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

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Date:   Sat, 23 Apr 2022 08:44:03 -0700
From:   Todd Kjos <tkjos@...gle.com>
To:     "Fabio M. De Francesco" <fmdefrancesco@...il.com>
Cc:     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()

On Sat, Apr 23, 2022 at 3:24 AM Fabio M. De Francesco
<fmdefrancesco@...il.com> wrote:
>
> 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>

Acked-by: Todd Kjos <tkjos@...gle.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);
>                 /*
> -                * 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;
> --
> 2.34.1
>