linux-kernel - Re: [RFC PATCH] getting misc stats/attributes via xattr API

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Date:   Wed, 11 May 2022 10:58:37 +0200
From:   Karel Zak <kzak@...hat.com>
To:     Dave Chinner <david@...morbit.com>
Cc:     Christian Brauner <brauner@...nel.org>,
        Miklos Szeredi <miklos@...redi.hu>,
        linux-fsdevel@...r.kernel.org, Theodore Ts'o <tytso@....edu>,
        Greg KH <gregkh@...uxfoundation.org>,
        linux-kernel@...r.kernel.org,
        Linux API <linux-api@...r.kernel.org>,
        linux-man <linux-man@...r.kernel.org>,
        LSM <linux-security-module@...r.kernel.org>,
        Ian Kent <raven@...maw.net>,
        David Howells <dhowells@...hat.com>,
        Linus Torvalds <torvalds@...ux-foundation.org>,
        Al Viro <viro@...iv.linux.org.uk>,
        Christian Brauner <christian@...uner.io>,
        Amir Goldstein <amir73il@...il.com>,
        James Bottomley <James.Bottomley@...senpartnership.com>
Subject: Re: [RFC PATCH] getting misc stats/attributes via xattr API

On Wed, May 11, 2022 at 09:25:52AM +1000, Dave Chinner wrote:
> > I'd love something like:
> > 
> > ssize_t sz;
> > fsinfo_query query[] = {
> >     { .request = FSINFO_MOUNT_PATH },
> >     { .request = FSINFO_PROPAGATION },
> >     { .request = FSINFO_CHILDREN_IDS },
> > };
> > 
> > sz = fsinfo(dfd, "", AT_EMPTY_PATH,
> >                 &query, ARRAY_SIZE(query),
> >                 buf, sizeof(buf));
> > 
> > for (p = buf; p < buf + sz; ) {
> > {
> >     fsinfo_entry *e = (struct fsinfo_entry) p;
> >     char *data = p + sizeof(struct fsinfo_entry);
> > 
> >     switch(e->request) {
> >     case FSINFO_MOUNT_PATH:
> >         printf("mountpoint %s\n", data);
> >         break;
> >     case FSINFO_PROPAGATION:
> >         printf("propagation %x\n", (uintptr_t) data);
> >         break;
> >     case FSINFO_CHILDREN_IDS:
> >         fsinfo_child *x = (fsinfo_child *) data;
> >         for (i = 0; i < e->count; i++) {
> >             printf("child: %d\n", x[i].mnt_id);
> >         }
> >         break;
> >     ...
> >     }
> > 
> >     p += sizeof(struct fsinfo_entry) + e->len;
> > }
> 
> That's pretty much what a multi-xattr get operation looks like.
> It's a bit more more intricate in the setup of the request/return
> buffer, but otherwise the structure of the code is the same.
> 
> I just don't see why we need special purpose interfaces like this
> for key/value information when small tweaks to the existing
> generic key/value interfaces can provide exactly the same
> functionality....

I don't say we need a new interface ;-) I'd be happy with whatever as
long as:

  * minimal strings parsing (wish than a requirement)
  * one syscall returns multiple key/value
  * can address mount table entries by ID
  * can ask for list of children (submounts)
  * extensible

if this will be possible with xattr (listxattr2(), or so) when great.

  Karel

-- 
 Karel Zak  <kzak@...hat.com>
 http://karelzak.blogspot.com