linux-kernel - Re: [PATCH v3 3/4] add statmount(2) syscall

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Message-ID: <CAJfpegt80_Tyto3QyD48V_yzHSghqg8AC_OPHEMPkDjEYCcisQ@mail.gmail.com>
Date:   Fri, 29 Sep 2023 11:10:04 +0200
From:   Miklos Szeredi <miklos@...redi.hu>
To:     Ian Kent <raven@...maw.net>
Cc:     Miklos Szeredi <mszeredi@...hat.com>,
        linux-fsdevel@...r.kernel.org, linux-kernel@...r.kernel.org,
        linux-api@...r.kernel.org, linux-man@...r.kernel.org,
        linux-security-module@...r.kernel.org, Karel Zak <kzak@...hat.com>,
        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>,
        Matthew House <mattlloydhouse@...il.com>,
        Florian Weimer <fweimer@...hat.com>,
        Arnd Bergmann <arnd@...db.de>
Subject: Re: [PATCH v3 3/4] add statmount(2) syscall

On Fri, 29 Sept 2023 at 02:42, Ian Kent <raven@...maw.net> wrote:
>
> On 28/9/23 21:01, Miklos Szeredi wrote:

> > +static struct vfsmount *lookup_mnt_in_ns(u64 id, struct mnt_namespace *ns)
> > +{
> > +     struct mount *mnt;
> > +     struct vfsmount *res = NULL;
> > +
> > +     lock_ns_list(ns);
> > +     list_for_each_entry(mnt, &ns->list, mnt_list) {
> > +             if (!mnt_is_cursor(mnt) && id == mnt->mnt_id_unique) {
> > +                     res = &mnt->mnt;
> > +                     break;
> > +             }
> > +     }
> > +     unlock_ns_list(ns);
> > +     return res;
> > +}
>
> Seems like we might need to consider making (struct mnt_namespace)->list
>
> a hashed list.

Yes, linear search needs to go.  A hash table is probably the easiest solution.

But I'd also consider replacing ns->list with an rbtree.  Not as
trivial as adding a system hash table and probably also slightly
slower, but it would have some advantages:

 - most space efficient (no overhead of hash buckets)

 - cursor can go away (f_pos can just contain last ID)

Thanks,
Miklos