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[2003:d8:2f18:7500:202e:b0f1:76d6:f9af]) by smtp.gmail.com with ESMTPSA id ffacd0b85a97d-3a892e5f923sm12917949f8f.89.2025.07.01.06.08.13 (version=TLS1_3 cipher=TLS_AES_128_GCM_SHA256 bits=128/128); Tue, 01 Jul 2025 06:08:14 -0700 (PDT) Message-ID: Date: Tue, 1 Jul 2025 15:08:12 +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] mm: support large mapping building for tmpfs To: Baolin Wang , akpm@linux-foundation.org, hughd@google.com Cc: ziy@nvidia.com, lorenzo.stoakes@oracle.com, Liam.Howlett@oracle.com, npache@redhat.com, ryan.roberts@arm.com, dev.jain@arm.com, baohua@kernel.org, vbabka@suse.cz, rppt@kernel.org, surenb@google.com, mhocko@suse.com, linux-mm@kvack.org, linux-kernel@vger.kernel.org References: 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 01.07.25 10:40, Baolin Wang wrote: Nit: talking about "large mappings" is confusing. Did you actually mean: "mm: fault in complete folios instead of individual pages for tmpfs" I suggest not talking about "large mappings" anywhere in this patch description, and instead talking about mapping multiple consecutive pages of a tmpfs folios at once instead. > After commit acd7ccb284b8 ("mm: shmem: add large folio support for tmpfs"), > tmpfs can also support large folio allocation (not just PMD-sized large > folios). > > However, when accessing tmpfs via mmap(), although tmpfs supports large folios, > we still establish mappings at the base page granularity, which is unreasonable. > > We can establish large mappings according to the size of the large folio. On one > hand, this can reduce the overhead of page faults; on the other hand, it can > leverage hardware architecture optimizations to reduce TLB misses, such as > contiguous PTEs on the ARM architecture. The latter would still apply if faulting in each individual page I guess. cont-pte will try to auto-optimize IIRC. > > Moreover, since the user has already added the 'huge=' option when mounting tmpfs > to allow for large folio allocation, establishing large folios' mapping is expected > and will not surprise users by inflating the RSS of the process. Hm, are we sure about that? Also, how does fault_around_bytes interact here? > > In order to support large mappings for tmpfs, besides checking VMA limits and > PMD pagetable limits, it is also necessary to check if the linear page offset > of the VMA is order-aligned within the file. Why? This only applies to PMD mappings. See below. > > Performance test: > I created a 1G tmpfs file, populated with 64K large folios, and accessed it > sequentially via mmap(). I observed a significant performance improvement: > > Before the patch: > real 0m0.214s > user 0m0.012s > sys 0m0.203s > > After the patch: > real 0m0.025s > user 0m0.000s > sys 0m0.024s > > Signed-off-by: Baolin Wang > --- > mm/memory.c | 13 +++++++++---- > 1 file changed, 9 insertions(+), 4 deletions(-) > > diff --git a/mm/memory.c b/mm/memory.c > index 0f9b32a20e5b..6385a9385a9b 100644 > --- a/mm/memory.c > +++ b/mm/memory.c > @@ -5383,10 +5383,10 @@ vm_fault_t finish_fault(struct vm_fault *vmf) > > /* > * Using per-page fault to maintain the uffd semantics, and same > - * approach also applies to non-anonymous-shmem faults to avoid > + * approach also applies to non shmem/tmpfs faults to avoid > * inflating the RSS of the process. > */ > - if (!vma_is_anon_shmem(vma) || unlikely(userfaultfd_armed(vma)) || > + if (!vma_is_shmem(vma) || unlikely(userfaultfd_armed(vma)) || > unlikely(needs_fallback)) { > nr_pages = 1; > } else if (nr_pages > 1) { > @@ -5395,15 +5395,20 @@ vm_fault_t finish_fault(struct vm_fault *vmf) > pgoff_t vma_off = vmf->pgoff - vmf->vma->vm_pgoff; > /* The index of the entry in the pagetable for fault page. */ > pgoff_t pte_off = pte_index(vmf->address); > + unsigned long hpage_size = PAGE_SIZE << folio_order(folio); > > /* > * Fallback to per-page fault in case the folio size in page > - * cache beyond the VMA limits and PMD pagetable limits. > + * cache beyond the VMA limits or PMD pagetable limits. And > + * also check if the linear page offset of vma is order-aligned > + * within the file for tmpfs. > */ > if (unlikely(vma_off < idx || > vma_off + (nr_pages - idx) > vma_pages(vma) || > pte_off < idx || > - pte_off + (nr_pages - idx) > PTRS_PER_PTE)) { > + pte_off + (nr_pages - idx) > PTRS_PER_PTE) || > + !IS_ALIGNED((vma->vm_start >> PAGE_SHIFT) - vma->vm_pgoff, > + hpage_size >> PAGE_SHIFT)) { Again, why? Shouldn't set_pte_range() just do the right thing? set_ptes() doesn't have any such restriction. Also see the arm64 variant where we call contpte_set_ptes(mm, addr, ptep, pte, nr); There, I think we perform checks whether whether we can set the cont-pte bit IIUC. if (((addr | next | (pfn << PAGE_SHIFT)) & ~CONT_PTE_MASK) == 0) pte = pte_mkcont(pte); else pte = pte_mknoncont(pte); -- Cheers, David / dhildenb