From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: from mx2.suse.de ([195.135.220.15]:47834 "EHLO mx2.suse.de" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1750902AbeFTFZ6 (ORCPT ); Wed, 20 Jun 2018 01:25:58 -0400 Received: from relay1.suse.de (charybdis-ext-too.suse.de [195.135.220.254]) by mx2.suse.de (Postfix) with ESMTP id 847A4AB37 for ; Wed, 20 Jun 2018 05:25:57 +0000 (UTC) Subject: Re: [PATCH v3] btrfs: Don't remove block group still has pinned down bytes To: Qu Wenruo , linux-btrfs@vger.kernel.org References: <20180615013523.2807-1-wqu@suse.com> From: Nikolay Borisov Message-ID: <2ec3bf9b-cc0f-a027-b17c-04b3b237be88@suse.com> Date: Wed, 20 Jun 2018 08:25:56 +0300 MIME-Version: 1.0 In-Reply-To: <20180615013523.2807-1-wqu@suse.com> Content-Type: text/plain; charset=utf-8 Sender: linux-btrfs-owner@vger.kernel.org List-ID: On 15.06.2018 04:35, Qu Wenruo wrote: > [BUG] > Under certain KVM load and LTP tests, we are possible to hit the > following calltrace if quota is enabled: > ------ > BTRFS critical (device vda2): unable to find logical 8820195328 length 4096 > BTRFS critical (device vda2): unable to find logical 8820195328 length 4096 > ------------[ cut here ]------------ > WARNING: CPU: 0 PID: 49 at ../block/blk-core.c:172 blk_status_to_errno+0x1a/0x30 > CPU: 0 PID: 49 Comm: kworker/u2:1 Not tainted 4.12.14-15-default #1 SLE15 (unreleased) > Hardware name: QEMU Standard PC (i440FX + PIIX, 1996), BIOS 1.0.0-prebuilt.qemu-project.org 04/01/2014 > Workqueue: btrfs-endio-write btrfs_endio_write_helper [btrfs] > task: ffff9f827b340bc0 task.stack: ffffb4f8c0304000 > RIP: 0010:blk_status_to_errno+0x1a/0x30 > Call Trace: > submit_extent_page+0x191/0x270 [btrfs] > ? btrfs_create_repair_bio+0x130/0x130 [btrfs] > __do_readpage+0x2d2/0x810 [btrfs] > ? btrfs_create_repair_bio+0x130/0x130 [btrfs] > ? run_one_async_done+0xc0/0xc0 [btrfs] > __extent_read_full_page+0xe7/0x100 [btrfs] > ? run_one_async_done+0xc0/0xc0 [btrfs] > read_extent_buffer_pages+0x1ab/0x2d0 [btrfs] > ? run_one_async_done+0xc0/0xc0 [btrfs] > btree_read_extent_buffer_pages+0x94/0xf0 [btrfs] > read_tree_block+0x31/0x60 [btrfs] > read_block_for_search.isra.35+0xf0/0x2e0 [btrfs] > btrfs_search_slot+0x46b/0xa00 [btrfs] > ? kmem_cache_alloc+0x1a8/0x510 > ? btrfs_get_token_32+0x5b/0x120 [btrfs] > find_parent_nodes+0x11d/0xeb0 [btrfs] > ? leaf_space_used+0xb8/0xd0 [btrfs] > ? btrfs_leaf_free_space+0x49/0x90 [btrfs] > ? btrfs_find_all_roots_safe+0x93/0x100 [btrfs] > btrfs_find_all_roots_safe+0x93/0x100 [btrfs] > btrfs_find_all_roots+0x45/0x60 [btrfs] > btrfs_qgroup_trace_extent_post+0x20/0x40 [btrfs] > btrfs_add_delayed_data_ref+0x1a3/0x1d0 [btrfs] > btrfs_alloc_reserved_file_extent+0x38/0x40 [btrfs] > insert_reserved_file_extent.constprop.71+0x289/0x2e0 [btrfs] > btrfs_finish_ordered_io+0x2f4/0x7f0 [btrfs] > ? pick_next_task_fair+0x2cd/0x530 > ? __switch_to+0x92/0x4b0 > btrfs_worker_helper+0x81/0x300 [btrfs] > process_one_work+0x1da/0x3f0 > worker_thread+0x2b/0x3f0 > ? process_one_work+0x3f0/0x3f0 > kthread+0x11a/0x130 > ? kthread_create_on_node+0x40/0x40 > ret_from_fork+0x35/0x40 > Code: 00 00 5b c3 0f 1f 00 66 2e 0f 1f 84 00 00 00 00 00 0f 1f 44 00 00 40 80 ff 0c 40 0f b6 c7 77 0b 48 c1 