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From:   David Howells <dhowells@redhat.com>
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To:     Miklos Szeredi <miklos@szeredi.hu>
Cc:     dhowells@redhat.com, Alexander Viro <viro@zeniv.linux.org.uk>,
        Matthew Wilcox <willy@infradead.org>,
        Ian Kent <raven@themaw.net>,
        Linus Torvalds <torvalds@linux-foundation.org>,
        linux-fsdevel@vger.kernel.org, linux-kernel@vger.kernel.org
Subject: Re: [RFC][PATCH 0/3] vfs: Use an xarray instead of inserted bookmarks to scan mount list
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Date:   Mon, 15 Mar 2021 13:41:48 +0000
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Miklos Szeredi <miklos@szeredi.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.

David