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Sat, 4 May 2019 08:29:35 +0000 (UTC) Subject: Re: [PATCH RFC] btrfs: reflink: Flush before reflink any extent to prevent NOCOW write falling back to CoW without data reservation To: Qu Wenruo Cc: fdmanana@gmail.com, Qu Wenruo , linux-btrfs References: <20190503010852.10342-1-wqu@suse.com> <1e36e9e2-dbd3-3ab0-b908-25cfdf1d310d@gmx.com> 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: <5e194c87-736a-ae0d-c7d5-d31420415a32@suse.com> Date: Sat, 4 May 2019 11:29:33 +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: Content-Type: text/plain; charset=utf-8 Content-Language: en-US Content-Transfer-Encoding: 8bit Sender: linux-btrfs-owner@vger.kernel.org Precedence: bulk List-ID: X-Mailing-List: linux-btrfs@vger.kernel.org On 3.05.19 г. 13:45 ч., Filipe Manana wrote: > On Fri, May 3, 2019 at 11:18 AM Qu Wenruo wrote: >> >> >> >> On 2019/5/3 下午5:21, Filipe Manana wrote: >>> On Fri, May 3, 2019 at 2:46 AM Qu Wenruo wrote: >>> >>> What a great subject. The "reflink:" part is unnecessary, since the >>> rest of the subject already mentions it, that makes it a bit shorter. >>> >>>> >>>> [BUG] >>>> The following command can lead to unexpected data COW: >>>> >>>> #!/bin/bash >>>> >>>> dev=/dev/test/test >>>> mnt=/mnt/btrfs >>>> >>>> mkfs.btrfs -f $dev -b 1G > /dev/null >>>> mount $dev $mnt -o nospace_cache >>>> >>>> xfs_io -f -c "falloc 8k 24k" -c "pwrite 12k 8k" $mnt/file1 >>>> xfs_io -c "reflink $mnt/file1 8k 0 4k" $mnt/file1 >>>> umount $dev >>>> >>>> The result extent will be >>>> >>>> item 7 key (257 EXTENT_DATA 4096) itemoff 15760 itemsize 53 >>>> generation 6 type 2 (prealloc) >>>> prealloc data disk byte 13631488 nr 28672 >>>> item 8 key (257 EXTENT_DATA 12288) itemoff 15707 itemsize 53 >>>> generation 6 type 1 (regular) >>>> extent data disk byte 13660160 nr 12288 <<< COW >>>> item 9 key (257 EXTENT_DATA 24576) itemoff 15654 itemsize 53 >>>> generation 6 type 2 (prealloc) >>>> prealloc data disk byte 13631488 nr 28672 >>>> >>>> Currently we always reserve space even for NOCOW buffered write, thus >>> >>> I would add 'data' between 'reserve' and 'space', to be clear. >>> >>>> under most case it shouldn't cause anything wrong even we fall back to >>>> COW. >>> >>> even we ... -> even if we fallback to COW when running delalloc / >>> starting writeback. >>> >>>> >>>> However when we're out of data space, we fall back to skip data space if >>>> we can do NOCOW write. >>> >>> we fall back to skip data space ... -> we fallback to NOCOW write >>> without reserving data space. >>> >>>> >>>> If such behavior happens under that case, we could hit the following >>>> problems: >>> >>>> - data space bytes_may_use underflow >>>> This will cause kernel warning. >>> >>> Ok. >>> >>>> >>>> - ENOSPC at delalloc time >>> >>> at delalloc time - that is an ambiguous term you use through the change log. >> >> In fact, I have a lot of uncertain terminology through kernel. >> >> Things like flush get referred multiple times in different context (e.g. >> filemap flush, flushoncommit, super block flush). >> >> If we have a terminology list, we can't be more happy to follow. > > So, some is kernel wide while others are btrfs specific. > > A buffered write creates dealloc - copies data to pages, marks the > pages as dirty and tags the range in the extent io tree as dellaloc. > Running delalloc, flushes writeback (starts IO for the dirty pages and > tags them as under writeback) and does other necessary things (like > reserving extents). > When writeback finishes, we add a task to a work queue to run > btrfs_finish_ordered_io - after that happens we say that the ordered > extent completed. > > It can get ambiguous very often. That's why I have created the following document which (tries) to explain this: https://github.com/btrfs/btrfs-dev-docs/blob/master/delalloc.txt It's not perfect but it's better than nothing, feel free to contribute improvements. < snip>