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b=TpnCNeDkFmKf8caNwk0vroIg025H+aS+gYvM6CtupjAtLu+gXm/BExjdya8nvjpWB t+ckV8jBREMsltwvhamM2VNB7rNhmEodT7Be0ZnXinw6l8X+JH67BSNgGcub1ombmp hbW8neetl2m2yRHXCi0UYPIgnzZrPGpoe6/dKcQ5mbRxaJM3PhMEi9/PxFHwJMdabN yVSH39t/34uF1d1e2fhRIiZqz3hfHe8nPVbCCa6eo/Ua1aHszkZGqI10ZzHw9KKAD/ h9FyFTGX4r4NpQ07MmS2w1RJVj4hw8e+PSnpFz8D5L1qB/XaOvfoxipGgMNwxaayDq 37G8bUkgklBRA== Message-ID: <54b5a63d-34fc-4bd8-beaf-19673c8b6ad7@kernel.org> Date: Fri, 9 Oct 2026 20:00:51 +0200 MIME-Version: 1.0 User-Agent: Mozilla Thunderbird Subject: Re: [PATCH] mm/huge_memory: avoid transient none PMDs during lazyfree reclaim To: Kyle Zeng , linux-mm@kvack.org Cc: linux-kernel@vger.kernel.org, Andrew Morton , Zi Yan , Baolin Wang , Nico Pache , Ryan Roberts , Dev Jain , Barry Song , Lance Yang , Usama Arif , Kiryl Shutsemau , stable@vger.kernel.org References: <20261009165214.40212-2-kylebot@openai.com> 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: <20261009165214.40212-2-kylebot@openai.com> Content-Type: text/plain; charset=UTF-8 Content-Transfer-Encoding: 7bit X-Rspamd-Server: rspam04 X-Rspamd-Queue-Id: 53C83140009 X-Rspam-User: X-Stat-Signature: f1po53fw3dc89nmibkdbrgqn3a94grzi X-HE-Tag: 1791568859-529018 X-HE-Meta: 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 9fDl35AA 01fowY/L5CymOwARTngDMlZ3jO47mlm3hjq4ujySwAqQ13+QpbXD1QLgvN5vVsy3EamiUiUzYNq8FezumfQQiEp2lFn6gGTSieYpfA5F39YffZzIFPCUnCxaqcEvmxYo3BVyPDdgKuShYgj31M6g4Dcai8nZ+FWwn4QNm/TcvLNxciYbsGk9jcQLabKLBKMrkeo63kNMGmjomwYgLmN5TkB7gB/OjG+DUQ68bzdFd89lkJ3WX2NdfL+hn8VyoGD2cxvVneCsPVu+37Aka/SuSJGev1ueDBimVHqx63oFEPHYB/qUElsuKn6jXAYHIi453nl99w6Ta2wl5BO5OJT51eNiA2se2sdCGDONp Sender: owner-linux-mm@kvack.org Precedence: bulk X-Loop: owner-majordomo@kvack.org List-ID: List-Subscribe: List-Unsubscribe: On 10/9/26 18:52, Kyle Zeng wrote: > __discard_anon_folio_pmd_locked() clears and flushes a huge PMD before > checking whether the folio can be discarded. If the folio was redirtied > or has unexpected references, it restores the original PMD. Reclaim holds > the PMD lock and the anon_vma read lock, but not the owning mmap_lock. > > Both zap_pmd_range() and mremap's get_old_pmd() can skip a none PMD > without taking its lock. A concurrent munmap() or whole-VMA > MREMAP_DONTUNMAP can therefore miss the PMD and later unlink the source > VMA from its anon_vma, leaving a restored mapping behind. The anon_vma > write lock taken by unlink_anon_vmas() waits for reclaim to finish, but > does not repeat the skipped page-table walk. Once the remaining VMA > links are removed, the folio's positive mapcount no longer guarantees a > live anon_vma. Racing lazyfree reclaim against MREMAP_DONTUNMAP as an > unprivileged user reproduces a KASAN use-after-free in > folio_lock_anon_vma_read(). > > Use pmdp_invalidate() to keep a recognizable, non-none huge PMD while > the discard can still fail. Concurrent unmap and move operations then > have to synchronize on the PMD lock instead of skipping the mapping. > Clear the invalidated PMD only after the dirty and reference checks have > succeeded, before removing the rmap and withdrawing the deposited page > table. The full invalidation retains the TLB flush required for the > dirty and GUP-fast checks. > > Fixes: 735ecdfaf4e8 ("mm/vmscan: avoid split lazyfree THP during shrink_folio_list()") > Cc: stable@vger.kernel.org > Assisted-by: Codex:gpt-6-astra > Signed-off-by: Kyle Zeng > --- > mm/huge_memory.c | 11 +++++++++-- > 1 file changed, 9 insertions(+), 2 deletions(-) > > diff --git a/mm/huge_memory.c b/mm/huge_memory.c > index 1e5d68acf62a..20262195385a 100644 > --- a/mm/huge_memory.c > +++ b/mm/huge_memory.c > @@ -3569,11 +3569,17 @@ static bool __discard_anon_folio_pmd_locked(struct vm_area_struct *vma, > return false; > } > > - orig_pmd = pmdp_huge_clear_flush(vma, addr, pmdp); > + /* > + * The discard may fail, so keep the PMD non-none until we're > + * committed to discarding it. Otherwise, concurrent munmap() or > + * mremap() can skip the PMD without taking the PTL and later unlink > + * the VMA from its anon_vma despite a restored mapping. > + */ You can drop the entire comment. This is what we should do in most cases where the mapping might be restored/changed, for example, in set_pmd_migration_entry(). > + orig_pmd = pmdp_invalidate(vma, addr, pmdp); > > /* > * Syncing against concurrent GUP-fast: > - * - clear PMD; barrier; read refcount > + * - invalidate PMD; barrier; read refcount > * - inc refcount; barrier; read PMD > */ > smp_mb(); > @@ -3607,6 +3613,7 @@ static bool __discard_anon_folio_pmd_locked(struct vm_area_struct *vma, > return false; > } > > + pmdp_huge_get_and_clear(mm, addr, pmdp); > folio_remove_rmap_pmd(folio, pmd_page(orig_pmd), vma); > zap_deposited_table(mm, pmdp); > add_mm_counter(mm, MM_ANONPAGES, -HPAGE_PMD_NR); Thanks! Acked-by: David Hildenbrand (Arm) -- Cheers, David