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 Received: from vger.kernel.org (vger.kernel.org [23.128.96.18]) by smtp.lore.kernel.org (Postfix) with ESMTP id 2D98AC6FA81 for ; Fri, 2 Sep 2022 06:15:03 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S235435AbiIBGPA (ORCPT ); Fri, 2 Sep 2022 02:15:00 -0400 Received: from lindbergh.monkeyblade.net ([23.128.96.19]:55932 "EHLO lindbergh.monkeyblade.net" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S231732AbiIBGO5 (ORCPT ); Fri, 2 Sep 2022 02:14:57 -0400 Received: from ams.source.kernel.org (ams.source.kernel.org [IPv6:2604:1380:4601:e00::1]) by lindbergh.monkeyblade.net (Postfix) with ESMTPS id 822D113F31; Thu, 1 Sep 2022 23:14:54 -0700 (PDT) Received: from smtp.kernel.org (relay.kernel.org [52.25.139.140]) (using TLSv1.2 with cipher ECDHE-RSA-AES256-GCM-SHA384 (256/256 bits)) (No client certificate requested) by ams.source.kernel.org (Postfix) with ESMTPS id 89CD7B829E3; Fri, 2 Sep 2022 06:14:53 +0000 (UTC) Received: by smtp.kernel.org (Postfix) with ESMTPSA id E0BB9C433C1; Fri, 2 Sep 2022 06:14:51 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/simple; d=linuxfoundation.org; s=korg; t=1662099292; bh=LOvLX/ZFQZ0Ns/M+t5tNoiDylATQ2kmr4xjAuQQnlvM=; h=Date:From:To:Cc:Subject:References:In-Reply-To:From; b=NSPAvVpxAWAIJzkgYUGM+BldPQKFWvyfFlg15uj0TRv1tclTicaoT3Yq6CEUVDYyq t6lrqH9Q8cufzTcAurnoVWld5sHApZTV96jQ+3egeiXyXtAs9P1EpiyiuPIQXYP+sK 96q9sjspvGHLC1kmEvli+9z77j0qc2OXo7dMcbbw= Date: Fri, 2 Sep 2022 08:14:39 +0200 From: Greg KH To: Omar Sandoval Cc: linux-btrfs@vger.kernel.org, stable@vger.kernel.org, kernel-team@fb.com Subject: Re: [PATCH 5.15] btrfs: fix space cache corruption and potential double allocations Message-ID: References: MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: Precedence: bulk List-ID: X-Mailing-List: linux-btrfs@vger.kernel.org On Thu, Sep 01, 2022 at 11:03:35AM -0700, Omar Sandoval wrote: > From: Omar Sandoval > > commit ced8ecf026fd8084cf175530ff85c76d6085d715 upstream. > > When testing space_cache v2 on a large set of machines, we encountered a > few symptoms: > > 1. "unable to add free space :-17" (EEXIST) errors. > 2. Missing free space info items, sometimes caught with a "missing free > space info for X" error. > 3. Double-accounted space: ranges that were allocated in the extent tree > and also marked as free in the free space tree, ranges that were > marked as allocated twice in the extent tree, or ranges that were > marked as free twice in the free space tree. If the latter made it > onto disk, the next reboot would hit the BUG_ON() in > add_new_free_space(). > 4. On some hosts with no on-disk corruption or error messages, the > in-memory space cache (dumped with drgn) disagreed with the free > space tree. > > All of these symptoms have the same underlying cause: a race between > caching the free space for a block group and returning free space to the > in-memory space cache for pinned extents causes us to double-add a free > range to the space cache. This race exists when free space is cached > from the free space tree (space_cache=v2) or the extent tree > (nospace_cache, or space_cache=v1 if the cache needs to be regenerated). > struct btrfs_block_group::last_byte_to_unpin and struct > btrfs_block_group::progress are supposed to protect against this race, > but commit d0c2f4fa555e ("btrfs: make concurrent fsyncs wait less when > waiting for a transaction commit") subtly broke this by allowing > multiple transactions to be unpinning extents at the same time. > > Specifically, the race is as follows: > > 1. An extent is deleted from an uncached block group in transaction A. > 2. btrfs_commit_transaction() is called for transaction A. > 3. btrfs_run_delayed_refs() -> __btrfs_free_extent() runs the delayed > ref for the deleted extent. > 4. __btrfs_free_extent() -> do_free_extent_accounting() -> > add_to_free_space_tree() adds the deleted extent back to the