtFiAoBbo3suOiskNnjWxpQYF 2s+vEzO0819b9qH2A1qqNDIHHXvuH4bv/C2x0G2xZCGelBLYFtWBMmIQbgRBKGwJ1vQ== 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=ZhC50Xt2; 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 E651C606A33E for ; Fri, 13 Nov 2026 02:44:27 +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=ZhC50Xt2 Received: by smtp.kernel.org (Postfix) with ESMTPSA id E1ED41F000CB; Fri, 13 Nov 2026 02:44:25 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=kernel.org; s=k20260515; t=1794537867; bh=yKLpzjQ6J8yCja5oBDK9zH5qDXnAqD8YWwpc7xYunGw=; h=From:To:Cc:Subject:Date:In-Reply-To:References; b=ZhC50Xt29pZpBEIx90i62UZAEKht2CDEwamtv/dKk3juILVk6KUh6reLd9YiB7rZl q3XXCR1wejf+X6bRpCj0W6vbXUQd4fDe1rRCiqxXyOn3ZMxeyakJAV32xwokGJso1C e0VSPe9G87VrVZh7JVgJpuCGnebWTP47NrJ7zSvA5cu6rAX0hJCrCaPW9w1mA92lFh /wFLqJkWRVUfROYPeNW9pdF77eJQul3BIn4B6YCPwyGgFPucghQVgjL5Mcs3CYsUc8 2lIKomdFj0YuyB+C3FriGOr8kSm+SrtE2xcgtzFOBi9kJeko5LUnPT47TfIknTnlrW 9C7LE2BothMig== From: Sasha Levin To: stable@vger.kernel.org Cc: Johannes Weiner , Andrew Morton , Sashiko , 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 6.1.y] mm: page_alloc: do not give all non-blocking requests reserve access Date: Thu, 12 Nov 2026 21:44:24 -0500 Message-ID: <20261113024316.459926-1-sashal@kernel.org> X-Mailer: git-send-email 2.53.0 In-Reply-To: <20261109201720-failed-6.1-6ee3d87b9408@kernel.org> References: <20261109201720-failed-6.1-6ee3d87b9408@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 6ee3d87b940848aaed5d4e8d005c5f674bf2637a ] Patch series "mm: page_alloc: fix reserve access for non-blocking requests", v2. Two fixes for how non-blocking allocations interact with the highatomic reserve. There are prerequisites for Salvatore's "mm/page_alloc: avoid direct reclaim and compaction for costly __GFP_NORETRY allocations" [0]. Patch 1 narrows which requests may physically take highatomic pages, patch 2 narrows which requests may count them in their watermark check. Both are prerequisites for Salvatore's patch: it clears __GFP_DIRECT_RECLAIM for costly __GFP_NORETRY requests, which turns every large page cache folio allocation that reaches the slowpath into a non-blocking one. Without patch 1, those requests may fall back to the highatomic freelists reserved for atomic requests. Without patch 2, they count the free highatomic pages in their watermark check while being served from regular memory, so they can consume regular high-order blocks below the min watermark by the size of the reserve. GFP_NOWAIT requests hit both paths today, so these are existing problems worthy of their own stable backports; but Salvatore's patch would widen them significantly. Patch 2 reproduction: a GFP_NOWAIT burst under memory pressure can drain regular free memory to zero while the highatomic reserve sits idle. Measured on a 32G box with an anonymous working set, swap, and a full 290M reserve, three synthetic bursts drove the 28G Normal zone's regular free memory to 28M, 0.8M and 0.6M against a 60M min watermark. Swapout failed its swap table allocation inside reclaim, reclaim scanned 13M pages to free 200k, and page faults stalled for tens to hundreds of milliseconds. With the fix the burst stops at the min watermark and nothing else happens. This was found in review and reproduced synthetically; it has not been observed in production. GFP_ATOMIC order-0 requests have a milder version of the same problem: They are entitled to the reserve but are served regular memory