On Wed, Jun 26, 2019 at 12:05:25PM +0200, Christian Brauner wrote:

On Mon, Jun 24, 2019 at 03:08:45PM +0100, David Howells wrote:

quoted

Hi Al,

Here are a set of patches that adds a syscall, fsinfo(), that allows
attributes of a filesystem/superblock to be queried.  Attribute values are
of four basic types:

 (1) Version dependent-length structure (size defined by type).

 (2) Variable-length string (up to PAGE_SIZE).

 (3) Array of fixed-length structures (up to INT_MAX size).

 (4) Opaque blob (up to INT_MAX size).

Attributes can have multiple values in up to two dimensions and all the
values of a particular attribute must have the same type.

Note that the attribute values *are* allowed to vary between dentries
within a single superblock, depending on the specific dentry that you're
looking at.

I've tried to make the interface as light as possible, so integer/enum
attribute selector rather than string and the core does all the allocation
and extensibility support work rather than leaving that to the filesystems.
That means that for the first two attribute types, sb->s_op->fsinfo() may
assume that the provided buffer is always present and always big enough.

Further, this removes the possibility of the filesystem gaining access to the
userspace buffer.


fsinfo() allows a variety of information to be retrieved about a filesystem
and the mount topology:

 (1) General superblock attributes:

      - The amount of space/free space in a filesystem (as statfs()).
      - Filesystem identifiers (UUID, volume label, device numbers, ...)
      - The limits on a filesystem's capabilities
      - Information on supported statx fields and attributes and IOC flags.
      - A variety single-bit flags indicating supported capabilities.
      - Timestamp resolution and range.
      - Sources (as per mount(2), but fsconfig() allows multiple sources).
      - In-filesystem filename format information.
      - Filesystem parameters ("mount -o xxx"-type things).
      - LSM parameters (again "mount -o xxx"-type things).

 (2) Filesystem-specific superblock attributes:

      - Server names and addresses.
      - Cell name.

 (3) Filesystem configuration metadata attributes:

      - Filesystem parameter type descriptions.
      - Name -> parameter mappings.
      - Simple enumeration name -> value mappings.

 (4) Mount topology:

      - General information about a mount object.
      - Mount device name(s).
      - Children of a mount object and their relative paths.

 (5) Information about what the fsinfo() syscall itself supports, including
     the number of attibutes supported and the number of capability bits
     supported.

Phew, this patchset is a lot. It's good of course but can we please cut
some of the more advanced features such as querying by mount id,
submounts etc. pp. for now?
I feel this would help with review and since your interface is
extensible it's really not a big deal if we defer fancy features to
later cycles after people had more time to review and the interface has
seen some exposure.

The mount api changes over the last months have honestly been so huge
that any chance to make the changes smaller and easier to digest we
should take. (I'm really not complaining. Good that the work is done and
it's entirely ok that it's a lot of code.)

It would also be great if after you have dropped some stuff from this
patchset and gotten an Ack we could stuff it into linux-next for some
time because it hasn't been so far...

And I also very much recommend to remove any potential cross-dependency
between the fsinfo() and the notification patchset.
Ideally, I'd like to see fsinfo() to be completely independent to not
block it on something way more controversial.
Furthermore, I can't possibly keep the context of another huge patchset
not yet merged in the back of my mind while reviewing this patchset. :)

Christian