free > space tree. > 5. do_free_extent_accounting() -> btrfs_update_block_group() -> > btrfs_cache_block_group() queues up the block group to get cached. > block_group->progress is set to block_group->start. > 6. btrfs_commit_transaction() for transaction A calls > switch_commit_roots(). It sets block_group->last_byte_to_unpin to > block_group->progress, which is block_group->start because the block > group hasn't been cached yet. > 7. The caching thread gets to our block group. Since the commit roots > were already switched, load_free_space_tree() sees the deleted extent > as free and adds it to the space cache. It finishes caching and sets > block_group->progress to U64_MAX. > 8. btrfs_commit_transaction() advances transaction A to > TRANS_STATE_SUPER_COMMITTED. > 9. fsync calls btrfs_commit_transaction() for transaction B. Since > transaction A is already in TRANS_STATE_SUPER_COMMITTED and the > commit is for fsync, it advances. > 10. btrfs_commit_transaction() for transaction B calls > switch_commit_roots(). This time, the block group has already been > cached, so it sets block_group->last_byte_to_unpin to U64_MAX. > 11. btrfs_commit_transaction() for transaction A calls > btrfs_finish_extent_commit(), which calls unpin_extent_range() for > the deleted extent. It sees last_byte_to_unpin set to U64_MAX (by > transaction B!), so it adds the deleted extent to the space cache > again! > > This explains all of our symptoms above: > > * If the sequence of events is exactly as described above, when the free > space is re-added in step 11, it will fail with EEXIST. > * If another thread reallocates the deleted extent in between steps 7 > and 11, then step 11 will silently re-add that space to the space > cache as free even though it is actually allocated. Then, if that > space is allocated *again*, the free space tree will be corrupted > (namely, the wrong item will be deleted). > * If we don't catch this free space tree corruption, it will continue > to get worse as extents are deleted and reallocated. > > The v1 space_cache is synchronously loaded when an extent is deleted > (btrfs_update_block_group() with alloc=0 calls btrfs_cache_block_group() > with load_cache_only=1), so it is not normally affected by this bug. > However, as noted above, if we fail to load the space cache, we will > fall back to caching from the extent tree and may hit this bug. > > The easiest fix for this race is to also make caching from the free > space tree or extent tree synchronous. Josef tested this and found no > performance regressions. > > A few extra changes fall out of this change. Namely, this fix does the > following, with step 2 being the crucial fix: > > 1. Factor btrfs_caching_ctl_wait_done() out of > btrfs_wait_block_group_cache_done() to allow waiting on a caching_ctl > that we already hold a reference to. > 2. Change the call in btrfs_cache_block_group() of > btrfs_wait_space_cache_v1_finished() to > btrfs_caching_ctl_wait_done(), which makes us wait regardless of the > space_cache option. > 3. Delete the now unused btrfs_wait_space_cache_v1_finished() and > space_cache_v1_done(). > 4. Change btrfs_cache_block_group()'s `int load_cache_only` parameter to > `bool wait` to more accurately describe its new meaning. > 5. Change a few callers which had a separate call to > btrfs_wait_block_group_cache_done() to use wait = true instead. > 6. Make btrfs_wait_block_group_cache_done() static now that it's not > used outside of block-group.c anymore. > > Fixes: d0c2f4fa555e ("btrfs: make concurrent fsyncs wait less when waiting for a transaction commit") > CC: stable@vger.kernel.org # 5.12+ > Reviewed-by: Filipe Manana > Signed-off-by: Omar Sandoval > Signed-off-by: David Sterba > --- > Hi, > > This is the backport of commit ced8ecf026fd8084cf175530ff85c76d6085d715 > to the 5.15 stable branch. Please consider it for the next 5.15 stable > release. Now queued up, thanks. greg k-h