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[2003:d8:2f30:ec00:8f7e:58a4:ebf0:6a36]) by smtp.gmail.com with ESMTPSA id 5b1f17b1804b1-450064ae786sm24445705e9.23.2025.05.28.08.09.27 (version=TLS1_3 cipher=TLS_AES_128_GCM_SHA256 bits=128/128); Wed, 28 May 2025 08:09:27 -0700 (PDT) Message-ID: <629bb87e-c493-4069-866c-20e02c14ddcc@redhat.com> Date: Wed, 28 May 2025 17:09:26 +0200 Precedence: bulk X-Mailing-List: stable@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 User-Agent: Mozilla Thunderbird Subject: Re: [PATCH v3] mm/hugetlb: fix a deadlock with pagecache_folio and hugetlb_fault_mutex_table To: Peter Xu , Oscar Salvador Cc: Gavin Guo , linux-mm@kvack.org, linux-kernel@vger.kernel.org, muchun.song@linux.dev, akpm@linux-foundation.org, mike.kravetz@oracle.com, kernel-dev@igalia.com, stable@vger.kernel.org, Hugh Dickins , Florent Revest , Gavin Shan References: <20250528023326.3499204-1-gavinguo@igalia.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 ZW5icmFuZCA8ZGF2aWRAcmVkaGF0LmNvbT7CwZgEEwEIAEICGwMGCwkIBwMCBhUIAgkKCwQW AgMBAh4BAheAAhkBFiEEG9nKrXNcTDpGDfzKTd4Q9wD/g1oFAl8Ox4kFCRKpKXgACgkQTd4Q 9wD/g1oHcA//a6Tj7SBNjFNM1iNhWUo1lxAja0lpSodSnB2g4FCZ4R61SBR4l/psBL73xktp rDHrx4aSpwkRP6Epu6mLvhlfjmkRG4OynJ5HG1gfv7RJJfnUdUM1z5kdS8JBrOhMJS2c/gPf wv1TGRq2XdMPnfY2o0CxRqpcLkx4vBODvJGl2mQyJF/gPepdDfcT8/PY9BJ7FL6Hrq1gnAo4 3Iv9qV0JiT2wmZciNyYQhmA1V6dyTRiQ4YAc31zOo2IM+xisPzeSHgw3ONY/XhYvfZ9r7W1l pNQdc2G+o4Di9NPFHQQhDw3YTRR1opJaTlRDzxYxzU6ZnUUBghxt9cwUWTpfCktkMZiPSDGd KgQBjnweV2jw9UOTxjb4LXqDjmSNkjDdQUOU69jGMUXgihvo4zhYcMX8F5gWdRtMR7DzW/YE BgVcyxNkMIXoY1aYj6npHYiNQesQlqjU6azjbH70/SXKM5tNRplgW8TNprMDuntdvV9wNkFs 9TyM02V5aWxFfI42+aivc4KEw69SE9KXwC7FSf5wXzuTot97N9Phj/Z3+jx443jo2NR34XgF 89cct7wJMjOF7bBefo0fPPZQuIma0Zym71cP61OP/i11ahNye6HGKfxGCOcs5wW9kRQEk8P9 M/k2wt3mt/fCQnuP/mWutNPt95w9wSsUyATLmtNrwccz63XOwU0EVcufkQEQAOfX3n0g0fZz Bgm/S2zF/kxQKCEKP8ID+Vz8sy2GpDvveBq4H2Y34XWsT1zLJdvqPI4af4ZSMxuerWjXbVWb T6d4odQIG0fKx4F8NccDqbgHeZRNajXeeJ3R7gAzvWvQNLz4piHrO/B4tf8svmRBL0ZB5P5A 2uhdwLU3NZuK22zpNn4is87BPWF8HhY0L5fafgDMOqnf4guJVJPYNPhUFzXUbPqOKOkL8ojk CXxkOFHAbjstSK5Ca3fKquY3rdX3DNo+EL7FvAiw1mUtS+5GeYE+RMnDCsVFm/C7kY8c2d0G NWkB9pJM5+mnIoFNxy7YBcldYATVeOHoY4LyaUWNnAvFYWp08dHWfZo9WCiJMuTfgtH9tc75 7QanMVdPt6fDK8UUXIBLQ2TWr/sQKE9xtFuEmoQGlE1l6bGaDnnMLcYu+Asp3kDT0w4zYGsx 5r6XQVRH4+5N6eHZiaeYtFOujp5n+pjBaQK7wUUjDilPQ5QMzIuCL4YjVoylWiBNknvQWBXS lQCWmavOT9sttGQXdPCC5ynI+1ymZC1ORZKANLnRAb0NH/UCzcsstw2TAkFnMEbo9Zu9w7Kv AxBQXWeXhJI9XQssfrf4Gusdqx8nPEpfOqCtbbwJMATbHyqLt7/oz/5deGuwxgb65pWIzufa N7eop7uh+6bezi+rugUI+w6DABEBAAHCwXwEGAEIACYCGwwWIQQb2cqtc1xMOkYN/MpN3hD3 AP+DWgUCXw7HsgUJEqkpoQAKCRBN3hD3AP+DWrrpD/4qS3dyVRxDcDHIlmguXjC1Q5tZTwNB boaBTPHSy/Nksu0eY7x6HfQJ3xajVH32Ms6t1trDQmPx2iP5+7iDsb7OKAb5eOS8h+BEBDeq 3ecsQDv0fFJOA9ag5O3LLNk+3x3q7e0uo06XMaY7UHS341ozXUUI7wC7iKfoUTv03iO9El5f XpNMx/YrIMduZ2+nd9Di7o5+KIwlb2mAB9sTNHdMrXesX8eBL6T9b+MZJk+mZuPxKNVfEQMQ a5SxUEADIPQTPNvBewdeI80yeOCrN+Zzwy/Mrx9EPeu59Y5vSJOx/z6OUImD/GhX7Xvkt3kq Er5KTrJz3++B6SH9pum9PuoE/k+nntJkNMmQpR4MCBaV/J9gIOPGodDKnjdng+mXliF3Ptu6 3oxc2RCyGzTlxyMwuc2U5Q7KtUNTdDe8T0uE+9b8BLMVQDDfJjqY0VVqSUwImzTDLX9S4g/8 kC4HRcclk8hpyhY2jKGluZO0awwTIMgVEzmTyBphDg/Gx7dZU1Xf8HFuE+UZ5UDHDTnwgv7E th6RC9+WrhDNspZ9fJjKWRbveQgUFCpe1sa77LAw+XFrKmBHXp9ZVIe90RMe2tRL06BGiRZr jPrnvUsUUsjRoRNJjKKA/REq+sAnhkNPPZ/NNMjaZ5b8Tovi8C0tmxiCHaQYqj7G2rgnT0kt WNyWQQ== Organization: Red Hat In-Reply-To: Content-Type: text/plain; charset=UTF-8; format=flowed Content-Transfer-Encoding: 7bit On 28.05.25 17:03, Peter Xu wrote: > On Wed, May 28, 2025 at 11:27:46AM +0200, Oscar Salvador wrote: >> On Wed, May 28, 2025 at 10:33:26AM +0800, Gavin Guo wrote: >>> There is ABBA dead locking scenario happening