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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 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 X-Rspam-User: X-Rspamd-Server: rspam01 X-Rspamd-Queue-Id: 8306880014 X-Stat-Signature: w18qo3sdbsm9t61xyjsxp6dowrurwwok X-HE-Tag: 1780931918-863302 X-HE-Meta: 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 vn2vs1M0 2GrbhNbkMRXlHC8tsZ83+mq6xwCKeSH4p3Q769AC3JWnvCBTcyyX6hFmsbLLmKuqr+q5OkHZk6e1awWyIdVqhEO/ys1/OdQC9sfr8EIMOPoS5l+sVJtHJ01B+cAeutHGs0NKSPSqd0rl6zIOmbxtCCunlxN44pGGYj5KedRI96tqPq9qMVkgv74Gylg0EO8g8CDOtWbgFC8m4p/fPaEp6rEVSD8MFwYhGR8GmLu1bnI065aHYZReQoFV5Iq91GRw/ieQehRWGiMzhIe98hgpzdhuC9wu9MXMUbVsC+NoLq0N2FVB6J0d7/43hDKxd3MCzuLtrgB1KOGebxSb8Yu8IdFO1bnryD4umMQC2yk3ipq6HomsUvYba6kme1ov9QV6x6AoG Sender: owner-linux-mm@kvack.org Precedence: bulk X-Loop: owner-majordomo@kvack.org List-ID: List-Subscribe: List-Unsubscribe: > 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