linux-kernel - Re: [PATCH v2] fs: make sure to fail try_to_unlazy() and try_to_unlazy() for LOOKUP

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Message-ID: <CAGudoHFLV5sHE1UBXR5BtPHUghnroA=m59D6yBknWnZz0mkS7A@mail.gmail.com>
Date: Wed, 17 Dec 2025 11:13:22 +0100
From: Mateusz Guzik <mjguzik@...il.com>
To: Al Viro <viro@...iv.linux.org.uk>
Cc: brauner@...nel.org, jack@...e.cz, linux-kernel@...r.kernel.org, 
	linux-fsdevel@...r.kernel.org, clm@...a.com
Subject: Re: [PATCH v2] fs: make sure to fail try_to_unlazy() and
 try_to_unlazy() for LOOKUP_CACHED

On Wed, Dec 17, 2025 at 11:05 AM Al Viro <viro@...iv.linux.org.uk> wrote:
>
> On Wed, Dec 17, 2025 at 10:11:04AM +0100, Mateusz Guzik wrote:
> > On Wed, Dec 17, 2025 at 10:07 AM Al Viro <viro@...iv.linux.org.uk> wrote:
> > >
> > > On Wed, Dec 17, 2025 at 09:47:04AM +0100, Mateusz Guzik wrote:
> > > > One remaining weirdness is terminate_walk() walking the symlink stack
> > > > after drop_links().
> > >
> > > What weirdness?  If we are not in RCU mode, we need to drop symlink bodies
> > > *and* drop symlink references?
> >
> > One would expect a routine named drop_links() would handle the
> > entirety of clean up of symlinks.
> >
> > Seeing how it only handles some of it, it should be renamed to better
> > indicate what it is doing, but that's a potential clean up for later.
>
> Take a look at the callers.  All 3 of them.
>
> 1) terminate_walk(): drop all symlink bodies, in non-RCU mode drop
> all paths as well.
>
> 2) a couple in legitimize_links(): *always* called in RCU mode.  That's
> the whole point - trying to grab references to a bunch of dentries/mounts,
> so that we could continue in non-RCU mode from that point on.  What should
> we do if we'd grabbed some of those references, but failed halfway through
> the stack?
>
> We *can't* do path_put() there - not under rcu_read_lock().  And we can't
> delay dropping the link bodies past rcu_read_unlock().
>
> Note that this state has
>         nd->depth link bodies in stack, all need to be droped before
> rcu_read_unlock()
>         first K link references in stack that need to be dropped after
> rcu_read_unlock()
>         nd->depth - K link references in stack that do _not_ need to
> be dropped.
>
> Solution: have link bodies dropped, callbacks cleared and nd->depth
> reset to K.  The caller of legitimate_links() immediately drops out
> of RCU mode and we proceed to terminate_walk(), same as we would
> on an error in non-RCU mode.
>
> This case is on a slow path; we could microoptimize it, but result
> would be really harder to understand.

I'm not arguing for drop_links() to change behavior, but for it to be
renamed to something which indicates there is still potential
symlink-related clean up to do.

As an outsider, a routine named drop_${whatever} normally suggests the
${whatever} is fully taken care of after the call, which is not the
case here.