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[2003:d8:2f3c:3a00:5662:26b3:3e5d:438e]) by smtp.gmail.com with ESMTPSA id 5b1f17b1804b1-454dd537785sm52075745e9.23.2025.07.11.09.08.54 (version=TLS1_3 cipher=TLS_AES_128_GCM_SHA256 bits=128/128); Fri, 11 Jul 2025 09:08:54 -0700 (PDT) Message-ID: Date: Fri, 11 Jul 2025 18:08:53 +0200 MIME-Version: 1.0 User-Agent: Mozilla Thunderbird Subject: Re: [PATCH] mm/filemap: Align last_index to folio size To: Youling Tang , Matthew Wilcox , Andrew Morton Cc: Jan Kara , linux-fsdevel@vger.kernel.org, linux-mm@kvack.org, linux-kernel@vger.kernel.org, chizhiling@163.com, Youling Tang , Chi Zhiling , Ryan Roberts References: <20250711055509.91587-1-youling.tang@linux.dev> From: David Hildenbrand 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: <20250711055509.91587-1-youling.tang@linux.dev> X-Mimecast-Spam-Score: 0 X-Mimecast-MFC-PROC-ID: aAU9APRM8Y_byyyFTqrWA1m1lDMRdgp04AQ7fxFxDKs_1752250137 X-Mimecast-Originator: redhat.com Content-Language: en-US Content-Type: text/plain; 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On 11.07.25 07:55, Youling Tang wrote: > From: Youling Tang > > On XFS systems with pagesize=4K, blocksize=16K, and CONFIG_TRANSPARENT_HUGEPAGE > enabled, We observed the following readahead behaviors: > # echo 3 > /proc/sys/vm/drop_caches > # dd if=test of=/dev/null bs=64k count=1 > # ./tools/mm/page-types -r -L -f /mnt/xfs/test > foffset offset flags > 0 136d4c __RU_l_________H______t_________________F_1 > 1 136d4d __RU_l__________T_____t_________________F_1 > 2 136d4e __RU_l__________T_____t_________________F_1 > 3 136d4f __RU_l__________T_____t_________________F_1 > ... > c 136bb8 __RU_l_________H______t_________________F_1 > d 136bb9 __RU_l__________T_____t_________________F_1 > e 136bba __RU_l__________T_____t_________________F_1 > f 136bbb __RU_l__________T_____t_________________F_1 <-- first read > 10 13c2cc ___U_l_________H______t______________I__F_1 <-- readahead flag > 11 13c2cd ___U_l__________T_____t______________I__F_1 > 12 13c2ce ___U_l__________T_____t______________I__F_1 > 13 13c2cf ___U_l__________T_____t______________I__F_1 > ... > 1c 1405d4 ___U_l_________H______t_________________F_1 > 1d 1405d5 ___U_l__________T_____t_________________F_1 > 1e 1405d6 ___U_l__________T_____t_________________F_1 > 1f 1405d7 ___U_l__________T_____t_________________F_1 > [ra_size = 32, req_count = 16, async_size = 16] > > # echo 3 > /proc/sys/vm/drop_caches > # dd if=test of=/dev/null bs=60k count=1 > # ./page-types -r -L -f /mnt/xfs/test > foffset offset flags > 0 136048 __RU_l_________H______t_________________F_1 > ... > c 110a40 __RU_l_________H______t_________________F_1 > d 110a41 __RU_l__________T_____t_________________F_1 > e 110a42 __RU_l__________T_____t_________________F_1 <-- first read > f 110a43 __RU_l__________T_____t_________________F_1 <-- first readahead flag > 10 13e7a8 ___U_l_________H______t_________________F_1 > ... > 20 137a00 ___U_l_________H______t_______P______I__F_1 <-- second readahead flag (20 - 2f) > 21 137a01 ___U_l__________T_____t_______P______I__F_1 > ... > 3f 10d4af ___U_l__________T_____t_______P_________F_1 > [first readahead: ra_size = 32, req_count = 15, async_size = 17] > > When reading 64k data (same for 61-63k range, where last_index is page-aligned > in filemap_get_pages()), 128k readahead is triggered via page_cache_sync_ra() > and the PG_readahead flag is set on the next folio (the one containing 0x10 page). > > When reading 60k data, 128k readahead is also triggered via page_cache_sync_ra(). > However, in this case the readahead