Re: [PATCH 06/21] VFS: Introduce a superblock configuration context [ver #3]

From: Miklos Szeredi
Date: Wed May 17 2017 - 03:54:29 EST

On Tue, May 16, 2017 at 6:33 PM, David Howells <dhowells@xxxxxxxxxx> wrote:
> Miklos Szeredi <mszeredi@xxxxxxxxxx> wrote:
>> One way to split this large patch up into more managable chunks would be:
>> 1) common infrastructure
>> 2) new mount related changes
>> 3) reconfig (remount) related changes
>> Would that work?
> The problem is that remount seems to generally use the same parsing code as
> the new-mount entry point.

You are talking about fs code?

I was just referring to this single patch, not the others.

> Before considering how to split it, can we consider whether to roll patches 20
> and 21 into the preceding patches?
>> (a) new mount with new super block created
>> (b) new mount with existing super block reused
>> (c) remount
> (b) is internal-only at the moment, used by NFS submounts as triggered by
> automounts. There isn't currently any way to supply mount options to this.

And all blockdev based fs.

>> 2) modify options ("foo" turns option on, "nofoo" turns it off)
> Not all options are binary and some options may be mandatory.

Right, I was simplifying.

>> The surprising thing here is that we do (a) and (b) via the same route
>> and (a) and (c) via a different ones. This doesn't feel right.
> You need to look at it like this:
> Case Options Ref Call Modify
> super sget super
> ======= ======= ======= ======= =======
> a Y - Y -
> b - Y Y -
> c Y [1] - Y
> [1] We don't have a separate reference sb, only the one we're going to modify,
> but we can preload the sb_config from that.
> (a) and (b) have the same action.
>> i) options that determine the sb instance (such as the blockdev or
>> the server IP address)
>> ii) subpath: this can determine the sb as well as the subtree to use
>> iii) options that can be changed while sb in use
>> iv) ???
> Ah - but some of these options have to be set *inside* sget() or before the
> superblock becomes live, even the ones that can be changed in-flight.

That would be the "???" category. Any concrete examples?

>> Would it make sense to make the "new mount" case be
>> A) find or create sb based on (i) and (ii) options
>> B) reconfigure the resulting sb based on (iii) options
> You would *have* to do the reconfiguration before making the superblock live
> to prevent config/use races,

Indeed, the actual attaching into the mount namespace would be the
final step. I deliberately left it out, because it's largely
orthogonal to the superblock config question.

> and some options in (iii) may be required during
> sget(), or even before you get as far as calling sget() (say you need to
> access a server).
>> This would make legacy new mount be: (A) + if new then (B). And
>> legacy remount just (B).
> It's not obvious that this is sufficiently equivalent from your brief
> description.

It's not obvious. I'm just trying to explore the design space to fix
as much idiocy in the current one as possible.

>> Also I think silently ignoring options is not always the right answer.
> Example?

mount /dev/sda -oacl /mnt
mount /dev/sda -onoacl /mnt2

> Do you mean like the NFS 'sloppy' option? I've noted that that might be best
> handled in userspace.
>> > + int (*remount_fs_sc) (struct super_block *, struct sb_config *);
>> How about reconfig_fs() or just reconfig()?
> Sure.
>> > + (*) struct dentry *(*mount)(struct sb_config *sc);
>> I'd be much happier with "get_root()" or something.
> Changed in patch 21 to ->get_tree() as suggested by Al. Having looked over
> the code, I'm tempted to change it back to ->mount() as being more obvious.

Some definitions of mount:

- to increase in amount or extent
- to seat oneself (as on a horse) for riding
- to climb on top of for copulation
- to launch and carry out (something, such as an assault or a campaign)
- to attach to a support

No really good match for what this method is doing. We could call it
->get_tree_to_mount(), but calling it just ->mount() implies that it's
doing the mounting, which it is not.

>> > + err = parse_monolithic_mount_data(sc, data);
>> > + if (err < 0)
>> > + goto err_sc;
>> If filesystem defines ->monolithic_mount_data() who is responsible for
>> calling the security hook?
> Which security hook? security_sb_remount()?
> Note this code has changed in patch 20. I should update security_sb_remount()
> to take an sb_config and call it in all paths.
>> Largely duplicated do_new_mount_sc(). What's the point?
> Legacy vs new. Fixed in patch 20.
>> Lots of these are not superblock options, and should be moved over to
>> the forbidden ones. Look at do_mount() for a hint.
> I still have to support legacy mount option parsing. Do I actually see these
> in legacy mount(2)? Or are they weeded out by mount(8)?

You are thinking on the wrong level. Of course mount(2) needs to
handle MS_NOSUID et al. But it's doing it now, and it isn't parsing
"nosuid", just translating MS_NOSUID to MNT_NOSUID. For the fsopen()
case you won't need to parse "nosuid" because that's a flag for

The only thing fsmount() should take from the sc is the root_dentry.
It should be equivalent to what currently is a bind mount, except it
should be able to fully configure the new mount.

I'm still hoping we can move subpath handling completely to fsmount()
in which case it would just take a struct super_block. But that would
have to start with lots of filesystem work (not just NFS but CEPH,
CIFS, etc..).