Re: [RFC v3 0/2] vfs / btrfs: add support for ustat()

[Jeff Mahoney]

On 8/15/14 5:29 AM, Al Viro wrote:
> On Thu, Aug 14, 2014 at 07:58:56PM -0700, Luis R. Rodriguez wrote:
> 
>> Christoph had noted that this seemed associated to the problem
>> that the btrfs uses different assignments for st_dev than s_dev,
>> but much as I'd like to see that changed based on discussions so
>> far its unclear if this is going to be possible unless strong
>> commitment is reached.

Resurrecting a dead thread since we've been carrying this patch anyway
since then.

> Explain, please.  Whose commitment and commitment to what, exactly?
> Having different ->st_dev values for different files on the same
> fs is a bloody bad idea; why does btrfs do that at all?  If nothing else,
> it breaks the usual "are those two files on the same fs?" tests...

It's because btrfs snapshots would have inode number collisions.
Changing the inode numbers for snapshots would negate a big benefit of
btrfs snapshots: the quick creation and lightweight on-disk
representation due to metadata sharing.

The thing is that ustat() used to work.  Your commit 0ee5dc676a5f8
(btrfs: kill magical embedded struct superblock) had a regression:
Since it replaced the superblock with a simple dev_t, it rendered the
device no longer discoverable by user_get_super.  We need a list_head to
attach for searching.

There's an argument that this is hacky.  It's valid.  The only other
feedback I've heard is to use a real superblock for subvolumes to do
this instead.  That doesn't work either, due to things like freeze/thaw
and inode writeback.  Ultimately, what we need is a single file system
with multiple namespaces.  Years ago we just needed different inode
namespaces, but as people have started adopting btrfs for containers, we
need more than that.  I've heard requests for per-subvolume security
contexts.  I'd imagine user namespaces are on someone's wish list.  A
working df can be done with ->d_automount, but the way btrfs handles
having a "canonical" subvolume location has always been a way to avoid
directory loops.  I'd like to just automount subvolumes everywhere
they're referenced.  One solution, for which I have no code yet, is to
have something like a superblock-light that we can hang things like a
security context, a user namespace, and an anonymous dev.  Most file
systems would have just one.  Btrfs would have one per subvolume.

That's a big project with a bunch of discussion.  So for now, I'd like
to move this patch forward while we (I) work on the bigger issue.

BTW, in this same thread, Christoph said:> Again, NAK.  Make btrfs
report the proper anon dev_t in stat and
> everything will just work.

We do.  We did then too.  But what doesn't work is a user doing stat()
and then using the dev_t to call ustat().

-Jeff

-- 
Jeff Mahoney
SUSE Labs

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