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b=hIHqXnlD6LBlrIWncM1iS+Ssr4Dtp/DrEcECJJWIQIL5j1SRatmszSYTEahxRkEtK SHtbnuQP1HtGCPU8p1fNEFNInOf93shaNULGIDKVpXCEUMOdnJYjT4ywKxdZ/GikvL JTkg8oYJHU0Ai4hN37+LKivtX0rTTg0f/OUv+VuflsrujYJvOxzWJFZ6JySChD2LI8 V0cv8EUqNZ640GHcmE+E17DvRMVnt9ASeb+DcSZ2EcyJDxEF7t5N7WZ2S6LG7Z+Gqd B/C0qMpD4o+sh6GUK67lbxybqde09mUTVJz0iVQeENBmGhCEt9Q7uHy7Py7De8gZUy 7ZASSzkq9CYgA== Message-ID: Date: Wed, 8 Jul 2026 11:03:52 +0200 MIME-Version: 1.0 User-Agent: Mozilla Thunderbird Subject: Re: [PATCH v2] mm/hugetlb: fix swap entry corruption when clearing uffd-wp at fork() To: Kiryl Shutsemau , akpm@linux-foundation.org Cc: muchun.song@linux.dev, osalvador@suse.de, peterx@redhat.com, linux-mm@kvack.org, linux-kernel@vger.kernel.org, stable@vger.kernel.org, kernel-team@meta.com, kas@kernel.org References: <20260708090110.136162-1-kirill@shutemov.name> From: "David Hildenbrand (Arm)" Content-Language: en-US Autocrypt: addr=david@kernel.org; keydata= xsFNBFXLn5EBEAC+zYvAFJxCBY9Tr1xZgcESmxVNI/0ffzE/ZQOiHJl6mGkmA1R7/uUpiCjJ dBrn+lhhOYjjNefFQou6478faXE6o2AhmebqT4KiQoUQFV4R7y1KMEKoSyy8hQaK1umALTdL QZLQMzNE74ap+GDK0wnacPQFpcG1AE9RMq3aeErY5tujekBS32jfC/7AnH7I0v1v1TbbK3Gp XNeiN4QroO+5qaSr0ID2sz5jtBLRb15RMre27E1ImpaIv2Jw8NJgW0k/D1RyKCwaTsgRdwuK Kx/Y91XuSBdz0uOyU/S8kM1+ag0wvsGlpBVxRR/xw/E8M7TEwuCZQArqqTCmkG6HGcXFT0V9 PXFNNgV5jXMQRwU0O/ztJIQqsE5LsUomE//bLwzj9IVsaQpKDqW6TAPjcdBDPLHvriq7kGjt WhVhdl0qEYB8lkBEU7V2Yb+SYhmhpDrti9Fq1EsmhiHSkxJcGREoMK/63r9WLZYI3+4W2rAc UucZa4OT27U5ZISjNg3Ev0rxU5UH2/pT4wJCfxwocmqaRr6UYmrtZmND89X0KigoFD/XSeVv jwBRNjPAubK9/k5NoRrYqztM9W6sJqrH8+UWZ1Idd/DdmogJh0gNC0+N42Za9yBRURfIdKSb B3JfpUqcWwE7vUaYrHG1nw54pLUoPG6sAA7Mehl3nd4pZUALHwARAQABzS5EYXZpZCBIaWxk ZW5icmFuZCAoQ3VycmVudCkgPGRhdmlkQGtlcm5lbC5vcmc+wsGQBBMBCAA6AhsDBQkmWAik AgsJBBUKCQgCFgICHgUCF4AWIQQb2cqtc1xMOkYN/MpN3hD3AP+DWgUCaYJt/AIZAQAKCRBN 3hD3AP+DWriiD/9BLGEKG+N8L2AXhikJg6YmXom9ytRwPqDgpHpVg2xdhopoWdMRXjzOrIKD g4LSnFaKneQD0hZhoArEeamG5tyo32xoRsPwkbpIzL0OKSZ8G6mVbFGpjmyDLQCAxteXCLXz ZI0VbsuJKelYnKcXWOIndOrNRvE5eoOfTt2XfBnAapxMYY2IsV+qaUXlO63GgfIOg8RBaj7x 3NxkI3rV0SHhI4GU9K6jCvGghxeS1QX6L/XI9mfAYaIwGy5B68kF26piAVYv/QZDEVIpo3t7 /fjSpxKT8plJH6rhhR0epy8dWRHk3qT5tk2P85twasdloWtkMZ7FsCJRKWscm1BLpsDn6EQ4 jeMHECiY9kGKKi8dQpv3FRyo2QApZ49NNDbwcR0ZndK0XFo15iH708H5Qja/8TuXCwnPWAcJ DQoNIDFyaxe26Rx3ZwUkRALa3iPcVjE0//TrQ4KnFf+lMBSrS33xDDBfevW9+Dk6IISmDH1R