From: Qu Wenruo <wqu@xxxxxxxx> commit 851fd730a743e072badaf67caf39883e32439431 upstream. [BUG] When a lot of subvolumes are created, there is a user report about transaction aborted: BTRFS: Transaction aborted (error -24) WARNING: CPU: 17 PID: 17041 at fs/btrfs/transaction.c:1576 create_pending_snapshot+0xbc4/0xd10 [btrfs] RIP: 0010:create_pending_snapshot+0xbc4/0xd10 [btrfs] Call Trace: create_pending_snapshots+0x82/0xa0 [btrfs] btrfs_commit_transaction+0x275/0x8c0 [btrfs] btrfs_mksubvol+0x4b9/0x500 [btrfs] btrfs_ioctl_snap_create_transid+0x174/0x180 [btrfs] btrfs_ioctl_snap_create_v2+0x11c/0x180 [btrfs] btrfs_ioctl+0x11a4/0x2da0 [btrfs] do_vfs_ioctl+0xa9/0x640 ksys_ioctl+0x67/0x90 __x64_sys_ioctl+0x1a/0x20 do_syscall_64+0x5a/0x110 entry_SYSCALL_64_after_hwframe+0x44/0xa9 ---[ end trace 33f2f83f3d5250e9 ]--- BTRFS: error (device sda1) in create_pending_snapshot:1576: errno=-24 unknown BTRFS info (device sda1): forced readonly BTRFS warning (device sda1): Skipping commit of aborted transaction. BTRFS: error (device sda1) in cleanup_transaction:1831: errno=-24 unknown [CAUSE] The error is EMFILE (Too many files open) and comes from the anonymous block device allocation. The ids are in a shared pool of size 1<<20. The ids are assigned to live subvolumes, ie. the root structure exists in memory (eg. after creation or after the root appears in some path). The pool could be exhausted if the numbers are not reclaimed fast enough, after subvolume deletion or if other system component uses the anon block devices. [WORKAROUND] Since it's not possible to completely solve the problem, we can only minimize the time the id is allocated to a subvolume root. Firstly, we can reduce the use of anon_dev by trees that are not subvolume roots, like data reloc tree. This patch will do extra check on root objectid, to skip roots that don't need anon_dev. Currently it's only data reloc tree and orphan roots. Reported-by: Greed Rong <greedrong@xxxxxxxxx> Link: https://lore.kernel.org/linux-btrfs/CA+UqX+NTrZ6boGnWHhSeZmEY5J76CTqmYjO2S+=tHJX7nb9DPw@xxxxxxxxxxxxxx/ CC: stable@xxxxxxxxxxxxxxx # 4.4+ Reviewed-by: Josef Bacik <josef@xxxxxxxxxxxxxx> Signed-off-by: Qu Wenruo <wqu@xxxxxxxx> Reviewed-by: David Sterba <dsterba@xxxxxxxx> Signed-off-by: David Sterba <dsterba@xxxxxxxx> Signed-off-by: Greg Kroah-Hartman <gregkh@xxxxxxxxxxxxxxxxxxx> --- fs/btrfs/disk-io.c | 13 ++++++++++--- 1 file changed, 10 insertions(+), 3 deletions(-) --- a/fs/btrfs/disk-io.c +++ b/fs/btrfs/disk-io.c @@ -1428,9 +1428,16 @@ static int btrfs_init_fs_root(struct btr spin_lock_init(&root->ino_cache_lock); init_waitqueue_head(&root->ino_cache_wait); - ret = get_anon_bdev(&root->anon_dev); - if (ret) - goto fail; + /* + * Don't assign anonymous block device to roots that are not exposed to + * userspace, the id pool is limited to 1M + */ + if (is_fstree(root->root_key.objectid) && + btrfs_root_refs(&root->root_item) > 0) { + ret = get_anon_bdev(&root->anon_dev); + if (ret) + goto fail; + } mutex_lock(&root->objectid_mutex); ret = btrfs_find_highest_objectid(root,
