[PATCH v15 6/9] namei: LOOKUP_{IN_ROOT,BENEATH}: permit limited ".." resolution

[Al Viro]

On Tue, Nov 05, 2019 at 08:05:50PM +1100, Aleksa Sarai wrote:

> One other possible alternative (which previous versions of this patch
> used) would be to check with path_is_under() if there was a racing
> rename or mount (after re-taking the relevant seqlocks). While this does
> work, it results in possible O(n*m) behaviour if there are many renames
> or mounts occuring *anywhere on the system*.

BTW, do you realize that open-by-fhandle (or working nfsd, for that matter)
will trigger arseloads of write_seqlock(&rename_lock) simply on d_splice_alias()
bringing disconnected subtrees in contact with parent?