linux-kernel - Re: [PATCH] fs: support filename refcount without atomics

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Message-ID: <CAGudoHGina_OHsbP_oz5UAtXKoKQqhv-tB6Ok63rRQHThPuy+Q@mail.gmail.com>
Date: Fri, 7 Mar 2025 17:25:35 +0100
From: Mateusz Guzik <mjguzik@...il.com>
To: Jens Axboe <axboe@...nel.dk>
Cc: brauner@...nel.org, viro@...iv.linux.org.uk, jack@...e.cz, 
	linux-kernel@...r.kernel.org, linux-fsdevel@...r.kernel.org, 
	io-uring@...r.kernel.org, audit@...r.kernel.org
Subject: Re: [PATCH] fs: support filename refcount without atomics

On Fri, Mar 7, 2025 at 5:18 PM Jens Axboe <axboe@...nel.dk> wrote:
>
> > +static inline void makeatomicname(struct filename *name)
> > +{
> > +     VFS_BUG_ON(IS_ERR_OR_NULL(name));
> > +     /*
> > +      * The name can legitimately already be atomic if it was cached by audit.
> > +      * If switching the refcount to atomic, we need not to know we are the
> > +      * only non-atomic user.
> > +      */
> > +     VFS_BUG_ON(name->owner != current && !name->is_atomic);
> > +     /*
> > +      * Don't bother branching, this is a store to an already dirtied cacheline.
> > +      */
> > +     name->is_atomic = true;
> > +}
>
> Should this not depend on audit being enabled? io_uring without audit is
> fine.
>

I thought about it, but then I got worried about transitions from
disabled to enabled -- will they suddenly start looking here? Should
this test for audit_enabled, audit_dummy_context() or something else?
I did not want to bother analyzing this.

I'll note though this would be an optimization on top of the current
code, so I don't think it *blocks* the patch.

-- 
Mateusz Guzik <mjguzik gmail.com>