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smtp.subspace.kernel.org; dkim=pass header.d=kernel.org header.i=@kernel.org header.a=rsa-sha256 header.s=k20260515 header.b=JpAFqAMm; 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 5A13B606A31C; Thu, 29 Oct 2026 15:51:49 +0000 (UTC) Authentication-Results: smtp.subspace.kernel.org; dkim=pass (2048-bit key, unprotected) header.d=kernel.org header.i=@kernel.org header.a=rsa-sha256 header.s=k20260515 header.b=JpAFqAMm Received: by smtp.kernel.org (Postfix) with ESMTPSA id B6CAE1F01816; Thu, 29 Oct 2026 15:51:44 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=kernel.org; s=k20260515; t=1793289108; bh=fqt04aVmWCFFMW5PhNUnGvgQrBeB0qH6TmYzshEFSoo=; h=Date:Subject:To:Cc:References:From:In-Reply-To; b=JpAFqAMmtKjz2YNA1PCteNNZKLQs6+Kbf3XFPbl5KJzUPOekANKjzlLumikEZG/GC kyvzIS1lMX/dB23SwAo8ObKjij0MkZSGJXHCpJcSmMkD2ZiM/2zw/MgQt4V85bYnI4 EHP+FEgtUrKS3ncKCRvUu3BL2PDXOhefQr/vJTPEww0seLm7MLu+nYFvjr5pCgCFco R/YA1jKSKnGcq8ioaXtqLJw2Mp3D4uGmEdW57c7J0G+y67HRe82W6JgCqNchkaVMuy ItAIGB0PsekMUWRi+ZP9SkN6dQM3dYGNgqVRp/6lcGDi3ZqV0NxezVxQY22uZxlL5j iN6uKrHV3gzbA== Message-ID: <7efdc32c-9df4-40b2-84fa-1ad90196b861@kernel.org> Date: Thu, 29 Oct 2026 16:51:43 +0100 X-Mailing-List: linux-kernel@vger.kernel.org List-Id: List-Post: List-Help: List-Subscribe: List-Unsubscribe: List-Owner: Precedence: list MIME-Version: 1.0 User-Agent: Mozilla Thunderbird Subject: Re: [PATCH v4 4/8] mm/mmzone: read each zoneref slot once when filtering by nodemask Content-Language: en-US To: "Gregory Price (Meta)" , linux-mm@kvack.org Cc: linux-kernel@vger.kernel.org, kernel-team@meta.com, muchun.song@linux.dev, osalvador@suse.de, david@kernel.org, akpm@linux-foundation.org, ziy@nvidia.com, joshua.hahnjy@gmail.com, rakie.kim@sk.com, ying.huang@linux.alibaba.com, ljs@kernel.org, liam@infradead.org, rppt@kernel.org, surenb@google.com, mhocko@suse.com, brendan.jackman@linux.dev, hannes@cmpxchg.org, mgorman@techsingularity.net, anshuman.khandual@arm.com, stable@vger.kernel.org References: <20261028211930.3504634-1-gourry@gourry.net> <20261028211930.3504634-5-gourry@gourry.net> From: "Vlastimil Babka (SUSE)" Autocrypt: addr=vbabka@kernel.org; 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charset=UTF-8 Content-Transfer-Encoding: 7bit On 10/28/26 22:19, Gregory Price (Meta) wrote: > __next_zones_zonelist() tests a slot's zone for NULL, then reads the > slot again to look up the zone's node. Offlining a zone's last block > rebuilds the zonelists in place, so the second read can return the > terminator: > > CPU 0 CPU 1 > __next_zones_zonelist(nodemask) > zonelist_zone(slot 1) = NORMAL > offline_pages() > build_all_zonelists() > [MOVABLE, NORMAL, NULL] > -> [NORMAL, NULL] > zref_in_nodemask(slot 1) > zone_to_nid(NULL) > > Current GCC and clang merge the two reads, but nothing requires it. > With mm/mmzone.o built with -fno-inline -fno-ipa-sra, holding a > nodemask allocation between the reads while the node's last movable > block goes offline oopses every time: > > BUG: kernel NULL pointer dereference, address: 0000000000000060 > RIP: 0010:zone_to_nid+0x0/0x10 > Call Trace: > zref_in_nodemask+0x9/0x20 > __next_zones_zonelist+0x23/0x70 > prepare_alloc_pages.constprop.0+0x16c/0x190 > __alloc_frozen_pages_noprof+0xaf/0x330 > > Read the slot once and test that zone. A NULL zone still ends the walk. > > Fixes: 11cd8638c37f ("mm, page_alloc: remove stop_machine from build_all_zonelists") > Cc: stable@vger.kernel.org > Signed-off-by: Gregory Price (Meta) Oh, zonelist_node_idx() reads nid from the zone, it's not part of zoneref as I thought. So it's the same after this patch, just with READ_ONCE(). Reviewed-by: Vlastimil Babka (SUSE) > --- > mm/mmzone.c | 11 ++++++----- > 1 file changed, 6 insertions(+), 5 deletions(-) > > diff --git a/mm/mmzone.c b/mm/mmzone.c > index 9cc9ef588580..009f1dd75dde 100644 > --- a/mm/mmzone.c > +++ b/mm/mmzone.c > @@ -43,13 +43,14 @@ struct zone *next_zone(struct zone *zone) > return zone; > } > > -static inline int zref_in_nodemask(struct zoneref *zref, > - const nodemask_t *nodes) > +/* A NULL zone terminates the zonelist, so it ends the search. */ > +static inline bool zone_in_nodemask(const struct zone *zone, > + const nodemask_t *nodes) > { > #ifdef CONFIG_NUMA > - return node_isset(zonelist_node_idx(zref), *nodes); > + return !zone || node_isset(zone_to_nid(zone), *nodes); > #else > - return 1; > + return true; > #endif /* CONFIG_NUMA */ > } > > @@ -67,7 +68,7 @@ struct zoneref *__next_zones_zonelist(struct zoneref *z, > z++; > else > while (zonelist_zone_idx(z) > highest_zoneidx || > - (zonelist_zone(z) && !zref_in_nodemask(z, nodes))) > + !zone_in_nodemask(READ_ONCE(z->zone), nodes)) > z++; > > return z;