Re: [PATCH v4 6/9] fs: report per-sb io stats

[Miklos Szeredi <miklos@szeredi.hu>]

On Sat, 5 Mar 2022 at 17:04, Amir Goldstein <amir73il@gmail.com> wrote:
>
> Show optional collected per-sb io stats in /proc/<pid>/mountstats
> for filesystems that do not implement their own show_stats() method
> and have generic per-sb stats enabled.

I still think it's wrong to extend the /proc/*/mount* family of
interfaces.  The big issue is that the kernel has to generate this
info for *all* mounts, even though the user may be just looking for
information on a specific mount.  Your workaround is to enable the
info generation for only a subset of fs types, but this doesn't solve
the fundamental issue.

So let's please implement a per-mount interface.  Yes, it's a much
bigger project, but one which needs to be done at one point, and which
would be generally useful.   There was tons of discussion around this
when dhowells tried to create one, and there was a suggestion by Linus
which I think all parties found acceptable:

 - return basic info in a binary format (similar to e.g. statx)
 - after the fix binary structure allow misc info to be added using an
ascii format

And since generating the info may be expensive in some cases, the
interface would need to allow selective queries (which statx does for
binary fields, but for ascii ones this is a new challenge).

I think allowing the user to query the list of supported fields should
also be possible (again, statx supports this).

So there are a number of requirements for this interface, and I'm not
quite sure what the best solution is.   I can try to put something
together if there are no objections...

Thanks,
Miklos