first, so the fix there has to change the watermark check and the freelist order together. Salvatore's change doesn't interact with this, though; fixing that is left for a separate patch. This patch (of 2): 281dd25c1a01 ("mm/page_alloc: let GFP_ATOMIC order-0 allocs access highatomic reserves") accidentally gave all non-blocking allocation requests access to highatomic reserves, which includes GFP_NOWAIT and other sites clearing __GFP_DIRECT_RECLAIM. While this improved atomic request success rate, it's unintentionally broad and can actually worsen highatomic requests by squandering the reserves on requests that don't need it. There are concurrent efforts to clear __GFP_DIRECT_RECLAIM altogether for costly order __GFP_NORETRY requests, which would then also fall into this exemption and deplete reserves even faster. What GFP_ATOMIC has over the others is __GFP_HIGH, which alloc_flags_slowpath() translates to ALLOC_MIN_RESERVE. Narrow the exemption to contexts that have both ALLOC_NON_BLOCK | ALLOC_MIN_RESERVE. Link: https://lore.kernel.org/20261001082152.2879289-1-dipiets@amazon.it [0] Link: https://lore.kernel.org/linux-mm/arFBWvzXPYrKuIYi@cmpxchg.org/ [1] Link: https://lore.kernel.org/linux-mm/20261001082152.2879289-1-dipiets@amazon.it/ [2] Link: https://lore.kernel.org/20261028171719.2043421-1-hannes@cmpxchg.org Link: https://lore.kernel.org/20261028171719.2043421-2-hannes@cmpxchg.org Fixes: 281dd25c1a01 ("mm/page_alloc: let GFP_ATOMIC order-0 allocs access highatomic reserves") Signed-off-by: Johannes Weiner Signed-off-by: Andrew Morton Reported-by: Sashiko Closes: https://sashiko.dev/#/patchset/20260904115629.3993331-1-dipiets@amazon.it 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: regular free memory to zero while the highatomic reserve sits idle. Measured on a 32G box with an anonymous working set, swap, and a full 290M reserve, three synthetic bursts drove the 28G Normal zone's regular free memory to 28M, 0.8M and 0.6M against a 60M min watermark. Swapout failed its swap table allocation inside reclaim, reclaim scanned 13M pages to free 200k, and page faults stalled for tens to hundreds of milliseconds. [ sashal: placed ALLOC_MASK_ATOMIC in the existing internal header instead of the separate page allocation header ] Signed-off-by: Sasha Levin --- mm/internal.h | 3 +++ mm/page_alloc.c | 4 +++- 2 files changed, 6 insertions(+), 1 deletion(-) diff --git a/mm/internal.h b/mm/internal.h index 3e3dfc8e0bc6a..a3811e6265b18 100644 --- a/mm/internal.h +++ b/mm/internal.h @@ -797,6 +797,9 @@ unsigned int reclaim_clean_pages_from_list(struct zone *zone, /* Flags that allow allocations below the min watermark. */ #define ALLOC_RESERVES (ALLOC_NON_BLOCK|ALLOC_MIN_RESERVE|ALLOC_HIGHATOMIC|ALLOC_OOM) +/* Flags that mean GFP_ATOMIC */ +#define ALLOC_MASK_ATOMIC (ALLOC_NON_BLOCK|ALLOC_MIN_RESERVE) + enum ttu_flags; struct tlbflush_unmap_batch; diff --git a/mm/page_alloc.c b/mm/page_alloc.c index de91843b80fad..edafc2c27d407 100644 --- a/mm/page_alloc.c +++ b/mm/page_alloc.c @@ -3777,7 +3777,9 @@ struct page *rmqueue_buddy(struct zone *preferred_zone, struct zone *zone, * reserves as failing now is worse than failing a * high-order atomic allocation in the future. */ - if (!page && (alloc_flags & (ALLOC_OOM|ALLOC_NON_BLOCK))) + if (!page && + ((alloc_flags & ALLOC_OOM) || + (alloc_flags & ALLOC_MASK_ATOMIC) == ALLOC_MASK_ATOMIC)) page = __rmqueue_smallest(zone, order, MIGRATE_HIGHATOMIC); if (!page) { -- 2.53.0