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Date:   Sun, 2 Oct 2022 01:06:22 +0100
From:   Al Viro <viro@zeniv.linux.org.uk>
To:     Brian Foster <bfoster@redhat.com>
Cc:     linux-fsdevel@vger.kernel.org, Ian Kent <raven@themaw.net>
Subject: Re: [PATCH v2 RESEND] namei: clear nd->root.mnt before O_CREAT unlazy
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On Fri, Sep 23, 2022 at 10:03:34AM -0400, Brian Foster wrote:

> incompatible with O_CREAT. Otherwise the tradeoff for this change is
> that this may impact behavior when an absolute path O_CREAT lookup
> lands on a symlink that contains another absolute path. The unlazy
> sequence of the create lookup now clears the nd->root mount pointer,
> which means that once we read said link via step_into(), the
> subsequent nd_jump_root() calls into set_root() to grab the mount
> pointer again (from refwalk mode). This is historical behavior for
> O_CREAT and less common than the current behavior of a typical
> create lookup unnecessarily legitimizing the root dentry.

I'm not worried about the overhead of retrieving the root again;
using the different values for beginning and the end of pathwalk,
OTOH...

It's probably OK, but it makes analysis harder.  Do we have a real-world
testcases where the contention would be observable?