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

[Ian Kent]

On 29/9/23 17:10, Miklos Szeredi wrote:
> On Fri, 29 Sept 2023 at 02:42, Ian Kent <raven@xxxxxxxxxx> 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)

I guess that would be ok.

Avoiding the cursor is a big plus.


An rbtree is used in kernfs and its readdir function is rather painful so

I wonder what the implications might be for other enumeration needs.


Ian