From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: X-Spam-Checker-Version: SpamAssassin 3.4.0 (2014-02-07) on aws-us-west-2-korg-lkml-1.web.codeaurora.org X-Spam-Level: X-Spam-Status: No, score=-1.0 required=3.0 tests=HEADER_FROM_DIFFERENT_DOMAINS, MAILING_LIST_MULTI,SPF_PASS autolearn=ham autolearn_force=no version=3.4.0 Received: from mail.kernel.org (mail.kernel.org [198.145.29.99]) by smtp.lore.kernel.org (Postfix) with ESMTP id 5E057C43387 for ; Thu, 10 Jan 2019 07:03:13 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [209.132.180.67]) by mail.kernel.org (Postfix) with ESMTP id 2FDB320657 for ; Thu, 10 Jan 2019 07:03:13 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1727344AbfAJHDM (ORCPT ); Thu, 10 Jan 2019 02:03:12 -0500 Received: from mx2.suse.de ([195.135.220.15]:36844 "EHLO mx1.suse.de" rhost-flags-OK-OK-OK-FAIL) by vger.kernel.org with ESMTP id S1727220AbfAJHDL (ORCPT ); Thu, 10 Jan 2019 02:03:11 -0500 X-Virus-Scanned: by amavisd-new at test-mx.suse.de Received: from relay1.suse.de (unknown [195.135.220.254]) by mx1.suse.de (Postfix) with ESMTP id F063FACBC; Thu, 10 Jan 2019 07:03:09 +0000 (UTC) Subject: Re: [PATCH] Btrfs: avoid deadlock with memory reclaim due to allocation of devices To: dsterba@suse.cz, fdmanana@kernel.org, linux-btrfs@vger.kernel.org References: <20181213211725.14832-1-fdmanana@kernel.org> <20190109182653.GN23615@twin.jikos.cz> 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: <8b224e15-24f1-e269-cec3-52fd7ef39f18@suse.com> Date: Thu, 10 Jan 2019 09:03:08 +0200 User-Agent: Mozilla/5.0 (X11; Linux x86_64; rv:60.0) Gecko/20100101 Thunderbird/60.2.1 MIME-Version: 1.0 In-Reply-To: <20190109182653.GN23615@twin.jikos.cz> 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 9.01.19 г. 20:26 ч., David Sterba wrote: > On Thu, Dec 13, 2018 at 09:17:25PM +0000, fdmanana@kernel.org wrote: >> - if (list_empty(&fs_devices->resized_devices)) >> - return; >> - >> - mutex_lock(&fs_devices->device_list_mutex); >> mutex_lock(&fs_info->chunk_mutex); >> list_for_each_entry_safe(curr, next, &fs_devices->resized_devices, >> resized_list) { >> @@ -7309,7 +7306,6 @@ void btrfs_update_commit_device_size(struct btrfs_fs_info *fs_info) >> curr->commit_total_bytes = curr->disk_total_bytes; > > I'm not sure about removing the device_list_mutex that's said to protect > the commit_total_bytes (comment in struct btrfs_device). > > Otherwise the logic is ok, the double lock could happen as you describe. > > btrfs_update_commit_device_size is called from btrfs_commit_transaction, > at the same time as commit_bytes_used. The latter is handled in a > similar way in btrfs_update_commit_device_bytes_used, but does not take > the device_list_mutex. > > commit_total_bytes is checked several times (eg. in write_dev_supers) to > see if writing the superblock copy is still within the device range. > > So, without the protected change, it's theoretically possible that a > stale value is used for the test and the superblock is either written > though it should not, and the other way around. But can it really, btrfs_[grow|shrink]_device happen under transaction and their modification of the device_disk_total_bytes (the value assigned to commit_total_bytes) always happen under chunk_mutex. Also the updates to both values are really owned by the transaction, so even if grow/shrink modify those value they will queue those changes in a new transaction, hence write_dev_super will see consistent value in the current transaction. I have a patch from Jeff (which is part of a bigger work) that actually unifies the resize/device size change lists into a single single and makes the code a bit easier to grok, nevertheless the above explanation is still correct even without this patch. > > This would require a resize racing at the time of the check. Grow and > shrink seem to take chunk_mutex while adjusting all the total/size > values, but it's not actually easy to follow as sometimes there are > helpers like btrfs_device_set_total_bytes used and sometimes it's direct > access. > > That the device_list_mutex can be safely dropped probably follows from > the simple fact that btrfs_update_commit_device_bytes_used is called > before write_dev_supers in the same context. > > But this sounds too simple, given that there are locks taken and > released and btrfs_write_and_wait_transaction called between. > > Referencing this code: > > 2201 btrfs_update_commit_device_size(fs_info); > 2202 btrfs_update_commit_device_bytes_used(cur_trans); > 2203 > 2204 clear_bit(BTRFS_FS_LOG1_ERR, &fs_info->flags); > 2205 clear_bit(BTRFS_FS_LOG2_ERR, &fs_info->flags); > 2206 > 2207 btrfs_trans_release_chunk_metadata(trans); > 2208 > 2209 spin_lock(&fs_info->trans_lock); > 2210 cur_trans->state = TRANS_STATE_UNBLOCKED; > 2211 fs_info->running_transaction = NULL; > 2212 spin_unlock(&fs_info->trans_lock); > 2213 mutex_unlock(&fs_info->reloc_mutex); > 2214 > 2215 wake_up(&fs_info->transaction_wait); > 2216 > 2217 ret = btrfs_write_and_wait_transaction(trans); > 2218 if (ret) { > 2219 btrfs_handle_fs_error(fs_info, ret, > 2220 "Error while writing out transaction"); > 2221 mutex_unlock(&fs_info->tree_log_mutex); > 2222 goto scrub_continue; > 2223 } > 2224 > 2225 ret = write_all_supers(fs_info, 0); >