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[2003:cb:c730:4300:18eb:6c63:a196:d3a2]) by smtp.gmail.com with ESMTPSA id 5b1f17b1804b1-432b0566544sm166915385e9.24.2024.11.11.02.33.39 (version=TLS1_3 cipher=TLS_AES_128_GCM_SHA256 bits=128/128); Mon, 11 Nov 2024 02:33:40 -0800 (PST) Message-ID: <85cfc467-320f-4388-b027-2cbad85dfbed@redhat.com> Date: Mon, 11 Nov 2024 11:33:38 +0100 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 v2] mm/readahead: Fix large folio support in async readahead To: Yafang Shao , willy@infradead.org, akpm@linux-foundation.org Cc: linux-mm@kvack.org, stable@vger.kernel.org References: <20241108141710.9721-1-laoar.shao@gmail.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: <20241108141710.9721-1-laoar.shao@gmail.com> Content-Type: text/plain; charset=UTF-8; format=flowed Content-Transfer-Encoding: 8bit On 08.11.24 15:17, Yafang Shao wrote: > When testing large folio support with XFS on our servers, we observed that > only a few large folios are mapped when reading large files via mmap. > After a thorough analysis, I identified it was caused by the > `/sys/block/*/queue/read_ahead_kb` setting. On our test servers, this > parameter is set to 128KB. After I tune it to 2MB, the large folio can > work as expected. However, I believe the large folio behavior should not be > dependent on the value of read_ahead_kb. It would be more robust if the > kernel can automatically adopt to it. Now I am extremely confused. Documentation/ABI/stable/sysfs-block: "[RW] Maximum number of kilobytes to read-ahead for filesystems on this block device." So, with your patch, will we also be changing the readahead size to exceed that, or simply allocate larger folios and not exceeding the readahead size (e.g., leaving them partially non-filled)? If you're also changing the readahead behavior to exceed the configuration parameter it would sound to me like "I am pushing the brake pedal and my care brakes; fix the brakes to adopt whether to brake automatically" :) Likely I am missing something here, and how the read_ahead_kb parameter is used after your patch. > > With /sys/block/*/queue/read_ahead_kb set to 128KB and performing a > sequential read on a 1GB file using MADV_HUGEPAGE, the differences in > /proc/meminfo are as follows: > > - before this patch > FileHugePages: 18432 kB > FilePmdMapped: 4096 kB > > - after this patch > FileHugePages: 1067008 kB > FilePmdMapped: 1048576 kB > > This shows that after applying the patch, the entire 1GB file is mapped to > huge pages. The stable list is CCed, as without this patch, large folios > don’t function optimally in the readahead path. >> It's worth noting that if read_ahead_kb is set to a larger value that isn't > aligned with huge page sizes (e.g., 4MB + 128KB), it may still fail to map > to hugepages. > > Fixes: 4687fdbb805a ("mm/filemap: Support VM_HUGEPAGE for file mappings") > Suggested-by: Matthew Wilcox > Signed-off-by: Yafang Shao > Cc: stable@vger.kernel.org > > --- > mm/readahead.c | 2 ++ > 1 file changed, 2 insertions(+) > > Changes: > v1->v2: > - Drop the align (Matthew) > - Improve commit log (Andrew) > > RFC->v1: https://lore.kernel.org/linux-mm/20241106092114.8408-1-laoar.shao@gmail.com/ > - Simplify the code as suggested by Matthew > > RFC: https://lore.kernel.org/linux-mm/20241104143015.34684-1-laoar.shao@gmail.com/ > > diff --git a/mm/readahead.c b/mm/readahead.c > index 3dc6c7a128dd..9b8a48e736c6 100644 > --- a/mm/readahead.c > +++ b/mm/readahead.c > @@ -385,6 +385,8 @@ static unsigned long get_next_ra_size(struct file_ra_state *ra, > return 4 * cur; > if (cur <= max / 2) > return 2 * cur; > + if (cur > max) > + return cur; > return max; Maybe something like return max_t(unsigned long, cur, max); might be more readable (likely "max()" cannot be used because of the local variable name "max" ...). ... but it's rather weird having a "max" and then returning something larger than the "max" ... especially with code like "ra->size = get_next_ra_size(ra, max_pages);" Maybe we can improve that by renaming "max_pages" / "max" to what it actually is supposed to be (which I haven't quite understood yet). -- Cheers, David / dhildenb