On Tue, May 03, 2022 at 05:39:46PM +0300, Amir Goldstein wrote:

On Tue, May 3, 2022 at 3:23 PM Miklos Szeredi [off-list ref] wrote:

quoted

This is a simplification of the getvalues(2) prototype and moving it to the
getxattr(2) interface, as suggested by Dave.

The patch itself just adds the possibility to retrieve a single line of
/proc/$$/mountinfo (which was the basic requirement from which the fsinfo
patchset grew out of).

But this should be able to serve Amir's per-sb iostats, as well as a host of
other cases where some statistic needs to be retrieved from some object.  Note:
a filesystem object often represents other kinds of objects (such as processes
in /proc) so this is not limited to fs attributes.

This also opens up the interface to setting attributes via setxattr(2).

After some pondering I made the namespace so:

: - root
bar - an attribute
foo: - a folder (can contain attributes and/or folders)

The contents of a folder is represented by a null separated list of names.

Examples:

$ getfattr -etext -n ":" .
# file: .
:="mnt:\000mntns:"

$ getfattr -etext -n ":mnt:" .
# file: .
:mnt:="info"

$ getfattr -etext -n ":mnt:info" .
# file: .
:mnt:info="21 1 254:0 / / rw,relatime - ext4 /dev/root rw\012"

$ getfattr -etext -n ":mntns:" .
# file: .
:mntns:="21:\00022:\00024:\00025:\00023:\00026:\00027:\00028:\00029:\00030:\00031:"

$ getfattr -etext -n ":mntns:28:" .
# file: .
:mntns:28:="info"

Comments?

I like that :)

It should be noted that while this API mandates text keys,
it does not mandate text values, so for example, sb iostats could be
exported as text or as binary struct, or as individual text/binary records or
all of the above.

Ugh, no, that would be a total mess.  Don't go exporting random binary
structs depending on the file, that's going to be completely
unmaintainable.  As it is, this is going to be hard enough with random
text fields.

As for this format, it needs to be required to be documented in
Documentation/ABI/ for each entry and key type so that we have a chance
of knowing what is going on and tracking how things are working and
validating stuff.

thanks,

greg k-h