Re: [PATCH 4/4] btrfs: free anon_dev earlier to prevent exhausting anonymous block device pool

[Josef Bacik <josef@toxicpanda.com>]

On 6/15/20 10:17 PM, Qu Wenruo wrote:
> [BUG]
> When a lot of subvolumes are created, there is a user report about
> transaction aborted:
> 
>    ------------[ cut here ]------------
>    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 root cause is we don't have unlimited resource for anonymous block
> device number.
> The anonymous block device pool only contains 1<<20 devices, and is
> shared across a several fses, like ceph and overlayfs.
> 
> While btrfs has support for 1<<48 subvolumes, so it's just a problem of
> time to hit such limit.
> 
> [WORKAROUND]
> Here we can free btrfs_root::anon_dev as long as the subvolume is no
> longer visible to users.
> 
> By freeing it earlier we reclaim the anon_dev quicker, hopefully to
> reduce the chance of exhausting the pool.
> 

Why isn't this happening as part of the root teardown once all the references to 
it are gone?  Thanks,

Josef