Re: [PATCH 1/4] btrfs: disk-io: don't allocate anonymous block device for user invisible roots

[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]
> Since it's not possible to completely solve the problem, we can only
> workaround it.
> 
> Firstly, we can reduce the user of anon_dev. Data reloc tree is not visible
> to users, thus it doesn't need anon_dev at all.
> 
> This patch will do extra check on root objectid, to rule out roots who
> don't need anon_dev.
> Although 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/
> Signed-off-by: Qu Wenruo <wqu@suse.com>

Reviewed-by: Josef Bacik <josef@toxicpanda.com>

Thanks,

Josef