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[2003:d8:2f4e:3200:c99d:a38b:3f3a:d4b3]) by smtp.gmail.com with ESMTPSA id 5b1f17b1804b1-475cae9253bsm74552055e9.1.2025.10.24.00.47.29 (version=TLS1_3 cipher=TLS_AES_128_GCM_SHA256 bits=128/128); Fri, 24 Oct 2025 00:47:29 -0700 (PDT) Message-ID: <9bfd8972-4111-4afb-b8a2-f514ae67f67f@redhat.com> Date: Fri, 24 Oct 2025 09:47:28 +0200 Precedence: bulk X-Mailing-List: linux-ext4@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 User-Agent: Mozilla Thunderbird Subject: Re: [PATCH] mm: do not install PMD mappings when handling a COW fault To: Zhang Yi , linux-mm@kvack.org Cc: linux-ext4@vger.kernel.org, linux-kernel@vger.kernel.org, yi.zhang@huawei.com, karol.wachowski@linux.intel.com, wangkefeng.wang@huawei.com, yangerkun@huawei.com References: <20251024015459.2824162-1-yi.zhang@huaweicloud.com> From: David Hildenbrand Content-Language: en-US Autocrypt: addr=david@redhat.com; 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 B3JfpUqcWwE7vUaYrHG1nw54pLUoPG6sAA7Mehl3nd4pZUALHwARAQABzSREYXZpZCBIaWxk ZW5icmFuZCA8ZGF2aWRAcmVkaGF0LmNvbT7CwZoEEwEIAEQCGwMCF4ACGQEFCwkIBwICIgIG FQoJCAsCBBYCAwECHgcWIQQb2cqtc1xMOkYN/MpN3hD3AP+DWgUCaJzangUJJlgIpAAKCRBN 3hD3AP+DWhAxD/9wcL0A+2rtaAmutaKTfxhTP0b4AAp1r/eLxjrbfbCCmh4pqzBhmSX/4z11 opn2KqcOsueRF1t2ENLOWzQu3Roiny2HOU7DajqB4dm1BVMaXQya5ae2ghzlJN9SIoopTWlR 0Af3hPj5E2PYvQhlcqeoehKlBo9rROJv/rjmr2x0yOM8qeTroH/ZzNlCtJ56AsE6Tvl+r7cW 3x7/Jq5WvWeudKrhFh7/yQ7eRvHCjd9bBrZTlgAfiHmX9AnCCPRPpNGNedV9Yty2Jnxhfmbv Pw37LA/jef8zlCDyUh2KCU1xVEOWqg15o1RtTyGV1nXV2O/mfuQJud5vIgzBvHhypc3p6VZJ lEf8YmT+Ol5P7SfCs5/uGdWUYQEMqOlg6w9R4Pe8d+mk8KGvfE9/zTwGg0nRgKqlQXrWRERv cuEwQbridlPAoQHrFWtwpgYMXx2TaZ3sihcIPo9uU5eBs0rf4mOERY75SK+Ekayv2ucTfjxr Kf014py2aoRJHuvy85ee/zIyLmve5hngZTTe3Wg3TInT9UTFzTPhItam6dZ1xqdTGHZYGU0O otRHcwLGt470grdiob6PfVTXoHlBvkWRadMhSuG4RORCDpq89vu5QralFNIf3EysNohoFy2A LYg2/D53xbU/aa4DDzBb5b1Rkg/udO1gZocVQWrDh6I2K3+cCs7BTQRVy5+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: <20251024015459.2824162-1-yi.zhang@huaweicloud.com> Content-Type: text/plain; charset=UTF-8; format=flowed Content-Transfer-Encoding: 7bit On 24.10.25 03:54, Zhang Yi wrote: > From: Zhang Yi > > During the ping of user pages in FOLL_LONGTERM on a COW VMA and a s/ping/pin/ or better "When pinning a page with FOLL_LONGTERM in a CoW VMA ..." > PMD-aligned (2MB on x86) large folio, follow_page_mask() failed to > obtain a valid anonymous page, resulting in an infinite loop issue. > The specific triggering process is as follows: > > 1. User call mmap with a 2MB size in MAP_PRIVATE mode for a file that > has a 2MB large folio installed in the page cache. > > addr = mmap(NULL, 2*1024*1024, PROT_READ, MAP_PRIVATE, file_fd, 0); > > 2. The kernel driver pass this mapped address to pin_user_pages_fast() > in FOLL_LONGTERM mode. > > pin_user_pages_fast(addr, 512, FOLL_LONGTERM, pages); > > -> pin_user_pages_fast() > | gup_fast_fallback() > | __gup_longterm_locked() > | __get_user_pages_locked() > | __get_user_pages() > | follow_page_mask() > | follow_p4d_mask() > | follow_pud_mask() > | follow_pmd_mask() //pmd_leaf(pmdval) is true because the > | //huge PMD is installed. This is normal > | //in the first round, but it shouldn't > | //happen in the second round. > | follow_huge_pmd() //require an anonymous page > | return -EMLINK; > | faultin_page() > | handle_mm_fault() > | wp_huge_pmd() //remove PMD and fall back to PTE > | handle_pte_fault() > | do_pte_missing() > | do_fault() > | do_read_fault() //FAULT_FLAG_WRITE is not set > | finish_fault() > | do_set_pmd() //install a huge PMD again, this is wrong!!! > | do_wp_page() //create private anonymous pages > <- goto retry; > > Due to an incorrectly large PMD set in do_read_fault(), > follow_pmd_mask() always returns -EMLINK, causing an infinite loop. > > David pointed out that we can preallocate a page table and remap the PMD > to be mapped by a PTE table in wp_huge_pmd() in the future. But now we > can avoid this issue by not installing PMD mappings when handling a COW > and unshare fault in do_set_pmd(). > > Fixes: a7f226604170 ("mm/gup: trigger FAULT_FLAG_UNSHARE when R/O-pinning a possibly shared anonymous page") > Reported-by: Karol Wachowski > Closes: https://lore.kernel.org/linux-ext4/844e5cd4-462e-4b88-b3b5-816465a3b7e3@linux.intel.com/ > Suggested-by: David Hildenbrand > Signed-off-by: Zhang Yi > --- > mm/memory.c | 5 +++++ > 1 file changed, 5 insertions(+) > > diff --git a/mm/memory.c b/mm/memory.c > index 0ba4f6b71847..0748a31367df 100644 > --- a/mm/memory.c > +++ b/mm/memory.c > @@ -5212,6 +5212,11 @@ vm_fault_t do_set_pmd(struct vm_fault *vmf, struct folio *folio, struct page *pa > if (!thp_vma_suitable_order(vma, haddr, PMD_ORDER)) > return ret; > > + /* We're about to trigger CoW, so never map it through a PMD. */ > + if (is_cow_mapping(vma->vm_flags) && > + (vmf->flags & (FAULT_FLAG_WRITE|FAULT_FLAG_UNSHARE))) > + return ret; > + > if (folio_order(folio) != HPAGE_PMD_ORDER) > return ret; > page = &folio->page; Acked-by: David Hildenbrand Thanks! -- Cheers David / dhildenb