Re: [PATCH/RFC] NFSD: handle BTRFS subvolumes better.

[Josef Bacik <josef@toxicpanda.com>]

On 7/15/21 6:37 PM, NeilBrown wrote:
> On Fri, 16 Jul 2021, Josef Bacik wrote:
>> On 7/15/21 1:24 PM, Christoph Hellwig wrote:
>>> On Thu, Jul 15, 2021 at 01:11:29PM -0400, Josef Bacik wrote:
>>>> Because there's no alternative.  We need a way to tell userspace they've
>>>> wandered into a different inode namespace.  There's no argument that what
>>>> we're doing is ugly, but there's never been a clear "do X instead".  Just a
>>>> lot of whinging that btrfs is broken.  This makes userspace happy and is
>>>> simple and straightforward.  I'm open to alternatives, but there have been 0
>>>> workable alternatives proposed in the last decade of complaining about it.
>>>
>>> Make sure we cross a vfsmount when crossing the "st_dev" domain so
>>> that it is properly reported.   Suggested many times and ignored all
>>> the time beause it requires a bit of work.
>>>
>>
>> You keep telling me this but forgetting that I did all this work when you
>> originally suggested it.  The problem I ran into was the automount stuff
>> requires that we have a completely different superblock for every vfsmount.
>> This is fine for things like nfs or samba where the automount literally points
>> to a completely different mount, but doesn't work for btrfs where it's on the
>> same file system.  If you have 1000 subvolumes and run sync() you're going to
>> write the superblock 1000 times for the same file system.  You are going to
>> reclaim inodes on the same file system 1000 times.  You are going to reclaim
>> dcache on the same filesytem 1000 times.  You are also going to pin 1000
>> dentries/inodes into memory whenever you wander into these things because the
>> super is going to hold them open.
>>
>> This is not a workable solution.  It's not a matter of simply tying into
>> existing infrastructure, we'd have to completely rework how the VFS deals with
>> this stuff in order to be reasonable.  And when I brought this up to Al he told
>> me I was insane and we absolutely had to have a different SB for every vfsmount,
>> which means we can't use vfsmount for this, which means we don't have any other
>> options.  Thanks,
> 
> When I was first looking at this, I thought that separate vfsmnts
> and auto-mounting was the way to go "just like NFS".  NFS still shares a
> lot between the multiple superblock - certainly it shares the same
> connection to the server.
> 
> But I dropped the idea when Bruce pointed out that nfsd is not set up to
> export auto-mounted filesystems.  It needs to be able to find a
> filesystem given a UUID (extracted from a filehandle), and it does this
> by walking through the mount table to find one that matches.  So unless
> all btrfs subvols were mounted all the time (which I wouldn't propose),
> it would need major work to fix.
> 
> NFSv4 describes the fsid as having a "major" and "minor" component.
> We've never treated these as having an important meaning - just extra
> bits to encode uniqueness in.  Maybe we should have used "major" for the
> vfsmnt, and kept "minor" for the subvol.....
> 
> The idea for a single vfsmnt exposing multiple inode-name-spaces does
> appeal to me.  The "st_dev" is just part of the name, and already a
> fairly blurry part.  Thanks to bind mounts, multiple mounts can have the
> same st_dev.  I see no intrinsic reason that a single mount should not
> have multiple fsids, provided that a coherent picture is provided to
> userspace which doesn't contain too many surprises.
> 

Ok so setting aside btrfs for the moment, how does NFS deal with exporting a 
directory that has multiple other file systems under that tree?  I assume the 
same sort of problem doesn't occur, but why is that?  Is it because it's a 
different vfsmount/sb or is there some other magic making this work?  Thanks,

Josef