Re: [PATCH] btrfs: prevent subvol with swapfile from being deleted

[Kaiwen Hu <kevinhu@synology.com>]

On 3/23/2022 3:39 AM, David Sterba wrote:
> On Tue, Mar 22, 2022 at 06:47:59PM +0800, Qu Wenruo wrote:
>> This looks a little weird to me.
>>
>> If the root is already dead, it means we should not be able to access
>> any file inside the subvolume.
>>
>> How could we go into btrfs_swap_activate() while the root is already dead?
>>
>> Or is there some special race I missed?
> The deletion has a few steps, eg. the dentry is removed, root is marked
> as dead and different thing locking protection.
>
> I was wondering about file descriptor access to the subvolume and
> calling swapon on that, but swapon/swapoff is a syscall and work with a
> path argument so that can't happen. I haven't checked in what order are
> the dentry removal and dead flag done, it could be theoretically
> possible that there's a short window where the dentry is accessible and
> subvolume already marked.


Thanks for david's help.  Yes, it is possible the subvolume is marked 
for deletion,  but still not remove yet.  So like 
|btrfs_ioctl_send()|doing, I check the dead-root mark before activating 
swapfile to prevent the race condition.


Thanks,

Kaiwen Hu