[RFC PATCH] getvalues(2) prototype

[Dave Chinner]

On Tue, Mar 22, 2022 at 08:27:12PM +0100, Miklos Szeredi wrote:
> Add a new userspace API that allows getting multiple short values in a
> single syscall.
> 
> This would be useful for the following reasons:
> 
> - Calling open/read/close for many small files is inefficient.  E.g. on my
>   desktop invoking lsof(1) results in ~60k open + read + close calls under
>   /proc and 90% of those are 128 bytes or less.

How does doing the open/read/close in a single syscall make this any
more efficient? All it saves is the overhead of a couple of
syscalls, it doesn't reduce any of the setup or teardown overhead
needed to read the data itself....

> - Interfaces for getting various attributes and statistics are fragmented.
>   For files we have basic stat, statx, extended attributes, file attributes
>   (for which there are two overlapping ioctl interfaces).  For mounts and
>   superblocks we have stat*fs as well as /proc/$PID/{mountinfo,mountstats}.
>   The latter also has the problem on not allowing queries on a specific
>   mount.

https://xkcd.com/927/

> - Some attributes are cheap to generate, some are expensive.  Allowing
>   userspace to select which ones it needs should allow optimizing queries.
> 
> - Adding an ascii namespace should allow easy extension and self
>   description.
> 
> - The values can be text or binary, whichever is fits best.
> 
> The interface definition is:
> 
> struct name_val {
> 	const char *name;	/* in */
> 	struct iovec value_in;	/* in */
> 	struct iovec value_out;	/* out */
> 	uint32_t error;		/* out */
> 	uint32_t reserved;
> };

Ahhh, XFS_IOC_ATTRMULTI_BY_HANDLE reborn. This is how xfsdump gets
and sets attributes efficiently when dumping and restoring files -
it's an interface that allows batches of xattr operations to be run
on a file in a single syscall.

I've said in the past when discussing things like statx() that maybe
everything should be addressable via the xattr namespace and
set/queried via xattr names regardless of how the filesystem stores
the data. The VFS/filesystem simply translates the name to the
storage location of the information. It might be held in xattrs, but
it could just be a flag bit in an inode field.

Then we just get named xattrs in batches from an open fd.

> int getvalues(int dfd, const char *path, struct name_val *vec, size_t num,
> 	      unsigned int flags);
> 
> @dfd and @path are used to lookup object $ORIGIN.  @vec contains @num
> name/value descriptors.  @flags contains lookup flags for @path.
> 
> The syscall returns the number of values filled or an error.
> 
> A single name/value descriptor has the following fields:
> 
> @name describes the object whose value is to be returned.  E.g.
> 
> mnt                    - list of mount parameters
> mnt:mountpoint         - the mountpoint of the mount of $ORIGIN
> mntns                  - list of mount ID's reachable from the current root
> mntns:21:parentid      - parent ID of the mount with ID of 21
> xattr:security.selinux - the security.selinux extended attribute
> data:foo/bar           - the data contained in file $ORIGIN/foo/bar

How are these different from just declaring new xattr namespaces for
these things. e.g. open any file and list the xattrs in the
xattr:mount.mnt namespace to get the list of mount parameters for
that mount.

Why do we need a new "xattr in everything but name" interface when
we could just extend the one we've already got and formalise a new,
cleaner version of xattr batch APIs that have been around for 20-odd
years already?

Cheers,

Dave.

-- 
Dave Chinner
david at fromorbit.com