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 236C43F23A3 for ; Mon, 8 Jun 2026 15:18:37 +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=1780931919; cv=none; b=ZL6kH8AkC0Ni0itOSj2xzh5OKaCOZt7bSQNo/3OJkAJJ6VDuLrcoinCIs6Tn663MOVLtWy7hKPsAroghwV6HnqcRUgDCrXZ7dKrYuNrB6hRSXlxweC9BhkzThMIW4mlEwKNV+bEnDef3UPbFTBNhC79fzxW3VTXyPFnQxWy1JIc= ARC-Message-Signature:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1780931919; c=relaxed/simple; bh=NN0Sip4q6f2Vm8H7hpuUr7rOTwxLejpo6KKjQr0R0no=; h=Message-ID:Date:MIME-Version:Subject:To:Cc:References:From: In-Reply-To:Content-Type; b=eHRw5k1kXqSwdTDZpeBYAOuZxuPvNp6b1T6GUioQj4mXlvXjiQxi/ZmDLjUyQ2OXE1zyttuVsEO3b8GA/OfHS+IFijFMTYme6BY7hgqJFftSdyutKKi0S02/PaHas2jN3B70CPLAuCc35PsdZsh0g9Enaj6cGIXI+kSq4nn5U38= ARC-Authentication-Results:i=1; smtp.subspace.kernel.org; dkim=pass (2048-bit key) header.d=kernel.org header.i=@kernel.org header.b=Q1uyet7h; 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="Q1uyet7h" Received: by smtp.kernel.org (Postfix) with ESMTPSA id A91FB1F00893; Mon, 8 Jun 2026 15:18:27 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=kernel.org; s=k20260515; t=1780931917; bh=tbanNgvzOTBZc/hxgfXOVnrR1EORd9B3hY+LTWCW/Mk=; h=Date:Subject:To:Cc:References:From:In-Reply-To; b=Q1uyet7hoyFl9IWg+nYeyondribXFjMAh2LCw6haQsZg1rJhTAMlv+zCb90FebntO q5mTQED8peHtkzPqcVgwLLSvEXsIWMn1DIKxjuVd7gu10UKcZh71cH+SLvzjkohKhq SN0oa+XNFw0/gOWQVaudBYL109MnmAyoDPtq6UkE3PD50OiTpQKbZfPyUiEr6H0IJJ VZpjsU4kZ56FWy9NMcYfhsFxN2Xd1097YYgrXqaichBKME1QVQnsH6eyl4XXENmY5o NYbvOzVd8dSXTToc4XMdKuYWUsgmrAp7aKi2GQVSZO9qCWR/Sd6WNyJEUCDHZ2rrWR 5cGCgnoMMjoBA== Message-ID: <7430d69a-616b-40cf-a2e8-6ef4e57b6857@kernel.org> Date: Mon, 8 Jun 2026 17:18:25 +0200 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 3/7] mm/migrate: skip data copy for already-copied folios To: "Garg, Shivank" , akpm@linux-foundation.org Cc: kinseyho@google.com, weixugc@google.com, ljs@kernel.org, Liam.Howlett@oracle.com, vbabka@kernel.org, willy@infradead.org, rppt@kernel.org, surenb@google.com, mhocko@suse.com, ziy@nvidia.com, matthew.brost@intel.com, joshua.hahnjy@gmail.com, rakie.kim@sk.com, byungchul@sk.com, gourry@gourry.net, ying.huang@linux.alibaba.com, apopple@nvidia.com, dave@stgolabs.net, rkodsara@amd.com, vkoul@kernel.org, bharata@amd.com, sj@kernel.org, rientjes@google.com, xuezhengchu@huawei.com, yiannis@zptcorp.com, dave.hansen@intel.com, hannes@cmpxchg.org, jhubbard@nvidia.com, peterx@redhat.com, riel@surriel.com, shakeel.butt@linux.dev, stalexan@redhat.com, tj@kernel.org, nifan.cxl@gmail.com, jic23@kernel.org, aneesh.kumar@kernel.org, nathan.lynch@amd.com, Frank.li@nxp.com, djbw@kernel.org, linux-kernel@vger.kernel.org, linux-mm@kvack.org References: <20260428155043.39251-2-shivankg@amd.com> <20260428155043.39251-8-shivankg@amd.com> <810e9a58-9c08-4f5e-af5f-866685ca09b2@kernel.org> <7e6a7626-5a1b-4672-91bd-85b0f4ee2ecb@amd.com> From: "David Hildenbrand (Arm)" Content-Language: en-US Autocrypt: addr=david@kernel.org; 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charset=UTF-8 Content-Transfer-Encoding: 7bit > Sashiko Comment: > Does expanding FOLIO_OLD_STATES to include BIT(2) silently corrupt anon_vma > pointers on 32-bit architectures? > The anon_vma pointer is packed with FOLIO_OLD_STATES inside dst->migrate_info. > To separate them, __migrate_folio_extract() uses info & ~FOLIO_OLD_STATES. > Since anon_vma objects are allocated from a kmem_cache with align=0, > ARCH_SLAB_MINALIGN defaults to 4 bytes on many 32-bit architectures. This > means a valid anon_vma pointer can legitimately end in 0x4 or 0xC (meaning > bit 2 is set). > When __migrate_folio_extract() masks the pointer with ~7, it will silently > clear bit 2 from the anon_vma pointer. Any subsequent call to put_anon_vma() > with this corrupted pointer could cause a use-after-free or a kernel panic. > -- > > #define ARCH_SLAB_MINALIGN __alignof__(unsigned long long) > > I initially assumed this to be always 8, confusing it to be same as size of > unsigned long long. > But the GCC docs note that alignment can be smaller in size: > > https://gcc.gnu.org/onlinedocs/gcc/Alignment.html > "For example, if the target machine requires a double value to be aligned on > an 8-byte boundary, then __alignof__ (double) is 8. This is true on many RISC > machines. On more traditional machine designs, __alignof__ (double) is 4 or > even 2." > > If my understanding is right, Sashiko concern is valid, and I can't safely > use BIT(2). 32bit makes this tricky indeed. And that's also the reason why FOLIO_MAPPING_FLAGS is currently limited to 2 bits. > I see few option from here. Either I can gate batch copy for CONFIG_64BIT, That's a bit nasty as we'll have to special case 32bit vs 64bit. > or I can force anon_vma_cachep to 8-byte alignment via kmem_cache_create() That's an option, but I would not do it just for this optimization. > align arg. Or I can change the migrate_folio() callback to pass already_copied > info to change to dst->migrate_info enum. Can you elaborate how that would look like? -- Cheers, David