Re: [PATCH] mm/highmem: Add notes about conversions from kmap{,_atomic}()

Fabio M. De Francesco
2022-12-07T23:03:51Z

On mercoledì 7 dicembre 2022 14:51:00 CET Sebastian Andrzej Siewior wrote:
> 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.

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

Again thanks for helping,

Fabio

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