linux-kernel - Re: [PATCH] mm/highmem: Add notes about conversions from kmap{,

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Message-ID: <Y5CaRLtwaYXXlGYS@linutronix.de>
Date:   Wed, 7 Dec 2022 14:51:00 +0100
From:   Sebastian Andrzej Siewior <bigeasy@...utronix.de>
To:     "Fabio M. De Francesco" <fmdefrancesco@...il.com>
Cc:     Jonathan Corbet <corbet@....net>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Ira Weiny <ira.weiny@...el.com>,
        Mike Rapoport <rppt@...nel.org>, linux-doc@...r.kernel.org,
        linux-kernel@...r.kernel.org, linux-mm@...ck.org,
        Mike Rapoport <rppt@...ux.ibm.com>,
        Peter Zijlstra <a.p.zijlstra@...llo.nl>,
        Thomas Gleixner <tglx@...utronix.de>
Subject: Re: [PATCH] mm/highmem: Add notes about conversions from
 kmap{,_atomic}()

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.
> > > 
> > > 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-mentioned
> > > cases, conversions should also explicitly disable page-faults and/or
> > > preemption"?
> > 
> > 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.
> 
> I never meant to suggest to _unconditionally_ disable page-faults 
> and/or preemption. I was only trying to say that developers must carefully 
> 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…

> If so, they must _explicitly_ disable preemption or page-faults or both 
> 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. 
So ideally it won't be used ;)

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

Correct and I assumed that you know all this. 

> I had probably used a bad wording when trying to say the same things that you 
> 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,
> 
> Fabio

Sebastian