linux-kernel - Re: [PATCH] virtio: console: Replace deprecated kmap_atomic with kmap_local

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Message-ID: <6781383df520_167faf2946@iweiny-mobl.notmuch>
Date: Fri, 10 Jan 2025 09:09:49 -0600
From: Ira Weiny <ira.weiny@...el.com>
To: David Reaver <me@...idreaver.com>, Amit Shah <amit@...nel.org>, "Arnd
 Bergmann" <arnd@...db.de>, Greg Kroah-Hartman <gregkh@...uxfoundation.org>
CC: David Reaver <me@...idreaver.com>, <virtualization@...ts.linux.dev>,
	<linux-kernel@...r.kernel.org>, Ira Weiny <ira.weiny@...el.com>, "Thomas
 Gleixner" <tglx@...utronix.de>
Subject: Re: [PATCH] virtio: console: Replace deprecated kmap_atomic with
 kmap_local_page

David Reaver 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.

Thanks for taking these on!  It is good to see some progress here.

> 
> 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 this patch, the only thing happening between
> mapping and unmapping the page is a memcpy, and I don't suspect it depends
> 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>
> ---
>  drivers/char/virtio_console.c | 4 ++--
>  1 file changed, 2 insertions(+), 2 deletions(-)
> 
> diff --git a/drivers/char/virtio_console.c b/drivers/char/virtio_console.c
> index c62b208b42f1..24442485e73e 100644
> --- a/drivers/char/virtio_console.c
> +++ b/drivers/char/virtio_console.c
> @@ -883,9 +883,9 @@ static int pipe_to_sg(struct pipe_inode_info *pipe, struct pipe_buffer *buf,
>  		if (len + offset > PAGE_SIZE)
>  			len = PAGE_SIZE - offset;
>  
> -		src = kmap_atomic(buf->page);
> +		src = kmap_local_page(buf->page);
>  		memcpy(page_address(page) + offset, src + buf->offset, len);
> -		kunmap_atomic(src);
> +		kunmap_local(src);

memcpy_to_page()

Ira

[snip]