From: Filipe Manana fdmanana@suse.com
commit 2166e5edce9ac1edf3b113d6091ef72fcac2d6c4 upstream.
We always preallocate a data extent for writing a free space cache, which causes writeback to always try the nocow path first, since the free space inode has the prealloc bit set in its flags.
However if the block group that contains the data extent for the space cache has been turned to RO mode due to a running scrub or balance for example, we have to fallback to the cow path. In that case once a new data extent is allocated we end up calling btrfs_add_reserved_bytes(), which decrements the counter named bytes_may_use from the data space_info object with the expection that this counter was previously incremented with the same amount (the size of the data extent).
However when we started writeout of the space cache at cache_save_setup(), we incremented the value of the bytes_may_use counter through a call to btrfs_check_data_free_space() and then decremented it through a call to btrfs_prealloc_file_range_trans() immediately after. So when starting the writeback if we fallback to cow mode we have to increment the counter bytes_may_use of the data space_info again to compensate for the extent allocation done by the cow path.
When this issue happens we are incorrectly decrementing the bytes_may_use counter and when its current value is smaller then the amount we try to subtract we end up with the following warning:
------------[ cut here ]------------ WARNING: CPU: 3 PID: 657 at fs/btrfs/space-info.h:115 btrfs_add_reserved_bytes+0x3d6/0x4e0 [btrfs] Modules linked in: btrfs blake2b_generic xor raid6_pq libcrc32c (...) CPU: 3 PID: 657 Comm: kworker/u8:7 Tainted: G W 5.6.0-rc7-btrfs-next-58 #5 Hardware name: QEMU Standard PC (i440FX + PIIX, 1996), BIOS rel-1.12.0-59-gc9ba5276e321-prebuilt.qemu.org 04/01/2014 Workqueue: writeback wb_workfn (flush-btrfs-1591) RIP: 0010:btrfs_add_reserved_bytes+0x3d6/0x4e0 [btrfs] Code: ff ff 48 (...) RSP: 0000:ffffa41608f13660 EFLAGS: 00010287 RAX: 0000000000001000 RBX: ffff9615b93ae400 RCX: 0000000000000000 RDX: 0000000000000002 RSI: 0000000000000000 RDI: ffff9615b96ab410 RBP: fffffffffffee000 R08: 0000000000000001 R09: 0000000000000000 R10: ffff961585e62a40 R11: 0000000000000000 R12: ffff9615b96ab400 R13: ffff9615a1a2a000 R14: 0000000000012000 R15: ffff9615b93ae400 FS: 0000000000000000(0000) GS:ffff9615bb200000(0000) knlGS:0000000000000000 CS: 0010 DS: 0000 ES: 0000 CR0: 0000000080050033 CR2: 000055cbbc2ae178 CR3: 0000000115794006 CR4: 00000000003606e0 DR0: 0000000000000000 DR1: 0000000000000000 DR2: 0000000000000000 DR3: 0000000000000000 DR6: 00000000fffe0ff0 DR7: 0000000000000400 Call Trace: find_free_extent+0x4a0/0x16c0 [btrfs] btrfs_reserve_extent+0x91/0x180 [btrfs] cow_file_range+0x12d/0x490 [btrfs] btrfs_run_delalloc_range+0x9f/0x6d0 [btrfs] ? find_lock_delalloc_range+0x221/0x250 [btrfs] writepage_delalloc+0xe8/0x150 [btrfs] __extent_writepage+0xe8/0x4c0 [btrfs] extent_write_cache_pages+0x237/0x530 [btrfs] extent_writepages+0x44/0xa0 [btrfs] do_writepages+0x23/0x80 __writeback_single_inode+0x59/0x700 writeback_sb_inodes+0x267/0x5f0 __writeback_inodes_wb+0x87/0xe0 wb_writeback+0x382/0x590 ? wb_workfn+0x4a2/0x6c0 wb_workfn+0x4a2/0x6c0 process_one_work+0x26d/0x6a0 worker_thread+0x4f/0x3e0 ? process_one_work+0x6a0/0x6a0 kthread+0x103/0x140 ? kthread_create_worker_on_cpu+0x70/0x70 ret_from_fork+0x3a/0x50 irq event stamp: 0 hardirqs last enabled at (0): [<0000000000000000>] 0x0 hardirqs last disabled at (0): [<ffffffffb2abdedf>] copy_process+0x74f/0x2020 softirqs last enabled at (0): [<ffffffffb2abdedf>] copy_process+0x74f/0x2020 softirqs last disabled at (0): [<0000000000000000>] 0x0 ---[ end trace bd7c03622e0b0a52 ]--- ------------[ cut here ]------------
So fix this by incrementing the bytes_may_use counter of the data space_info when we fallback to the cow path. If the cow path is successful the counter is decremented after extent allocation (by btrfs_add_reserved_bytes()), if it fails it ends up being decremented as well when clearing the delalloc range (extent_clear_unlock_delalloc()).
This could be triggered sporadically by the test case btrfs/061 from fstests.
Fixes: 82d5902d9c681b ("Btrfs: Support reading/writing on disk free ino cache") CC: stable@vger.kernel.org # 4.4+ Signed-off-by: Filipe Manana fdmanana@suse.com Signed-off-by: David Sterba dsterba@suse.com Signed-off-by: Greg Kroah-Hartman gregkh@linuxfoundation.org
--- fs/btrfs/inode.c | 20 +++++++++++++++----- 1 file changed, 15 insertions(+), 5 deletions(-)
--- a/fs/btrfs/inode.c +++ b/fs/btrfs/inode.c @@ -1327,6 +1327,8 @@ static int fallback_to_cow(struct inode const u64 start, const u64 end, int *page_started, unsigned long *nr_written) { + const bool is_space_ino = btrfs_is_free_space_inode(BTRFS_I(inode)); + const u64 range_bytes = end + 1 - start; struct extent_io_tree *io_tree = &BTRFS_I(inode)->io_tree; u64 range_start = start; u64 count; @@ -1354,19 +1356,27 @@ static int fallback_to_cow(struct inode * that if the COW path fails for any reason, it decrements (through * extent_clear_unlock_delalloc()) the bytes_may_use counter of the * data space info, which we incremented in the step above. + * + * If we need to fallback to cow and the inode corresponds to a free + * space cache inode, we must also increment bytes_may_use of the data + * space_info for the same reason. Space caches always get a prealloc + * extent for them, however scrub or balance may have set the block + * group that contains that extent to RO mode. */ - count = count_range_bits(io_tree, &range_start, end, end + 1 - start, + count = count_range_bits(io_tree, &range_start, end, range_bytes, EXTENT_NORESERVE, 0); - if (count > 0) { + if (count > 0 || is_space_ino) { + const u64 bytes = is_space_ino ? range_bytes : count; struct btrfs_fs_info *fs_info = BTRFS_I(inode)->root->fs_info; struct btrfs_space_info *sinfo = fs_info->data_sinfo;
spin_lock(&sinfo->lock); - btrfs_space_info_update_bytes_may_use(fs_info, sinfo, count); + btrfs_space_info_update_bytes_may_use(fs_info, sinfo, bytes); spin_unlock(&sinfo->lock);
- clear_extent_bit(io_tree, start, end, EXTENT_NORESERVE, 0, 0, - NULL); + if (count > 0) + clear_extent_bit(io_tree, start, end, EXTENT_NORESERVE, + 0, 0, NULL); }
return cow_file_range(inode, locked_page, start, end, page_started,