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Sat, 6 Apr 2019 06:09:14 +0000 (UTC) Subject: Re: Possible bio merging breakage in mp bio rework To: Ming Lei Cc: Jens Axboe , Omar Sandoval , linux-block@vger.kernel.org, LKML , linux-btrfs References: <59c19acf-999f-1911-b0b8-1a5cec8116c5@suse.com> <20190406001653.GA4805@ming.t460p> From: Nikolay Borisov Openpgp: preference=signencrypt Autocrypt: addr=nborisov@suse.com; prefer-encrypt=mutual; keydata= mQINBFiKBz4BEADNHZmqwhuN6EAzXj9SpPpH/nSSP8YgfwoOqwrP+JR4pIqRK0AWWeWCSwmZ T7g+RbfPFlmQp+EwFWOtABXlKC54zgSf+uulGwx5JAUFVUIRBmnHOYi/lUiE0yhpnb1KCA7f u/W+DkwGerXqhhe9TvQoGwgCKNfzFPZoM+gZrm+kWv03QLUCr210n4cwaCPJ0Nr9Z3c582xc bCUVbsjt7BN0CFa2BByulrx5xD9sDAYIqfLCcZetAqsTRGxM7LD0kh5WlKzOeAXj5r8DOrU2 GdZS33uKZI/kZJZVytSmZpswDsKhnGzRN1BANGP8sC+WD4eRXajOmNh2HL4P+meO1TlM3GLl EQd2shHFY0qjEo7wxKZI1RyZZ5AgJnSmehrPCyuIyVY210CbMaIKHUIsTqRgY5GaNME24w7h TyyVCy2qAM8fLJ4Vw5bycM/u5xfWm7gyTb9V1TkZ3o1MTrEsrcqFiRrBY94Rs0oQkZvunqia c+NprYSaOG1Cta14o94eMH271Kka/reEwSZkC7T+o9hZ4zi2CcLcY0DXj0qdId7vUKSJjEep c++s8ncFekh1MPhkOgNj8pk17OAESanmDwksmzh1j12lgA5lTFPrJeRNu6/isC2zyZhTwMWs k3LkcTa8ZXxh0RfWAqgx/ogKPk4ZxOXQEZetkEyTFghbRH2BIwARAQABtCNOaWtvbGF5IEJv cmlzb3YgPG5ib3Jpc292QHN1c2UuY29tPokCOAQTAQIAIgUCWIo48QIbAwYLCQgHAwIGFQgC CQoLBBYCAwECHgECF4AACgkQcb6CRuU/KFc0eg/9GLD3wTQz9iZHMFbjiqTCitD7B6dTLV1C ddZVlC8Hm/TophPts1bWZORAmYIihHHI1EIF19+bfIr46pvfTu0yFrJDLOADMDH+Ufzsfy2v HSqqWV/nOSWGXzh8bgg/ncLwrIdEwBQBN9SDS6aqsglagvwFD91UCg/TshLlRxD5BOnuzfzI Leyx2c6YmH7Oa1R4MX9Jo79SaKwdHt2yRN3SochVtxCyafDlZsE/efp21pMiaK1HoCOZTBp5 VzrIP85GATh18pN7YR9CuPxxN0V6IzT7IlhS4Jgj0NXh6vi1DlmKspr+FOevu4RVXqqcNTSS E2rycB2v6cttH21UUdu/0FtMBKh+rv8+yD49FxMYnTi1jwVzr208vDdRU2v7Ij/TxYt/v4O8 V+jNRKy5Fevca/1xroQBICXsNoFLr10X5IjmhAhqIH8Atpz/89ItS3+HWuE4BHB6RRLM0gy8 T7rN6ja+KegOGikp/VTwBlszhvfLhyoyjXI44Tf3oLSFM+8+qG3B7MNBHOt60CQlMkq0fGXd mm4xENl/SSeHsiomdveeq7cNGpHi6i6ntZK33XJLwvyf00PD7tip/GUj0Dic/ZUsoPSTF/mG EpuQiUZs8X2xjK/AS/l3wa4Kz2tlcOKSKpIpna7V1+CMNkNzaCOlbv7QwprAerKYywPCoOSC 7P25Ag0EWIoHPgEQAMiUqvRBZNvPvki34O/dcTodvLSyOmK/MMBDrzN8Cnk302XfnGlW/YAQ csMWISKKSpStc6tmD+2Y0z9WjyRqFr3EGfH1RXSv9Z1vmfPzU42jsdZn667UxrRcVQXUgoKg QYx055Q2FdUeaZSaivoIBD9WtJq/66UPXRRr4H/+Y5FaUZx+gWNGmBT6a0S/GQnHb9g3nonD jmDKGw+YO4P6aEMxyy3k9PstaoiyBXnzQASzdOi39BgWQuZfIQjN0aW+Dm8kOAfT5i/yk59h VV6v3NLHBjHVw9kHli3jwvsizIX9X2W8tb1SefaVxqvqO1132AO8V9CbE1DcVT8fzICvGi42 FoV/k0QOGwq+LmLf0t04Q0csEl+h69ZcqeBSQcIMm/Ir+NorfCr6HjrB6lW7giBkQl6hhomn