mm/util: use kmap_local_page() in memcmp_pages()

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

@@ -1047,11 +1047,11 @@ int __weak memcmp_pages(struct page *page1, struct page *page2)
 	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;
 }