linux-kernel - Re: [PATCH 2/3] binder: Use kmap_local_page() in binder_alloc_copy_user_to

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Date:   Sat, 23 Apr 2022 08:43:27 -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 2/3] binder: Use kmap_local_page() in binder_alloc_copy_user_to_buffer()

On Sat, Apr 23, 2022 at 3:24 AM Fabio M. De Francesco
<fmdefrancesco@...il.com> wrote:
>
> The use of kmap() is being deprecated in favor of kmap_local_page()
> where it is feasible. With kmap_local_page(), the mapping is per
> thread, CPU local and not globally visible.
>
> binder_alloc_copy_user_to_buffer() is a function where the use of
> kmap_local_page() in place of kmap() is correctly suited because
> the mapping is local to the thread.
>
> Therefore, use kmap_local_page() / kunmap_local().
>
> Signed-off-by: Fabio M. De Francesco <fmdefrancesco@...il.com>

Acked-by: Todd Kjos <tkjos@...gle.com>

> ---
>  drivers/android/binder_alloc.c | 4 ++--
>  1 file changed, 2 insertions(+), 2 deletions(-)
>
> diff --git a/drivers/android/binder_alloc.c b/drivers/android/binder_alloc.c
> index 0b3f2f569053..0875c463c002 100644
> --- a/drivers/android/binder_alloc.c
> +++ b/drivers/android/binder_alloc.c
> @@ -1217,9 +1217,9 @@ binder_alloc_copy_user_to_buffer(struct binder_alloc *alloc,
>                 page = binder_alloc_get_page(alloc, buffer,
>                                              buffer_offset, &pgoff);
>                 size = min_t(size_t, bytes, PAGE_SIZE - pgoff);
> -               kptr = kmap(page) + pgoff;
> +               kptr = kmap_local_page(page) + pgoff;
>                 ret = copy_from_user(kptr, from, size);
> -               kunmap(page);
> +               kunmap_local(kptr);
>                 if (ret)
>                         return bytes - size + ret;
>                 bytes -= size;
> --
> 2.34.1
>