l1mtDP6MTdbyYzEiBFcwQD4terc7S/8ELRRybWQHQp7sxQM/Lnuhs77MgY/e6c5AVWnMKd/z MKm4ru7A8+8gdHeydrRQSWDaVbfy3Hup0Ia76J9FaolnjB8YLUOJPdhI2vbvNCQ2ipxw3Y3c KhVIpGYqwdvFIiz0Fej7wnJICIrpJs/+XLQHyqcmERn3s/iWwBpeogrx2Lf8AGezqnv9woq7 OSoWlwXDJiUdaqPEB/HmGfqoRRN20jx+OOvuaBMPAPb+aKJyle8zABEBAAGJAh8EGAECAAkF AliKBz4CGwwACgkQcb6CRuU/KFdacg/+M3V3Ti9JYZEiIyVhqs+yHb6NMI1R0kkAmzsGQ1jU zSQUz9AVMR6T7v2fIETTT/f5Oout0+Hi9cY8uLpk8CWno9V9eR/B7Ifs2pAA8lh2nW43FFwp IDiSuDbH6oTLmiGCB206IvSuaQCp1fed8U6yuqGFcnf0ZpJm/sILG2ECdFK9RYnMIaeqlNQm iZicBY2lmlYFBEaMXHoy+K7nbOuizPWdUKoKHq+tmZ3iA+qL5s6Qlm4trH28/fPpFuOmgP8P K+7LpYLNSl1oQUr+WlqilPAuLcCo5Vdl7M7VFLMq4xxY/dY99aZx0ZJQYFx0w/6UkbDdFLzN upT7NIN68lZRucImffiWyN7CjH23X3Tni8bS9ubo7OON68NbPz1YIaYaHmnVQCjDyDXkQoKC R82Vf9mf5slj0Vlpf+/Wpsv/TH8X32ajva37oEQTkWNMsDxyw3aPSps6MaMafcN7k60y2Wk/ TCiLsRHFfMHFY6/lq/c0ZdOsGjgpIK0G0z6et9YU6MaPuKwNY4kBdjPNBwHreucrQVUdqRRm RcxmGC6ohvpqVGfhT48ZPZKZEWM+tZky0mO7bhZYxMXyVjBn4EoNTsXy1et9Y1dU3HVJ8fod 5UqrNrzIQFbdeM0/JqSLrtlTcXKJ7cYFa9ZM2AP7UIN9n1UWxq+OPY9YMOewVfYtL8M= Message-ID: <9ac6f2eb-069a-a02c-7863-e33cb00ad312@suse.com> Date: Sat, 6 Apr 2019 09:09:12 +0300 User-Agent: Mozilla/5.0 (X11; Linux x86_64; rv:60.0) Gecko/20100101 Thunderbird/60.6.1 MIME-Version: 1.0 In-Reply-To: <20190406001653.GA4805@ming.t460p> Content-Type: text/plain; charset=utf-8 Content-Language: en-US Content-Transfer-Encoding: 8bit Sender: linux-kernel-owner@vger.kernel.org Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On 6.04.19 г. 3:16 ч., Ming Lei wrote: > Hi Nikolay, > > On Fri, Apr 05, 2019 at 07:04:18PM +0300, Nikolay Borisov wrote: >> Hello Ming, >> >> Following the mp biovec rework what is the maximum >> data that a bio could contain? Should it be PAGE_SIZE * bio_vec > > There isn't any maximum data limit on the bio submitted from fs, > and block layer will make the final bio sent to driver correct > by applying all kinds of queue limit, such as max segment size, > max segment number, max sectors, ... > >> or something else? Currently I can see bios as large as 127 megs >> on sequential workloads, I got prompted to this since btrfs has a >> memory allocation that is dependent on the data in the bio and this >> particular memory allocation started failing with order 6 allocs. > > Could you share us the code? I don't see why order 6 allocs is a must. When a bio is submitted btrfs has to calculate the checksum for it, this happens in btrfs_csum_one_bio. Said checksums are stored in an kmalloc'ed