linux-kernel - Re: [PATCH v3] fs: make sure to fail try_to_unlazy() and try_to_unlazy() for LOOKUP

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Message-ID: <20251220085826.GB1712166@ZenIV>
Date: Sat, 20 Dec 2025 08:58:26 +0000
From: Al Viro <viro@...iv.linux.org.uk>
To: Mateusz Guzik <mjguzik@...il.com>
Cc: brauner@...nel.org, jack@...e.cz, linux-kernel@...r.kernel.org,
	linux-fsdevel@...r.kernel.org, clm@...a.com
Subject: Re: [PATCH v3] fs: make sure to fail try_to_unlazy() and
 try_to_unlazy() for LOOKUP_CACHED

On Sat, Dec 20, 2025 at 06:40:22AM +0100, Mateusz Guzik wrote:
> Otherwise the slowpath can be taken by the caller, defeating the flag.
> 
> This regressed after calls to legitimize_links() started being
> conditionally elided and stems from the routine always failing
> after seeing the flag, regardless if there were any links.
> 
> In order to address both the bug and the weird semantics make it illegal
> to call legitimize_links() with LOOKUP_CACHED and handle the problem at
> the two callsites.

I still don't get what's weird about the semantics involved, but
the only question I've got is the location of this VFS_BUG_ON().
A way to ensure that we don't forget to check LOOKUP_CACHED early,
in both (seriously similar) callers?

Anyway, it does fix the regression.