linux-kernel - Re: [PATCH linux-next] kernel/power: Use kmap_local

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Date:   Mon, 11 Dec 2023 17:04:04 +0100
From:   "Rafael J. Wysocki" <rafael@...nel.org>
To:     chenguanxi11234@....com
Cc:     rafael@...nel.org, pavel@....cz, len.brown@...el.com,
        linux-pm@...r.kernel.org, linux-kernel@...r.kernel.org,
        Chen Haonan <chen.haonan2@....com.cn>
Subject: Re: [PATCH linux-next] kernel/power: Use kmap_local_page() in snapshot.c

On Mon, Dec 11, 2023 at 3:52 PM <chenguanxi11234@....com> wrote:
>
> From: Chen Haonan <chen.haonan2@....com.cn>
>
> kmap_atomic() has been deprecated in favor of kmap_local_page().
>
> Each call to kmap_atomic() in the kernel creates a non-preemptable
> segment and disables the missing page exception. This can be one of
> the sources of unexpected latency. Therefore users should choose
> kmap_local_page() over kmap_atomic().

Do you realize that the code being modified runs with only one CPU
online and with interrupts off on that CPU?

> Signed-off-by: Chen Haonan <chen.haonan2@....com.cn>
> ---
>  kernel/power/snapshot.c | 12 ++++++------
>  1 file changed, 6 insertions(+), 6 deletions(-)
> diff --git a/kernel/power/snapshot.c b/kernel/power/snapshot.c
> index 50a15408c3fc..feef0d4d3288 100644
> --- a/kernel/power/snapshot.c
> +++ b/kernel/power/snapshot.c
> @@ -1487,11 +1487,11 @@ static bool copy_data_page(unsigned long dst_pfn, unsigned long src_pfn)
>         s_page = pfn_to_page(src_pfn);
>         d_page = pfn_to_page(dst_pfn);
>         if (PageHighMem(s_page)) {
> -               src = kmap_atomic(s_page);
> -               dst = kmap_atomic(d_page);
> +               src = kmap_local_page(s_page);
> +               dst = kmap_local_page(d_page);
>                 zeros_only = do_copy_page(dst, src);
> -               kunmap_atomic(dst);
> -               kunmap_atomic(src);
> +               kunmap_local(dst);
> +               kunmap_local(src);
>         } else {
>                 if (PageHighMem(d_page)) {
>                         /*
> @@ -1499,9 +1499,9 @@ static bool copy_data_page(unsigned long dst_pfn, unsigned long src_pfn)
>                          * data modified by kmap_atomic()
>                          */
>                         zeros_only = safe_copy_page(buffer, s_page);
> -                       dst = kmap_atomic(d_page);
> +                       dst = kmap_local_page(d_page);
>                         copy_page(dst, buffer);
> -                       kunmap_atomic(dst);
> +                       kunmap_local(dst);
>                 } else {
>                         zeros_only = safe_copy_page(page_address(d_page), s_page);
>                 }
> --
> 2.25.1
>