linux-kernel - Re: [RFC][PATCH 0/3] vfs: Use an xarray instead of inserted bookmarks to scan mount list

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Message-ID: <CAJfpegujN1KA=bS55RobUGphfUbY7BAnPke+dPdUnZbcWudwxQ@mail.gmail.com>
Date:   Mon, 15 Mar 2021 15:22:47 +0100
From:   Miklos Szeredi <miklos@...redi.hu>
To:     David Howells <dhowells@...hat.com>
Cc:     Alexander Viro <viro@...iv.linux.org.uk>,
        Matthew Wilcox <willy@...radead.org>,
        Ian Kent <raven@...maw.net>,
        Linus Torvalds <torvalds@...ux-foundation.org>,
        linux-fsdevel@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [RFC][PATCH 0/3] vfs: Use an xarray instead of inserted bookmarks
 to scan mount list

On Mon, Mar 15, 2021 at 2:41 PM David Howells <dhowells@...hat.com> wrote:
>
> Miklos Szeredi <miklos@...redi.hu> wrote:
>
> > >  (2) We can use the file position to represent the mnt_id and can jump to
> > >      it directly - ie. using seek() to jump to a mount object by its ID.
> >
> > What happens if the mount at the current position is removed?
>
> umount_tree() requires the namespace_sem to be writelocked, so that should be
> fine as the patches currently read-lock that whilst doing /proc/*/mount*
>
> I'm assuming that kern_unmount() won't be a problem as it is there to deal
> with mounts made by kern_mount() which don't get added to the mount list
> (mnt_ns is MNT_NS_INTERNAL).  kern_unmount_array() seems to be the same
> because overlayfs gives it mounts generated by clone_private_mount().  It
> might be worth putting a WARN_ON() in kern_unmount() to require this.
>
> When reading through proc, m_start() calls xas_find() which returns the entry
> at the starting index or, if not present, the next higher entry.

This will break the property of new mounts always being added to the
end of the list.  That's likely a regression for nerural based parsers
(i.e. people), probably less so for machine parsers.

Thanks,
Miklos

>
> David
>