Received-SPF: pass (google.com: domain of linux-kernel-owner@vger.kernel.org designates 2620:137:e000::1:20 as permitted sender) client-ip=2620:137:e000::1:20;
Date:   Wed, 7 Dec 2022 14:51:00 +0100
From:   Sebastian Andrzej Siewior <bigeasy@linutronix.de>
To:     "Fabio M. De Francesco" <fmdefrancesco@gmail.com>
Cc:     Jonathan Corbet <corbet@lwn.net>,
        Andrew Morton <akpm@linux-foundation.org>,
        Ira Weiny <ira.weiny@intel.com>,
        Mike Rapoport <rppt@kernel.org>, linux-doc@vger.kernel.org,
        linux-kernel@vger.kernel.org, linux-mm@kvack.org,
        Mike Rapoport <rppt@linux.ibm.com>,
        Peter Zijlstra <a.p.zijlstra@chello.nl>,
        Thomas Gleixner <tglx@linutronix.de>
Subject: Re: [PATCH] mm/highmem: Add notes about conversions from
 kmap{,_atomic}()
Message-ID: <Y5CaRLtwaYXXlGYS@linutronix.de>
References: <20221206070029.7342-1-fmdefrancesco@gmail.com>
 <2093077.OBFZWjSADL@suse>
 <Y5BIHdnP4yeJ8svL@linutronix.de>
 <2130641.irdbgypaU6@suse>
MIME-Version: 1.0
Content-Type: text/plain; charset=utf-8
Content-Disposition: inline
Content-Transfer-Encoding: quoted-printable
In-Reply-To: <2130641.irdbgypaU6@suse>
Precedence: bulk

On 2022-12-07 14:01:50 [+0100], Fabio M. De Francesco wrote:
> > > If so, I understand and I again agree with you. If not, I'm missing
> > > something; so please let me understand properly.
> > >=20
> > > Aside from the above, I'm not sure whether you deleted the last phrase
> > > before
> > > your suggestion. What about making it to become "For the above-mentio=
ned
> > > cases, conversions should also explicitly disable page-faults and/or
> > > preemption"?
> >=20
> > They need to disable preemption or page-faults or both if it is needed
> > (not unconditionally) and where it is needed. This means not
> > unconditionally over the whole kmap-ed section.
>=20
> I never meant to suggest to _unconditionally_ disable page-faults=20
> and/or preemption. I was only trying to say that developers must carefull=
y=20
> check whether or not the whole kmap-ed section depended on those side eff=
ects.

I know. That are the two condition that should be checked/ kept in mind
while replacing the code. Maybe I read it wrongly=E2=80=A6

> If so, they must _explicitly_ disable preemption or page-faults or both=
=20
> together with the use of kmap_local_page().

Right. The requirement for it should be probably documented in case it
is not obvious. For PREEMPT_RT it will become a problem if the preempt
disabled section additionally acquired a spinlock_t or allocated memory.=20
So ideally it won't be used ;)

>                                             Instead, if the section doesn=
't=20
> depend on preemption and/or page-faults disabling, they must only replace=
=20
> kmap_atomic() with kmap_local_page().

Correct and I assumed that you know all this.=20

> I had probably used a bad wording when trying to say the same things that=
 you=20
> wrote much more clearly.

Write it as you wish I just made a recommendation. If the wording is
crystal clear then there is less room for interpretations.

> Thanks,
>=20
> Fabio

Sebastian