linux-kernel - Re: [PATCH 3/3] binder: Use kmap_local_page() in binder_alloc_get

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Date:   Sun, 24 Apr 2022 11:39:57 +0200
From:   "Fabio M. De Francesco" <fmdefrancesco@...il.com>
To:     Todd Kjos <tkjos@...roid.com>, linux-kernel@...r.kernel.org,
        Christophe JAILLET <christophe.jaillet@...adoo.fr>
Cc:     Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        Arve Hjønnevåg <arve@...roid.com>,
        Martijn Coenen <maco@...roid.com>,
        Joel Fernandes <joel@...lfernandes.org>,
        Christian Brauner <brauner@...nel.org>,
        Hridya Valsaraju <hridya@...gle.com>,
        Suren Baghdasaryan <surenb@...gle.com>,
        Ira Weiny <ira.weiny@...el.com>
Subject: Re: [PATCH 3/3] binder: Use kmap_local_page() in binder_alloc_get_page()

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@...il.com>
> > ---
> >   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 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;
> 
>