flag is set on the 0xf page. Although the > requested read size (req_count) is 60k, the actual read will be aligned to > folio size (64k), which triggers the readahead flag and initiates asynchronous > readahead via page_cache_async_ra(). This results in two readahead operations > totaling 256k. > > The root cause is that when the requested size is smaller than the actual read > size (due to folio alignment), it triggers asynchronous readahead. By changing > last_index alignment from page size to folio size, we ensure the requested size > matches the actual read size, preventing the case where a single read operation > triggers two readahead operations. > > After applying the patch: > # echo 3 > /proc/sys/vm/drop_caches > # dd if=test of=/dev/null bs=60k count=1 > # ./page-types -r -L -f /mnt/xfs/test > foffset offset flags > 0 136d4c __RU_l_________H______t_________________F_1 > 1 136d4d __RU_l__________T_____t_________________F_1 > 2 136d4e __RU_l__________T_____t_________________F_1 > 3 136d4f __RU_l__________T_____t_________________F_1 > ... > c 136bb8 __RU_l_________H______t_________________F_1 > d 136bb9 __RU_l__________T_____t_________________F_1 > e 136bba __RU_l__________T_____t_________________F_1 <-- first read > f 136bbb __RU_l__________T_____t_________________F_1 > 10 13c2cc ___U_l_________H______t______________I__F_1 <-- readahead flag > 11 13c2cd ___U_l__________T_____t______________I__F_1 > 12 13c2ce ___U_l__________T_____t______________I__F_1 > 13 13c2cf ___U_l__________T_____t______________I__F_1 > ... > 1c 1405d4 ___U_l_________H______t_________________F_1 > 1d 1405d5 ___U_l__________T_____t_________________F_1 > 1e 1405d6 ___U_l__________T_____t_________________F_1 > 1f 1405d7 ___U_l__________T_____t_________________F_1 > [ra_size = 32, req_count = 16, async_size = 16] > > The same phenomenon will occur when reading from 49k to 64k. Set the readahead > flag to the next folio. > > Because the minimum order of folio in address_space equals the block size (at > least in xfs and bcachefs that already support bs > ps), having request_count > aligned to block size will not cause overread. > > Co-developed-by: Chi Zhiling > Signed-off-by: Chi Zhiling > Signed-off-by: Youling Tang > --- > include/linux/pagemap.h | 6 ++++++ > mm/filemap.c | 5 +++-- > 2 files changed, 9 insertions(+), 2 deletions(-) > > diff --git a/include/linux/pagemap.h b/include/linux/pagemap.h > index e63fbfbd5b0f..447bb264fd94 100644 > --- a/include/linux/pagemap.h > +++ b/include/linux/pagemap.h > @@ -480,6 +480,12 @@ mapping_min_folio_nrpages(struct address_space *mapping) > return 1UL << mapping_min_folio_order(mapping); > } > > +static inline unsigned long > +mapping_min_folio_nrbytes(struct address_space *mapping) > +{ > + return mapping_min_folio_nrpages(mapping) << PAGE_SHIFT; > +} > + > /** > * mapping_align_index() - Align index for this mapping. > * @mapping: The address_space. > diff --git a/mm/filemap.c b/mm/filemap.c > index 765dc5ef6d5a..56a8656b6f86 100644 > --- a/mm/filemap.c > +++ b/mm/filemap.c > @@ -2584,8 +2584,9 @@ static int filemap_get_pages(struct kiocb *iocb, size_t count, > unsigned int flags; > int err = 0; > > - /* "last_index" is the index of the page beyond the end of the read */ > - last_index = DIV_ROUND_UP(iocb->ki_pos + count, PAGE_SIZE); > + /* "last_index" is the index of the folio beyond the end of the read */ > + last_index = round_up(iocb->ki_pos + count, mapping_min_folio_nrbytes(mapping)); > + last_index >>= PAGE_SHIFT; > retry: > if (fatal_signal_pending(current)) > return -EINTR; -- Cheers, David / dhildenb