+ mm-util-use-kmap_local_page-in-memcmp_pages.patch added to mm-unstable branch

+ mm-util-use-kmap_local_page-in-memcmp_pages.patch added to mm-unstable branch

[Andrew Morton]

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

This patch will shortly appear at
     https://git.kernel.org/pub/scm/linux/kernel/git/akpm/25-new.git/tree/patches/mm-util-use-kmap_local_page-in-memcmp_pages.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 De Francesco <fabio.maria.de.francesco@intel.com>
Subject: mm/util: use kmap_local_page() in memcmp_pages()
Date: Mon, 20 Nov 2023 15:15:27 +0100

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

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

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 memcmp_pages(), the block of code between the mapping and un-mapping
does not depend on the above-mentioned side effects of kmap_aatomic(), so
that 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/20231120141554.6612-1-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/util.c |    8 ++++----
 1 file changed, 4 insertions(+), 4 deletions(-)

--- a/mm/util.c~mm-util-use-kmap_local_page-in-memcmp_pages
+++ a/mm/util.c
@@ -1047,11 +1047,11 @@ int __weak memcmp_pages(struct page *pag
 	char *addr1, *addr2;
 	int ret;
 
-	addr1 = kmap_atomic(page1);
-	addr2 = kmap_atomic(page2);
+	addr1 = kmap_local_page(page1);
+	addr2 = kmap_local_page(page2);
 	ret = memcmp(addr1, addr2, PAGE_SIZE);
-	kunmap_atomic(addr2);
-	kunmap_atomic(addr1);
+	kunmap_local(addr2);
+	kunmap_local(addr1);
 	return ret;
 }
 
_

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