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[2003:d8:2f4e:fd00:8e13:e3b5:90c8:1159]) by smtp.gmail.com with ESMTPSA id ffacd0b85a97d-3a6d0f1054bsm9455563f8f.9.2025.06.23.07.11.38 (version=TLS1_3 cipher=TLS_AES_128_GCM_SHA256 bits=128/128); Mon, 23 Jun 2025 07:11:39 -0700 (PDT) Message-ID: <35b03911-74c3-4626-aaa8-4c331c086f8f@redhat.com> Date: Mon, 23 Jun 2025 16:11:38 +0200 Precedence: bulk X-Mailing-List: linux-kernel@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 User-Agent: Mozilla Thunderbird Subject: Re: [PATCH v2 2/5] mm,hugetlb: Sort out folio locking in the faulting path To: Oscar Salvador , Andrew Morton Cc: Muchun Song , Peter Xu , Gavin Guo , linux-mm@kvack.org, linux-kernel@vger.kernel.org References: <20250620123014.29748-1-osalvador@suse.de> <20250620123014.29748-3-osalvador@suse.de> 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: <20250620123014.29748-3-osalvador@suse.de> Content-Type: text/plain; charset=UTF-8; format=flowed Content-Transfer-Encoding: 7bit On 20.06.25 14:30, Oscar Salvador wrote: > Recent conversations showed that there was a misunderstanding about why we > were locking the folio prior to call in hugetlb_wp(). > In fact, as soon as we have the folio mapped into the pagetables, we no longer > need to hold it locked, because we know that no concurrent truncation could have > happened. > There is only one case where the folio needs to be locked, and that is when we > are handling an anonymous folio, because hugetlb_wp() will check whether it can > re-use it exclusively for the process that is faulting it in. > > So, pass the folio locked to hugetlb_wp() when that is the case. > > Suggested-by: David Hildenbrand > Signed-off-by: Oscar Salvador > --- > mm/hugetlb.c | 43 +++++++++++++++++++++++++++++++++---------- > 1 file changed, 33 insertions(+), 10 deletions(-) > > diff --git a/mm/hugetlb.c b/mm/hugetlb.c > index 175edafeec67..1a5f713c1e4c 100644 > --- a/mm/hugetlb.c > +++ b/mm/hugetlb.c > @@ -6437,6 +6437,7 @@ static vm_fault_t hugetlb_no_page(struct address_space *mapping, > pte_t new_pte; > bool new_folio, new_pagecache_folio = false; > u32 hash = hugetlb_fault_mutex_hash(mapping, vmf->pgoff); > + bool folio_locked = true; > > /* > * Currently, we are forced to kill the process in the event the > @@ -6602,6 +6603,11 @@ static vm_fault_t hugetlb_no_page(struct address_space *mapping, > > hugetlb_count_add(pages_per_huge_page(h), mm); > if ((vmf->flags & FAULT_FLAG_WRITE) && !(vma->vm_flags & VM_SHARED)) { > + /* No need to lock file folios. See comment in hugetlb_fault() */ > + if (!anon_rmap) { > + folio_locked = false; > + folio_unlock(folio); > + } > /* Optimization, do the COW without a second fault */ > ret = hugetlb_wp(vmf); > } > @@ -6616,7 +6622,8 @@ static vm_fault_t hugetlb_no_page(struct address_space *mapping, > if (new_folio) > folio_set_hugetlb_migratable(folio); > > - folio_unlock(folio); > + if (folio_locked) > + folio_unlock(folio); > out: > hugetlb_vma_unlock_read(vma); > > @@ -6636,7 +6643,8 @@ static vm_fault_t hugetlb_no_page(struct address_space *mapping, > if (new_folio && !new_pagecache_folio) > restore_reserve_on_error(h, vma, vmf->address, folio); > > - folio_unlock(folio); > + if (folio_locked) > + folio_unlock(folio); > folio_put(folio); > goto out; > } > @@ -6670,7 +6678,7 @@ vm_fault_t hugetlb_fault(struct mm_struct *mm, struct vm_area_struct *vma, > { > vm_fault_t ret; > u32 hash; > - struct folio *folio; > + struct folio *folio = NULL; > struct hstate *h = hstate_vma(vma); > struct address_space *mapping; > struct vm_fault vmf = { > @@ -6687,6 +6695,7 @@ vm_fault_t hugetlb_fault(struct mm_struct *mm, struct vm_area_struct *vma, > * be hard to debug if called functions make assumptions > */ > }; > + bool folio_locked = false; > > /* > * Serialize hugepage allocation and instantiation, so that we don't > @@ -6801,13 +6810,24 @@ vm_fault_t hugetlb_fault(struct mm_struct *mm, struct vm_area_struct *vma, > /* Fallthrough to CoW */ > } > > - /* hugetlb_wp() requires page locks of pte_page(vmf.orig_pte) */ > - folio = page_folio(pte_page(vmf.orig_pte)); > - folio_lock(folio); > - folio_get(folio); > - > if (flags & (FAULT_FLAG_WRITE|FAULT_FLAG_UNSHARE)) { > if (!huge_pte_write(vmf.orig_pte)) { > + /* > + * Anonymous folios need to be lock since hugetlb_wp() > + * checks whether we can re-use the folio exclusively > + * for us in case we are the only user of it. > + */ Should we move that comment to hugetlb_wp() instead? And if we are already doing this PTL unlock dance now, why not do it in hugetlb_wp() instead so we can simplify this code? -- Cheers, David / dhildenb