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b=M5vUOTU/7y3rWRuGH/qGFr2AfvRDyZZTUDU2FNrcktQ8pgg1BBfe2oa0NAZ3GwHpf Tgh2MQ3iWGrssS+cQwky2NWwhSlBmiT6w6ateXiR46ILzX+L9LRV8aeCQvs/q4Ixnu PpG9HJYPi3q02fb9CmOoE0tUNgqPMIw4UAjk3DlfMe7k8Resz/K2IsMUXF77wROazU 3fA/yPlhn5JVXhf+i6F19wyLVwLfkwHg4yBgcCnd2sZzD+scRo8XSASFqtFXj0lwPc Tm6n/LHh4tntcifaC0VxK/s3ANE5VswGozCyGiUp7xlpiZXy3kgaTpnVhs9fcJ4TLm SsWvE6y8xYyOQ== Message-ID: Date: Mon, 8 Jun 2026 17:43:08 +0200 MIME-Version: 1.0 User-Agent: Mozilla Thunderbird Subject: Re: [PATCH 3/7] mm/migrate: skip data copy for already-copied folios To: Zi Yan Cc: "Garg, Shivank" , akpm@linux-foundation.org, 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, 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> <7430d69a-616b-40cf-a2e8-6ef4e57b6857@kernel.org> 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-Rspamd-Server: rspam06 X-Rspamd-Queue-Id: 755BF8000C X-Stat-Signature: jid4quixwsqofizrmn1wkzhys3u35486 X-Rspam-User: X-HE-Tag: 1780933400-223056 X-HE-Meta: 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 /avdJIjL hRK9Vksx46sBBxjjTcL5aVBgSyDiKw7xxp6Ul01/u3gYqU6Ucd6UUZQBS/W+dlNgK1CVM9xiGBwfnE8a0bls3IAUNeCIB0JLbn4/PqThxAv+FL6i+Cifla1ligRqNhta1PTQ7NgxAT9d59wG6wCJ56ZNrmBpps2pDg518Plcff2WiNAn/Mw1FYjARRizpYXKORhTdEaCcd2oWd/dSrDyMC14uX2xLeJRGpLWAjNfrzxE5zT/i+1cAHehjjsAmNAXrp8YLg+fNoYtufWDuWHT5+akSJXatYFvIHVjFn1o4PhgJeHLxXEXobpLwkuVeEDsR1EOLabpSAcsNUSx0osLpd7xNmC5ECynuu6iB3eGTfH3KaXoHJZz0wrot3GS2I2UxoES1 Sender: owner-linux-mm@kvack.org Precedence: bulk X-Loop: owner-majordomo@kvack.org List-ID: List-Subscribe: List-Unsubscribe: On 6/8/26 17:41, Zi Yan wrote: > On 8 Jun 2026, at 11:18, David Hildenbrand (Arm) wrote: > >>> 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. > > IIRC, multithreaded copy is already gated by CONFIG_HIGHMEM, otherwise > it needs to perform kmap_local() at each copying CPU, which complicates > the process. Then, this code will only used for 32bit without highmem, > I assume there will no page copy DMA on 32bit platform. Maybe it is not > too bad to limit this to 64bit. I'm more concerned of CONFIG_64BIT handling in the code, but if that can be avoided easily, fine with me. -- Cheers, David