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

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Message-ID: <fad918d3-6923-5bec-7830-5cddf7a725d6@wanadoo.fr>
Date:   Sat, 23 Apr 2022 18:02:48 +0200
From:   Christophe JAILLET <christophe.jaillet@...adoo.fr>
To:     "Fabio M. De Francesco" <fmdefrancesco@...il.com>,
        Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        Arve Hjønnevåg <arve@...roid.com>,
        Todd Kjos <tkjos@...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>, linux-kernel@...r.kernel.org
Subject: Re: [PATCH 3/3] binder: Use kmap_local_page() in
 binder_alloc_get_page()

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.

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;