Re: [PATCH] staging: media: atomisp: Use kmap_local_page() in hmm_store()

["Fabio M. De Francesco" <fmdefrancesco@gmail.com>]

On gioved? 14 aprile 2022 11:12:39 CEST Julia Lawall wrote:
> 
> On Thu, 14 Apr 2022, Fabio M. De Francesco wrote:
> 
> > On gioved? 14 aprile 2022 09:03:40 CEST Julia Lawall wrote:
> > >
> > > On Wed, 13 Apr 2022, Ira Weiny wrote:
> > >
> > > > On Wed, Apr 13, 2022 at 05:44:54PM -0700, Alison Schofield wrote:
> > > > > On Thu, Apr 14, 2022 at 12:55:31AM +0200, Fabio M. De Francesco
> > wrote:
> > > > > > The use of kmap() is being deprecated in favor of 
kmap_local_page()
> > > > > > where it is feasible. The same is true for kmap_atomic().
> > > > > >
> > > > > > In file pci/hmm/hmm.c, function hmm_store() test if we are in
> > atomic
> > > > > > context and, if so, it calls kmap_atomic(), if not, it calls
> > kmap().
> > > > > >
> > > > > > First of all, in_atomic() shouldn't be used in drivers. This 
macro
> > > > > > cannot always detect atomic context; in particular, it cannot 
know
> > > > > > about held spinlocks in non-preemptible kernels.
> > > > > >
> > > > > > Notwithstanding what it is said above, this code doesn't need 
to
> > care
> > > > > > whether or not it is executing in atomic context. It can simply 
use
> > > > > > kmap_local_page() / kunmap_local() that can instead do the 
mapping
> > /
> > > > > > unmapping regardless of the context.
> > > > > >
> > > > > > With kmap_local_page(), the mapping is per thread, CPU local 
and
> > not
> > > > > > globally visible. Therefore, hmm_store()() is a function where 
the
> > use
> > > > > > of kmap_local_page() in place of both kmap() and kmap_atomic() 
is
> > > > > > correctly suited.
> > > > > >
> > > > > > Convert the calls of kmap() / kunmap() and kmap_atomic() /
> > > > > > kunmap_atomic() to kmap_local_page() / kunmap_local() and drop 
the
> > > > > > unnecessary tests which test if the code is in atomic context.
> > > > > >
> > > > >
> > > > > Not specifically about this patch, but more generally about all
> > > > > such conversions - is there a 'proof' that shows this just works
> > > >
> > > > Just code inspection.  Most of them that I have done have been 
compile
> > tested
> > > > only.  Part of the key is that des is a local variable and is not
> > aliased by
> > > > anything outside this function.
> > >
> > > Typically, the concern about being in atomic context has to do with
> > > whether GFP_KERNEL or GFP_ATOMIC should be used, ie whether 
allocation
> > > can sleep.
> >
> > I'd add that the concern about being in atomic context has mainly to do
> > with calling whatever function that may sleep.
> >
> > Some time ago I analyzed a calls chain which, under spinlocks and with
> > IRQ's disabled, led to console_lock() which is annotated with
> > might_sleep(). It took about 8000 ms to recover when executing in a 4 
CPU /
> > 8 SMT System. Linus T. suggested to make this work asynchronous (commit
> > 1ee33b1ca2b8 ("tty: n_hdlc: make n_hdlc_tty_wakeup() asynchronous")).
> >
> > > It doesn't have to do with whether some data can be shared.
> >
> > Yes, FWIW I agree with you.
> >
> > > Is that the concern here?
> >
> > The concern here is about the locality of the pointer variable to which 
the
> > struct page has been mapped to. In atomic context we are not allowed to
> > kmap() (this is why in the code we had that in_atomic() test), instead 
we
> > can kmap_local_page() or kmap_atomic(). The latter is strongly 
discouraged
> > in favor of the former.
> 
> I have the impression that you are first agreeing with me and then
> contradicting me :).  Is your point that in general a concern about 
atomic
> context has to do with whether sleeping is allowed, but that the concern
> is something else here?  I'm not familiar with these kmap functions.

Yes the concern is something else here (sorry for my poor English). It was 
not my intention to contradict you :) 

The concern for "locality" here is that if the variable is shared between 
different functions there are cases where we cannot use kmap_local_page().
For example, those mappings with kmap_local_page() are per thread and CPU 
local.

I think that the best means to convey what I want to express is pointing to 
my patch to Documentation/vm/highmem.rst:

https://lore.kernel.org/lkml/20220412124003.10736-1-fmdefrancesco@gmail.com/

I hope that this can help to understand why we care of "locality" in this 
specific case where we use kmap_local_page().

Again, sorry for not being clear.

Thanks,

Fabio M. De Francesco

> thanks,
> julia
> 
> 
> >
> > Furthermore, Alison was asking if we can prove that these kinds of
> > conversions can actually work when we have not the hardware for 
testing. As
> > Ira wrote, code inspection is sufficient to prove it.
> >
> > Thanks,
> >
> > Fabio M. De Francesco