linux-kernel - Re: [PATCH v2] pm: Replace deprecated kmap_atomic() with kmap_local

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Message-ID: <CAJZ5v0gaaOwkiYH8-HuoAd3BXkruvi00MFp=bkwAUe+X76yggQ@mail.gmail.com>
Date: Tue, 18 Feb 2025 20:40:41 +0100
From: "Rafael J. Wysocki" <rafael@...nel.org>
To: me@...idreaver.com
Cc: "Rafael J . Wysocki" <rafael@...nel.org>, Pavel Machek <pavel@....cz>, Len Brown <len.brown@...el.com>, 
	linux-pm@...r.kernel.org, linux-kernel@...r.kernel.org, 
	Ira Weiny <ira.weiny@...el.com>
Subject: Re: [PATCH v2] pm: Replace deprecated kmap_atomic() with kmap_local_page()

On Sun, Jan 12, 2025 at 4:27 PM David Reaver <me@...idreaver.com> wrote:
>
> kmap_atomic() is deprecated and should be replaced with kmap_local_page()
> [1][2]. kmap_local_page() is faster in kernels with HIGHMEM enabled, can
> take page faults, and allows preemption.
>
> According to [2], this replacement is safe as long as the code between
> kmap_atomic() and kunmap_atomic() does not implicitly depend on disabling
> page faults or preemption. In all of the call sites in this patch, the only
> thing happening between mapping and unmapping pages is copy_page() calls,
> and I don't suspect they depend on disabling page faults or preemption.
>
> [1] https://lwn.net/Articles/836144/
> [2] https://docs.kernel.org/mm/highmem.html#temporary-virtual-mappings
>
> Signed-off-by: David Reaver <me@...idreaver.com>
> ---
>
> Apologies, I tried testing a CONFIG_HIGHMEM kernel in QEMU and clearly made
> an error because there is an obvious typo. Any pointers to test setups for
> testing CONFIG_HIGHMEM in a VM would be appreciated! Here is v2 of the
> patch.
>
> V1 -> V2: Fix typo kmap_page_local() -> kmap_local_page()
>
>  kernel/power/snapshot.c | 16 ++++++++--------
>  1 file changed, 8 insertions(+), 8 deletions(-)
>
> diff --git a/kernel/power/snapshot.c b/kernel/power/snapshot.c
> index c9fb559a6399..87f4dde4a49d 100644
> --- a/kernel/power/snapshot.c
> +++ b/kernel/power/snapshot.c
> @@ -2270,9 +2270,9 @@ int snapshot_read_next(struct snapshot_handle *handle)
>                          */
>                         void *kaddr;
>
> -                       kaddr = kmap_atomic(page);
> +                       kaddr = kmap_local_page(page);
>                         copy_page(buffer, kaddr);
> -                       kunmap_atomic(kaddr);
> +                       kunmap_local(kaddr);
>                         handle->buffer = buffer;
>                 } else {
>                         handle->buffer = page_address(page);
> @@ -2561,9 +2561,9 @@ static void copy_last_highmem_page(void)
>         if (last_highmem_page) {
>                 void *dst;
>
> -               dst = kmap_atomic(last_highmem_page);
> +               dst = kmap_local_page(last_highmem_page);
>                 copy_page(dst, buffer);
> -               kunmap_atomic(dst);
> +               kunmap_local(dst);
>                 last_highmem_page = NULL;
>         }
>  }
> @@ -2881,13 +2881,13 @@ static inline void swap_two_pages_data(struct page *p1, struct page *p2,
>  {
>         void *kaddr1, *kaddr2;
>
> -       kaddr1 = kmap_atomic(p1);
> -       kaddr2 = kmap_atomic(p2);
> +       kaddr1 = kmap_local_page(p1);
> +       kaddr2 = kmap_local_page(p2);
>         copy_page(buf, kaddr1);
>         copy_page(kaddr1, kaddr2);
>         copy_page(kaddr2, buf);
> -       kunmap_atomic(kaddr2);
> -       kunmap_atomic(kaddr1);
> +       kunmap_local(kaddr2);
> +       kunmap_local(kaddr1);
>  }
>
>  /**

Applied as 6.15 material, thanks!