From mboxrd@z Thu Jan 1 00:00:00 1970 Received: from smtp.kernel.org (aws-us-west-2-korg-mail-alma10-1.taild15c8.ts.net [100.103.45.18]) (using TLSv1.2 with cipher ECDHE-RSA-AES256-GCM-SHA384 (256/256 bits)) (No client certificate requested) by smtp.subspace.kernel.org (Postfix) with ESMTPS id 1F91E221FCD for ; Sun, 28 Jun 2026 09:22:28 +0000 (UTC) Authentication-Results: smtp.subspace.kernel.org; arc=none smtp.client-ip=100.103.45.18 ARC-Seal:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1782638550; cv=none; b=j+lvpKtofQ2ptJZeNlOTsxD0V1NfKLKuvBRbznRaShS+/XT1idlEC4sVZRhriE1P8N3TuRKj8Id5Su05c05TX9UPmSooyHBCQuUHFFR6Db+nBX8lz7Lj+Z02QnlAW0SEgOu72mpx3AeqP3Xn9q9zIW36H9X75JsTJWTey/1zuv4= ARC-Message-Signature:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1782638550; c=relaxed/simple; bh=C4tPJoDbDGW9clSVJgQVYRkd7xhUyH7AGTWgUBP8llk=; h=Message-ID:Date:MIME-Version:Subject:To:Cc:References:From: In-Reply-To:Content-Type; b=GTGMK4/M3FvVWAqcpiALLt7P7QpOMWDiU12Qe3M7DB3Fd6zYW24+2bUs5T45sWg+o0NAMb6EtXXWLn+hNegrFp0N0EovItCdzvF4wQJ6CpoFUBZYibVfj+2s9vT2A32NNSCGT9F9K/QXxSTyQ2kHcYTnheJLmjDVniaxUYLGtIw= ARC-Authentication-Results:i=1; smtp.subspace.kernel.org; dkim=pass (2048-bit key) header.d=kernel.org header.i=@kernel.org header.b=dX83W0lh; arc=none smtp.client-ip=100.103.45.18 Authentication-Results: smtp.subspace.kernel.org; dkim=pass (2048-bit key) header.d=kernel.org header.i=@kernel.org header.b="dX83W0lh" Received: by smtp.kernel.org (Postfix) with ESMTPSA id C13311F000E9; Sun, 28 Jun 2026 09:22:26 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=kernel.org; s=k20260515; t=1782638548; bh=UQ/aYEJVIRtbpfJ4OpCNdmweOjbbLLnGcukBKDZN/eU=; h=Date:Subject:To:Cc:References:From:In-Reply-To; b=dX83W0lh5V3fEptvqMtKYTtxrC6XcfuGCN0z+xqhJ3sKaN5fJQBCsJSGeAuPjRXb/ 3F40fcY0U7DIT9Jhr0R7j6XqcUIQXqDdi97TxDquT49q/WQj7F0zMl2yH5k3erXIbA +d1QktEA2hZNaHpCEQ4i7VN0m0j2ewQemXmXPJCW9wUT77+Xfdytfq7z+B+hxKgBl6 foyPcG+R4/3fPRMz95UDImUqzv1dXh702O+i6P3BKKwtkHtKsSyXmTgOMNpsho/WZn 916YhGzDV0Uqi+JOIICkzWqW8V4FGnYMU2XGT+WVznN4KdgOuPYtvy/T7I0PSs9fQ1 RBK6GcM2xS3vQ== Message-ID: <68122038-e8e0-47ed-82f8-cb6a23e4658e@kernel.org> Date: Sun, 28 Jun 2026 18:22:22 +0900 Precedence: bulk X-Mailing-List: linux-kernel@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 User-Agent: Mozilla Thunderbird Subject: Re: [PATCH] mm/slub: serve slabobj_ext array from a strictly larger kmalloc cache To: "Vlastimil Babka (SUSE)" , Shakeel Butt Cc: Andrew Morton , Roman Gushchin , Hao Li , Christoph Lameter , David Rientjes , Suren Baghdasaryan , Usama Arif , Meta kernel team , linux-mm@kvack.org, linux-kernel@vger.kernel.org, Danielle Costantino References: <20260625230029.703750-1-shakeel.butt@linux.dev> <62453403-954c-4cf1-8924-6d38184b0810@kernel.org> <09267187-6c85-438f-8791-4cce8d07892a@kernel.org> Content-Language: en-US From: Harry Yoo In-Reply-To: Content-Type: multipart/signed; micalg=pgp-sha256; protocol="application/pgp-signature"; boundary="------------OEDpvwo0aWJgefRP7EiOdTve" This is an OpenPGP/MIME signed message (RFC 4880 and 3156) --------------OEDpvwo0aWJgefRP7EiOdTve Content-Type: multipart/mixed; boundary="------------qCfhd81ooqA0slhNdxF44v7v"; protected-headers="v1" From: Harry Yoo To: "Vlastimil Babka (SUSE)" , Shakeel Butt Cc: Andrew Morton , Roman Gushchin , Hao Li , Christoph Lameter , David Rientjes , Suren Baghdasaryan , Usama Arif , Meta kernel team , linux-mm@kvack.org, linux-kernel@vger.kernel.org, Danielle Costantino Message-ID: <68122038-e8e0-47ed-82f8-cb6a23e4658e@kernel.org> Subject: Re: [PATCH] mm/slub: serve slabobj_ext array from a strictly larger kmalloc cache References: <20260625230029.703750-1-shakeel.butt@linux.dev> <62453403-954c-4cf1-8924-6d38184b0810@kernel.org> <09267187-6c85-438f-8791-4cce8d07892a@kernel.org> In-Reply-To: --------------qCfhd81ooqA0slhNdxF44v7v Content-Type: text/plain; charset=UTF-8 Content-Transfer-Encoding: quoted-printable On 6/28/26 4:47 PM, Vlastimil Babka (SUSE) wrote: > On 6/28/26 5:23 AM, Shakeel Butt wrote: >> On Sat, Jun 27, 2026 at 07:58:12PM -0700, Shakeel Butt wrote: >>> On Fri, Jun 26, 2026 at 07:11:33PM +0200, Vlastimil Babka (SUSE) wrot= e: >>> [...] >>>>>>> Fix it structurally by removing cycles of every shape: serve the = array >>>>>>> from a cache strictly larger than the one it describes whenever i= t would >>>>>>> otherwise come from the same or a smaller cache. Every reference= edge >>>>>>> then points from a smaller to a larger cache (here kmalloc-1k's a= rray >>>>>>> moves to kmalloc-2k), so the relation is a DAG and cannot contain= a cycle. >>>>>> >>>>>> This will fix the problem. >>>>>> >>>>>> But this will waste memory as we need smaller obj_exts array >>>>>> as the size gets larger. >>>>>> >>>>>> We should probably create a new kmalloc type to avoid cycles inste= ad? >>>>>> (needed only when memory profiling is enabled, though) >>>>>> >>>>>> That would also prevent recursion even further. >>>>> >>>>> Yes but I assume that would add kmem caches even for users not usin= g memory >>>>> profiling. Anyways, I think that is a separate discussion. Am I und= erstanding >>>>> correctly that you don't have any concerns with this approach? >>>> >>>> Umm, the memory waste is a concern? >>>> >>>> Minimally I'd now want to only do that size bumping when allocation >>>> profiling is enabled. Ideally that means both configured in and not = booted >>>> with "never". >>>> >>>> We probably should have done that already in 280ea9c3154b2. Because = AFAIU >>>> memcg-only obj_exts array don't have this issue (or maybe they do ha= ve the >>>> [1] issue? Harry?). But if memcg-only should keep avoiding the same = size >>>> bucket, it can keep what it was doing and only memalloc profiling wo= uld do >>>> the strictly larger thing. >>> >>> memcg should not have this issue as normal kmalloc caches do not serv= e memcg >>> charged objects.=20 >> >> I am wrong here as I went back and see d8df600b67d7. I was confused too :) > (8dafa9f5900c upstream) >=20 >>> >>> So here we can do dedicated caches as Harry suggested or make this si= ze bumping >>> very specialized as Vlastimil suggested. What do we want long term? O= rthogonally >=20 > Maybe long term we make kmem_buckets unconditional and use that. >=20 >>> we do want this fix to be backported easily to older stable kernels. = I will see >>> how does this narrowed down size bumping looks like. >>> >> >> BTW I think we need something like the following, right? >> >> if (mem_alloc_profiling_enabled()) { >> if (obj_exts_cache->object_size <=3D s->object_size) >> return s->object_size + 1; >> } else { >> if (obj_exts_cache->object_size =3D=3D s->object_size) >> return s->object_size + 1; >> } We should not add mem_alloc_profiling_enabled() check because, then we're not fixing this issue on SLUB_TINY, when the caller specifies __GFP_RECLAIMABLE|__GFP_ACCOUNT without memory allocation profiling. `if (!is_kmalloc_normal(s))` check already bails out when it doesn't need to bump the size. So Shakeel's original code will work fine. We're only pessimizing memory allocation profiling and SLUB_TINY && MEMCG users, but (as Vlastimil suggests off-list) it wouldn't make much sense to enable MEMCG on memory restricted systems anyway. (IIRC even raspberry pis don't enable the memory controller by default...) I think it's okay to fix the bug first, but we need to address the memory wastage issue sooner or later if companies (Meta and Google I guess?) are deploying kernels with memory allocation profiling on in production systems. Perhaps it's worth adding a comment like this, though: /* * Only bump the size when the object (not the obj_exts array) is * allocated from KMALLOC_NORMAL, either by memory allocation profiling * or memcg on SLUB_TINY with __GFP_RECLAIMABLE|__GFP_ACCOUNT. * Otherwise, obj_exts allocations cannot form a cycle between * kmalloc caches. */ if (!is_kmalloc_normal(s)) return sz; Thanks! --=20 Cheers, Harry / Hyeonggon --------------qCfhd81ooqA0slhNdxF44v7v-- --------------OEDpvwo0aWJgefRP7EiOdTve Content-Type: application/pgp-signature; name="OpenPGP_signature.asc" Content-Description: OpenPGP digital signature Content-Disposition: attachment; filename="OpenPGP_signature.asc" -----BEGIN PGP SIGNATURE----- iHUEARYKAB0WIQQQ1ub6gR5ogjaKRmOGXBN6rc5S1gUCakDnzgAKCRCGXBN6rc5S 1lQ1AQC0uWuA8kOqLnc7JcxHi6c8NiDIGEK82coLHYTGzOvjeQEAuQNNB5RBK/ac /JarFs8VAaTd1jCT3ZJkADc0Pv+l5Qk= =rUUx -----END PGP SIGNATURE----- --------------OEDpvwo0aWJgefRP7EiOdTve--