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

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Message-ID: <20250109035904.168345-1-me@davidreaver.com>
Date: Wed,  8 Jan 2025 19:59:02 -0800
From: David Reaver <me@...idreaver.com>
To: 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: [PATCH] virtio: console: Replace deprecated kmap_atomic with kmap_local_page

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 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);
 
 		sg_set_page(&(sgl->sg[sgl->n]), page, len, offset);
 	}