between hugetlb_fault() >>> and hugetlb_wp() on the pagecache folio's lock and hugetlb global mutex, >>> which is reproducible with syzkaller [1]. As below stack traces reveal, >>> process-1 tries to take the hugetlb global mutex (A3), but with the >>> pagecache folio's lock hold. Process-2 took the hugetlb global mutex but >>> tries to take the pagecache folio's lock. >>> >>> Process-1 Process-2 >>> ========= ========= >>> hugetlb_fault >>> mutex_lock (A1) >>> filemap_lock_hugetlb_folio (B1) >>> hugetlb_wp >>> alloc_hugetlb_folio #error >>> mutex_unlock (A2) >>> hugetlb_fault >>> mutex_lock (A4) >>> filemap_lock_hugetlb_folio (B4) >>> unmap_ref_private >>> mutex_lock (A3) >>> >>> Fix it by releasing the pagecache folio's lock at (A2) of process-1 so >>> that pagecache folio's lock is available to process-2 at (B4), to avoid >>> the deadlock. In process-1, a new variable is added to track if the >>> pagecache folio's lock has been released by its child function >>> hugetlb_wp() to avoid double releases on the lock in hugetlb_fault(). >>> The similar changes are applied to hugetlb_no_page(). >>> >>> Link: https://drive.google.com/file/d/1DVRnIW-vSayU5J1re9Ct_br3jJQU6Vpb/view?usp=drive_link [1] >>> Fixes: 40549ba8f8e0 ("hugetlb: use new vma_lock for pmd sharing synchronization") >>> Cc: >>> Cc: Hugh Dickins >>> Cc: Florent Revest >>> Reviewed-by: Gavin Shan >>> Signed-off-by: Gavin Guo >> ... >>> diff --git a/mm/hugetlb.c b/mm/hugetlb.c >>> index 6a3cf7935c14..560b9b35262a 100644 >>> --- a/mm/hugetlb.c >>> +++ b/mm/hugetlb.c >>> @@ -6137,7 +6137,8 @@ static void unmap_ref_private(struct mm_struct *mm, struct vm_area_struct *vma, >>> * Keep the pte_same checks anyway to make transition from the mutex easier. >>> */ >>> static vm_fault_t hugetlb_wp(struct folio *pagecache_folio, >>> - struct vm_fault *vmf) >>> + struct vm_fault *vmf, >>> + bool *pagecache_folio_locked) >>> { >>> struct vm_area_struct *vma = vmf->vma; >>> struct mm_struct *mm = vma->vm_mm; >>> @@ -6234,6 +6235,18 @@ static vm_fault_t hugetlb_wp(struct folio *pagecache_folio, >>> u32 hash; >>> >>> folio_put(old_folio); >>> + /* >>> + * The pagecache_folio has to be unlocked to avoid >>> + * deadlock and we won't re-lock it in hugetlb_wp(). The >>> + * pagecache_folio could be truncated after being >>> + * unlocked. So its state should not be reliable >>> + * subsequently. >>> + */ >>> + if (pagecache_folio) { >>> + folio_unlock(pagecache_folio); >>> + if (pagecache_folio_locked) >>> + *pagecache_folio_locked = false; >>> + } >> >> I am having a problem with this patch as I think it keeps carrying on an >> assumption that it is not true. >> >> I was discussing this matter yesterday with Peter Xu (CCed now), who has also some >> experience in this field. >> >> Exactly against what pagecache_folio's lock protects us when >> pagecache_folio != old_folio? >> >> There are two cases here: >> >> 1) pagecache_folio = old_folio (original page in the pagecache) >> 2) pagecache_folio != old_folio (original page has already been mapped >> privately and CoWed, old_folio contains >> the new folio) >> >> For case 1), we need to hold the lock because we are copying old_folio >> to the new one in hugetlb_wp(). That is clear. > > So I'm not 100% sure we need the folio lock even for copy; IIUC a refcount > would be enough? The introducing patches seem to talk about blocking concurrent migration / rmap walks. Maybe also concurrent fallocate(PUNCH_HOLE) is a problem regarding reservations? Not sure ... For 2) I am also not sure if we need need the pagecache folio locked; I doubt it ... but this code is not the easiest to follow. -- Cheers, David / dhildenb