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

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Date:   Sat, 30 Sep 2023 09:16:11 +0800
From:   Ian Kent <raven@...maw.net>
To:     Miklos Szeredi <miklos@...redi.hu>
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 29/9/23 17:10, Miklos Szeredi wrote:
> 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)

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