Re: [PATCH 3/3] binder: Use kmap_local_page() in binder_alloc_get_page()

[Todd Kjos]

On Sun, Apr 24, 2022 at 2:40 AM Fabio M. De Francesco
<fmdefrancesco@xxxxxxxxx> wrote:
>
> On sabato 23 aprile 2022 18:02:48 CEST Christophe JAILLET wrote:
> > Hi,
> >
> > Le 23/04/2022 à 12:24, Fabio M. De Francesco a écrit :
> > > The use of kmap_atomic() is being deprecated in favor of
> kmap_local_page()
> > > where it is feasible. Each call of kmap_atomic() in the kernel creates
> > > a non-preemptible section and disable pagefaults. This could be a
> source
> > > of unwanted latency, so it should be only used if it is absolutely
> > > required, otherwise kmap_local_page() should be preferred.
> > >
> > > With kmap_local_page(), the mapping is per thread, CPU local and not
> > > globally visible. Furthermore, the mapping can be acquired from any
> context
> > > (including interrupts).
> > >
> > > Therefore, use kmap_local_page() / kunmap_local() in place of
> > > kmap_atomic() / kunmap_atomic().
> > >
> > > Signed-off-by: Fabio M. De Francesco <fmdefrancesco@xxxxxxxxx>
> > > ---
> > >   drivers/android/binder_alloc.c | 6 +++---
> > >   1 file changed, 3 insertions(+), 3 deletions(-)
> > >
> > > diff --git a/drivers/android/binder_alloc.c b/drivers/android/
> binder_alloc.c
> > > index 0875c463c002..058595cc7ff0 100644
> > > --- a/drivers/android/binder_alloc.c
> > > +++ b/drivers/android/binder_alloc.c
> > > @@ -1250,17 +1250,17 @@ static int binder_alloc_do_buffer_copy(struct
> binder_alloc *alloc,
> > >             page = binder_alloc_get_page(alloc, buffer,
> > >                                          buffer_offset,
> &pgoff);
> > >             size = min_t(size_t, bytes, PAGE_SIZE - pgoff);
> > > -           base_ptr = kmap_atomic(page);
> > > +           base_ptr = kmap_local_page(page);
> > >             tmpptr = base_ptr + pgoff;
> > >             if (to_buffer)
> > >                     memcpy(tmpptr, ptr, size);
> > >             else
> > >                     memcpy(ptr, tmpptr, size);
> >
> > in the same spirit as patch 1/3, memcpy_to_page() and memcpy_from_page()
> > looks good candidate to avoid code duplication.
>
> Hello Christophe, Todd,
>
> I had thought to use memcpy_to_page() and memcpy_from_page() (exactly as I
> did in other conversions I have been working on during the latest couple of
> weeks).
>
> However, I decided to avoid to use them for I should also have deleted the
> comment which is before "kunmap_local(base_ptr);".
>
> I don't know how much Maintainers think it is necessary to make readers
> notice that "kunmap_local() takes care of flushing the cache []" (exactly
> as kunmap_atomic() does). Actually I'd delete that comment that looks
> redundant and unnecessary to me, but I cannot know if Todd wants it to
> remain there.
>
> @Todd: Can you please say what you think about this topic? Should I leave
> the patch as-is or I should use memcpy_{to,from}_page() and delete that
> comment?

I'm fine with using memcpy_{to,from}_page() and removing the comment.

>
> I won't send any v2 unless I have your confirmation.
>
> Thanks,
>
> Fabio
>
> >
> > Not checked in details, but looks mostly the same.
> >
> > Just my 2c.
> >
> > CJ
> >
> > >             /*
> > > -            * kunmap_atomic() takes care of flushing the cache
> > > +            * kunmap_local() takes care of flushing the cache
> > >              * if this device has VIVT cache arch
> > >              */
> > > -           kunmap_atomic(base_ptr);
> > > +           kunmap_local(base_ptr);
> > >             bytes -= size;
> > >             pgoff = 0;
> > >             ptr = ptr + size;
> >
> >
>
>
>
>