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    Subject[PATCH 4.9 171/212] btrfs: dont allocate anonymous block device for user invisible roots
    Date
    From: Qu Wenruo <wqu@suse.com>

    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@gmail.com>
    Link: https://lore.kernel.org/linux-btrfs/CA+UqX+NTrZ6boGnWHhSeZmEY5J76CTqmYjO2S+=tHJX7nb9DPw@mail.gmail.com/
    CC: stable@vger.kernel.org # 4.4+
    Reviewed-by: Josef Bacik <josef@toxicpanda.com>
    Signed-off-by: Qu Wenruo <wqu@suse.com>
    Reviewed-by: David Sterba <dsterba@suse.com>
    Signed-off-by: David Sterba <dsterba@suse.com>
    Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

    ---
    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
    @@ -1527,9 +1527,16 @@ int btrfs_init_fs_root(struct btrfs_root
    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,

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