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From:   "Fabio M. De Francesco" <fmdefrancesco@gmail.com>
To:     Sebastian Andrzej Siewior <bigeasy@linutronix.de>
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}()
Date:   Thu, 08 Dec 2022 00:03:29 +0100
Message-ID: <2263204.ElGaqSPkdT@suse>
In-Reply-To: <Y5CaRLtwaYXXlGYS@linutronix.de>
References: <20221206070029.7342-1-fmdefrancesco@gmail.com> <2130641.irdbgypaU6@suse> <Y5CaRLtwaYXXlGYS@linutronix.de>
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On mercoled=C3=AC 7 dicembre 2022 14:51:00 CET Sebastian Andrzej Siewior wr=
ote:
> 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 phr=
ase
> > > > before
> > > > your suggestion. What about making it to become "For the above-
mentioned
> > > > 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
> > and/or preemption. I was only trying to say that developers must carefu=
lly
> > check whether or not the whole kmap-ed section depended on those side
> > effects.
> 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
>=20
> > If so, they must _explicitly_ disable preemption or page-faults or both
> > together with the use of kmap_local_page().
>=20
> 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.
> So ideally it won't be used ;)
>=20
> >                                             Instead, if the section=20
doesn't
> >=20
> > depend on preemption and/or page-faults disabling, they must only repla=
ce
> > kmap_atomic() with kmap_local_page().
>=20
> Correct and I assumed that you know all this.
>=20
> > I had probably used a bad wording when trying to say the same things th=
at
> > you
> > wrote much more clearly.
>=20
> Write it as you wish I just made a recommendation. If the wording is
> crystal clear then there is less room for interpretations.

I just sent v2 of this patch.[1] I hope that now I left less room for=20
potential misinterpretation by merging your suggestion with the old text.

Again thanks for helping,

=46abio

[1] https://lore.kernel.org/lkml/20221207225308.8290-1-fmdefrancesco@gmail.=
com/T/