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b=jVd9KAcuMu0TNXZeoByHF3KAzOSxm8XMBVZ6mQ6QBVKKxJTc7/cm529qR6DbrR8Io rAoWeoJCZM+eN7ARUYqNxWblUf31m2r8SH6cjJX46DD/Ph0kY85iYkqO+a6eDEYMZC faitM5TpGFrED073qp0WLAr9ejfunVcNc6wbGLBDwxjfqJ6mj9NfeZqPmKsVicfzjB 2cSe6opxwp84fTugfEwBHskJZj9y016emGRNn7jtQLkyaSHF38KgsAmK7vfWQHA37k y0fbpezVJaTCm7/xo1L1ImRh6Rh1nLMVf8GNdqXYNgD2hNuaoKD6HTGx2FeZlD8QO4 5QAZSjCeKoROQ== Message-ID: Date: Fri, 25 Sep 2026 13:28:30 +0200 MIME-Version: 1.0 User-Agent: Mozilla Thunderbird Subject: Re: [PATCH] proc/task_mmu: handle special PMDs in clear_refs and pagemap To: Gregory Price , linux-mm@kvack.org Cc: linux-fsdevel@vger.kernel.org, linux-kernel@vger.kernel.org, kernel-team@meta.com, akpm@linux-foundation.org, liam@infradead.org, ljs@kernel.org, vbabka@kernel.org, jannh@google.com, pfalcato@suse.de, peterx@redhat.com, jgg@ziepe.ca, sashiko-bot , stable@vger.kernel.org References: <20260912122822.3348978-1-gourry@gourry.net> 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: <20260912122822.3348978-1-gourry@gourry.net> Content-Type: text/plain; charset=UTF-8 Content-Transfer-Encoding: 7bit X-Rspam-User: X-Stat-Signature: 8r4x1i4pyfgzwb11qryi64yxsgkt6w13 X-Rspamd-Server: rspam03 X-Rspamd-Queue-Id: CBB5D2000B X-HE-Tag: 1790344971-323208 X-HE-Meta: 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 J/xHAivY KTR4srNJBzPWut6DXYKmDMUfbjQg5l0bnMKvku9G+QW0vqakIAAxdsxPVvv+r0Oj2e+X9m03c8uM9dXHlKVNqutUBW0Qb9KP17vct3n7eUTrByHv+EKJ9E4kdRN6la8+41M73Mmg4yusqJzgjOUXk+Psgj1x42lJ/qx+ZfXIPoPT3OMtHpxzR38ZZV9iviRkYyoQj9sNZpi4NkNsIzUfk5vXirV3TW3rnNhI/WbVsyHdLJybRJQ6ZTy+nB+bybATG3/UrZwBdtS58EAN+MxE2QH2gdAzXzvrYk4I4Km7aBTW0ijhiSVeHU1izGpAUq1aL7/epRFJQPlLIDDWlJBWNr0ciEGb/jUesy42mVZRIzdU7DLty0fDSbfGMKtrZ2Kv4EYY2IXQE5eu4jposC4/rscK58z0yt15X0mRrd34i//0F5iI8XUQJ2tFA/LS48iB19HB6 Sender: owner-linux-mm@kvack.org Precedence: bulk X-Loop: owner-majordomo@kvack.org List-ID: List-Subscribe: List-Unsubscribe: On 9/12/26 14:28, Gregory Price wrote: > mshv_vtl_low can install a special PMD from a PFN supplied in the mmap file > offset. That PFN need not have a memmap entry. Reading /proc/PID/pagemap > or writing /proc/PID/clear_refs for such a mapping oopses the kernel. > > With a test driver mapping the PFN at the 1 TiB mark: > > echo 1 > /proc/self/clear_refs > > BUG: unable to handle page fault for address: fffffb5b80000008 > #PF: supervisor read access in kernel mode > #PF: error_code(0x0000) - not-present page > RIP: 0010:clear_refs_pte_range+0xb1/0x1e0 > Call Trace: > walk_pgd_range+0x50a/0xad0 > __walk_page_range+0x6a/0x1d0 > walk_page_range_mm_unsafe+0x193/0x230 > clear_refs_write+0x18e/0x3f0 > > pread(pagemap_fd, buf, 512 * 8, addr / PAGE_SIZE * 8) > > BUG: unable to handle page fault for address: fffff57840000008 > RIP: 0010:pagemap_pmd_range+0x3cf/0x6b0 > Call Trace: > walk_pgd_range+0x50a/0xad0 > __walk_page_range+0x6a/0x1d0 > walk_page_range_mm_unsafe+0x193/0x230 > pagemap_read+0x1dc/0x350 > > clear_refs_pte_range() passes the PMD to pmd_folio(), while > pagemap_pmd_range_thp() uses pmd_page() followed by page_folio(). > > Both paths assume the PFN has a struct page and dereference bad address. > > Use vm_normal_folio_pmd() and vm_normal_page_pmd() so clear_refs skips > folio operations and pagemap skips folio-derived flags for special PMDs. > This matches the PTE paths, which already use vm_normal_folio() and > vm_normal_page(). > > User-facing notes: > - Pagemap still reports PM_PRESENT and the PFN to a CAP_SYS_ADMIN reader. > > - For a special PMD backed by a valid memmap entry and for the huge > zero PMD, PM_FILE changes from set to clear. > > - PM_MMAP_EXCLUSIVE is already clear in both cases. > > - The output for an anonymous THP is unchanged. > > Tested in QEMU with the test driver on broken and fixed kernels. Both proc > operations oops before the fix and return 0 after it. > > Reported-by: sashiko-bot > Closes: https://sashiko.dev/#/patchset/20260912034833.2952750-1-gourry%40gourry.net > Fixes: 3c8e44c9b369 ("mm: mark special bits for huge pfn mappings when inject") > Cc: # v6.19+ > Assisted-by: LLM > Signed-off-by: Gregory Price > --- Simple fix, let's CC-stable indeed (unless I am too late and this is already upstream :) ) Acked-by: David Hildenbrand (Arm) -- Cheers, David