array, whose size is calculated as: 32 + bio_size / btrfs' block size (usually 4k). So for a 127mb bio that would be: 32 * ((134184960÷4096) * 4) = 127k. We'd make an order 3 allocation. Admittedly the code in btrfs should know better rather than make unbounded allocations without a fallback, but bio suddenly becoming rather unbounded in their size caught us offhand. > >> Further debugging showed that with the following xfs_io command line: >> >> >> xfs_io -f -c "pwrite -S 0x61 -b 4m 0 10g" /media/scratch/file1 >> >> I can easily see very large bios: >> >> [ 188.366540] kworker/-7 3.... 34847519us : btrfs_submit_bio_hook: bio: ffff8dffe9940bb0 bi_iter.bi_size = 134184960 bi_vcn: 28 bi_vcnt_max: 256 >> [ 188.367129] kworker/-658 2.... 34946536us : btrfs_submit_bio_hook: bio: ffff8dffe9940370 bi_iter.bi_size = 134246400 bi_vcn: 28 bi_vcnt_max: 256 >> [ 188.367714] kworker/-7 3.... 35107967us : btrfs_submit_bio_hook: bio: ffff8dffe9940bb0 bi_iter.bi_size = 134184960 bi_vcn: 30 bi_vcnt_max: 256 >> [ 188.368319] kworker/-658 2.... 35229894us : btrfs_submit_bio_hook: bio: ffff8dffe9940370 bi_iter.bi_size = 134246400 bi_vcn: 32 bi_vcnt_max: 256 >> [ 188.368909] kworker/-7 3.... 35374809us : btrfs_submit_bio_hook: bio: ffff8dffe9940bb0 bi_iter.bi_size = 134184960 bi_vcn: 25 bi_vcnt_max: 256 >> [ 188.369498] kworker/-658 2.... 35516194us : btrfs_submit_bio_hook: bio: ffff8dffe9940370 bi_iter.bi_size = 134246400 bi_vcn: 31 bi_vcnt_max: 256 >> [ 188.370086] kworker/-7 3.... 35663669us : btrfs_submit_bio_hook: bio: ffff8dffe9940bb0 bi_iter.bi_size = 134184960 bi_vcn: 32 bi_vcnt_max: 256 >> [ 188.370696] kworker/-658 2.... 35791006us : btrfs_submit_bio_hook: bio: ffff8dffe9940370 bi_iter.bi_size = 100655104 bi_vcn: 24 bi_vcnt_max: 256 >> [ 188.371335] kworker/-658 2.... 35816114us : btrfs_submit_bio_hook: bio: ffff8dffe99434f0 bi_iter.bi_size = 33591296 bi_vcn: 5 bi_vcnt_max: 256 >> >> >> So that's 127 megs in a single bio? This stems from the new merging logic. >> 07173c3ec276 ("block: enable multipage bvecs") made it so that physically >> contiguous pages added to the bio would just modify bi_iter.bi_size and the >> initial page's bio_vec's bv_len. There's no longer the >> page == bv->bv_page portion of the check. > > bio_add_page() tries best to put physically contiguous pages into one bvec, and > I don't see anything is wrong in the log. > > Could you show us what the real problem is? > > Thanks, > Ming >