e0 04 8b 80 00 bf c8 bd c3 <0f> 0b b8 fb ff ff ff c3 0f 1f 40 00 66 2e 0f 1f 84 00 00 00 00 > ---[ end trace f079fb809e7a862b ]--- > BTRFS critical (device vda2): unable to find logical 8820195328 length 16384 > BTRFS: error (device vda2) in btrfs_finish_ordered_io:3023: errno=-5 IO failure > BTRFS info (device vda2): forced readonly > BTRFS error (device vda2): pending csums is 2887680 > ------ > > [CAUSE] > It's caused by race with block group auto removal like the following > case: > - There is a meta block group X, which has only one tree block > The tree block belongs to fs tree 257. > - In current transaction, some operation modified fs tree 257 > The tree block get CoWed, so the block group X is empty, and marked as > unused, queued to be deleted. > - Some workload (like fsync) wakes up cleaner_kthread() > Which will call btrfs_deleted_unused_bgs() to remove unused block > groups. > So block group X along its chunk map get removed. > - Some delalloc work finished for fs tree 257 > Quota needs to get the original reference of the extent, which will > reads tree blocks of commit root of 257. > Then since the chunk map get removed, above warning get triggered. > > [FIX] > Just teach btrfs_delete_unused_bgs() to skip block group who still has > pinned bytes. > > However there is a minor side effect, since currently we only queue > empty blocks at update_block_group(), and such empty block group with > pinned bytes won't go through update_block_group() again, such block > group won't be removed, until it get new extent allocated and removed. > > But please note that, there are more problems related to extent > allocator with block group auto removal. > > Even a block group is marked unused, extent allocator can still allocate > new extents from unused block group. > Thus delaying block group to next transaction won't work. > (Extents get allocated in current transaction, and removed again in next > transaction). Isn't this easily solvable by making the allocator check whether the block group it has chosen is part of the unused_bgs_list ? > > So the root fix need to co-operate with extent allocator. > But current fix should be enough for this particular bug. > > Signed-off-by: Qu Wenruo LGTM, Reviewed-by: Nikolay Borisov > --- > changelog: > v2: > Commit message update, to better indicate how pinned byte is used in > btrfs and why it's related to quota. > v3: > Commit message update, further explaining the bug with an example. > And added the side effect of the fix, and possible further fix. > --- > fs/btrfs/extent-tree.c | 2 +- > 1 file changed, 1 insertion(+), 1 deletion(-) > > diff --git a/fs/btrfs/extent-tree.c b/fs/btrfs/extent-tree.c > index f190023386a9..7d14c4ca8232 100644 > --- a/fs/btrfs/extent-tree.c > +++ b/fs/btrfs/extent-tree.c > @@ -10675,7 +10675,7 @@ void btrfs_delete_unused_bgs(struct btrfs_fs_info *fs_info) > /* Don't want to race with allocators so take the groups_sem */ > down_write(&space_info->groups_sem); > spin_lock(&block_group->lock); > - if (block_group->reserved || > + if (block_group->reserved || block_group->pinned || > btrfs_block_group_used(&block_group->item) || > block_group->ro || > list_is_singular(&block_group->list)) { >