HFq2jpkN+FX/PE8eVhV68B2DsAPZ5rUwyCKUXPTJ/irrCCmAAb5Jpv11S7hUSpqtM/6oVESC 3z/7CzrVtRODzLtNgV4r5EI+wAv/3PgJLlMwgJM90Fb3CB2IgbxhjvmB1WNdvXACVydx55V7 LPPKodSTF29rlnQAf9HLgCphuuSrrPn5VQDaYZl4N/7zc2wcWM7BTQRVy5+RARAA59fefSDR 9nMGCb9LbMX+TFAoIQo/wgP5XPyzLYakO+94GrgfZjfhdaxPXMsl2+o8jhp/hlIzG56taNdt VZtPp3ih1AgbR8rHgXw1xwOpuAd5lE1qNd54ndHuADO9a9A0vPimIes78Hi1/yy+ZEEvRkHk /kDa6F3AtTc1m4rbbOk2fiKzzsE9YXweFjQvl9p+AMw6qd/iC4lUk9g0+FQXNdRs+o4o6Qvy iOQJfGQ4UcBuOy1IrkJrd8qq5jet1fcM2j4QvsW8CLDWZS1L7kZ5gT5EycMKxUWb8LuRjxzZ 3QY1aQH2kkzn6acigU3HLtgFyV1gBNV44ehjgvJpRY2cC8VhanTx0dZ9mj1YKIky5N+C0f21 zvntBqcxV0+3p8MrxRRcgEtDZNav+xAoT3G0W4SahAaUTWXpsZoOecwtxi74CyneQNPTDjNg azHmvpdBVEfj7k3p4dmJp5i0U66Onmf6mMFpArvBRSMOKU9DlAzMi4IvhiNWjKVaIE2Se9BY FdKVAJaZq85P2y20ZBd08ILnKcj7XKZkLU5FkoA0udEBvQ0f9QLNyyy3DZMCQWcwRuj1m73D sq8DEFBdZ5eEkj1dCyx+t/ga6x2rHyc8Sl86oK1tvAkwBNsfKou3v+jP/l14a7DGBvrmlYjO 59o3t6inu6H7pt7OL6u6BQj7DoMAEQEAAcLBfAQYAQgAJgIbDBYhBBvZyq1zXEw6Rg38yk3e EPcA/4NaBQJonNqrBQkmWAihAAoJEE3eEPcA/4NaKtMQALAJ8PzprBEXbXcEXwDKQu+P/vts IfUb1UNMfMV76BicGa5NCZnJNQASDP/+bFg6O3gx5NbhHHPeaWz/VxlOmYHokHodOvtL0WCC 8A5PEP8tOk6029Z+J+xUcMrJClNVFpzVvOpb1lCbhjwAV465Hy+NUSbbUiRxdzNQtLtgZzOV Zw7jxUCs4UUZLQTCuBpFgb15bBxYZ/BL9MbzxPxvfUQIPbnzQMcqtpUs21CMK2PdfCh5c4gS sDci6D5/ZIBw94UQWmGpM/O1ilGXde2ZzzGYl64glmccD8e87OnEgKnH3FbnJnT4iJchtSvx yJNi1+t0+qDti4m88+/9IuPqCKb6Stl+s2dnLtJNrjXBGJtsQG/sRpqsJz5x1/2nPJSRMsx9 5YfqbdrJSOFXDzZ8/r82HgQEtUvlSXNaXCa95ez0UkOG7+bDm2b3s0XahBQeLVCH0mw3RAQg r7xDAYKIrAwfHHmMTnBQDPJwVqxJjVNr7yBic4yfzVWGCGNE4DnOW0vcIeoyhy9vnIa3w1uZ 3iyY2Nsd7JxfKu1PRhCGwXzRw5TlfEsoRI7V9A8isUCoqE2Dzh3FvYHVeX4Us+bRL/oqareJ CIFqgYMyvHj7Q06kTKmauOe4Nf0l0qEkIuIzfoLJ3qr5UyXc2hLtWyT9Ir+lYlX9efqh7mOY qIws/H2t In-Reply-To: <20260708090110.136162-1-kirill@shutemov.name> Content-Type: text/plain; charset=UTF-8 Content-Transfer-Encoding: 7bit X-Rspam-User: X-Rspamd-Server: rspam10 X-Rspamd-Queue-Id: C6C9640008 X-Stat-Signature: kp7n6osqgt4aci9s1b3rkbb117qwg94y X-HE-Tag: 1783501438-335010 X-HE-Meta: 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 c+ml8QrM x7yLnj/OJnHk8Fxf7utVDCTTXU3VcmBtUPnb8zZI3B9yP6N4gr6DGQUahjqNf/1wSlFu8tR5VV/DcLPYZhJe3AAWPJfh8vO0uTKNP7+snFsP/LoU2ApLQ9lMgpIw1iupbq3TPRLQJXL8gr2LIC+WonblfZjhDyMm1jRYB/wJYiRwv06f7BM9tKfoUyh/2QxE4IeT+EzizEnopQMSehVOOzl8XWtjYgvb6/IiqY3T3A0Bna7Dhj532FG1KV3rEYsVXpi0vSjsXoKJ0L5Eo8tHjAvwfF0vi7Yd58ZOo8IbgJDQ6Nu7G93KAvparb+OzOxCTpdHr Sender: owner-linux-mm@kvack.org