+ mm-ksm-use-kmap_local_page-in-calc_checksum.patch added to mm-unstable branch

+ mm-ksm-use-kmap_local_page-in-calc_checksum.patch added to mm-unstable branch

[Andrew Morton]

The patch titled
     Subject: mm/ksm: use kmap_local_page() in calc_checksum()
has been added to the -mm mm-unstable branch.  Its filename is
     mm-ksm-use-kmap_local_page-in-calc_checksum.patch

This patch will shortly appear at
     https://git.kernel.org/pub/scm/linux/kernel/git/akpm/25-new.git/tree/patches/mm-ksm-use-kmap_local_page-in-calc_checksum.patch

This patch will later appear in the mm-unstable branch at
    git://git.kernel.org/pub/scm/linux/kernel/git/akpm/mm

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------------------------------------------------------
From: "Fabio M. De Francesco" <fabio.maria.de.francesco@intel.com>
Subject: mm/ksm: use kmap_local_page() in calc_checksum()
Date: Mon, 20 Nov 2023 15:18:44 +0100

kmap_atomic() has been deprecated in favor of kmap_local_page().

Therefore, replace kmap_atomic() with kmap_local_page() in
calc_checksum().

kmap_atomic() is implemented like a kmap_local_page() which also disables
page-faults and preemption (the latter only in !PREEMPT_RT kernels).  The
kernel virtual addresses returned by these two API are only valid in the
context of the callers (i.e., they cannot be handed to other threads).

With kmap_local_page() the mappings are per thread and CPU local like in
kmap_atomic(); however, they can handle page-faults and can be called from
any context (including interrupts).  The tasks that call kmap_local_page()
can be preempted and, when they are scheduled to run again, the kernel
virtual addresses are restored and are still valid.

In calc_checksum(), the block of code between the mapping and un-mapping
does not depend on the above-mentioned side effects of kmap_aatomic(), so
that a mere replacements of the old API with the new one is all that is
required (i.e., there is no need to explicitly call pagefault_disable()
and/or preempt_disable()).

Link: https://lkml.kernel.org/r/20231120141855.6761-1-fmdefrancesco@gmail.com
Signed-off-by: Fabio M. De Francesco <fabio.maria.de.francesco@linux.intel.com>
Cc: Ira Weiny <ira.weiny@intel.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
---

 mm/ksm.c |    4 ++--
 1 file changed, 2 insertions(+), 2 deletions(-)

--- a/mm/ksm.c~mm-ksm-use-kmap_local_page-in-calc_checksum
+++ a/mm/ksm.c
@@ -1099,9 +1099,9 @@ error:
 static u32 calc_checksum(struct page *page)
 {
 	u32 checksum;
-	void *addr = kmap_atomic(page);
+	void *addr = kmap_local_page(page);
 	checksum = xxhash(addr, PAGE_SIZE, 0);
-	kunmap_atomic(addr);
+	kunmap_local(addr);
 	return checksum;
 }
 
_

Patches currently in -mm which might be from fabio.maria.de.francesco@intel.com are

mm-util-use-kmap_local_page-in-memcmp_pages.patch
mm-ksm-use-kmap_local_page-in-calc_checksum.patch