Precedence: bulk X-Loop: owner-majordomo@kvack.org List-ID: List-Subscribe: List-Unsubscribe: On 7/8/26 11:01, Kiryl Shutsemau wrote: > From: "Kiryl Shutsemau (Meta)" > > copy_hugetlb_page_range() clears the uffd-wp bit of migration and > hwpoison entries with huge_pte_clear_uffd_wp(), which operates on the > present-PTE bit position. Swap entries keep the uffd-wp state elsewhere > -- the migration branch reads and sets it with pte_swp_uffd_wp() and > pte_swp_mkuffd_wp() -- and the present-PTE position falls into the swap > payload. On x86-64 it lands in the inverted swap offset, where a > naturally-aligned hugetlb PFN always has the affected bit set, so the > clear advances the encoded PFN by two pages. > > No userfaultfd needs to be involved: the clear is guarded only by the > child VMA not being uffd-wp registered, so a plain fork() with an > in-flight hugetlb migration entry (or a poisoned hugetlb page) corrupts > the entry copied into the child. Instrumenting the clear and forking > after MADV_HWPOISON on a 2MB anon hugetlb page shows: > > offset before=120e00 > offset after =120e02 > > The fallout is mostly latent: rmap walks match migration entries by > folio range and remove_migration_pte() rebuilds the PTE from the folio, > so a within-folio PFN skew heals once migration completes. But any path > that re-encodes the corrupted offset -- e.g. hugetlb_change_protection() > rewriting a writable migration entry via > make_readable_migration_entry(swp_offset(entry)) -- propagates it. > > Migration entries legitimately carry uffd-wp, so clear it with > pte_swp_clear_uffd_wp(), matching copy_nonpresent_pte() and > move_huge_pte(). > > A hwpoison entry, on the other hand, never carries the uffd-wp bit: it > is installed fresh by make_hwpoison_entry() (try_to_unmap_one() does not > preserve uffd-wp on the hwpoison path) and hugetlb_change_protection() > leaves hwpoison entries untouched. There was nothing to clear there, > only the corruption, so drop the clear entirely. > > Reported-by: Sashiko AI review > Closes: https://lore.kernel.org/all/20260703140011.99E601F000E9@smtp.kernel.org/ > Suggested-by: David Hildenbrand > Fixes: bc70fbf269fd ("mm/hugetlb: handle uffd-wp during fork()") > Cc: stable@vger.kernel.org > Signed-off-by: Kiryl Shutsemau > Assisted-by: Claude:claude-fable-5 > --- Thanks! Acked-by: David Hildenbrand (Arm) -- Cheers, David