arm64: mte: Tags-aware aware memcmp_pages() implementation

When the Memory Tagging Extension is enabled, two pages are identical
only if both their data and tags are identical.

Make the generic memcmp_pages() a __weak function and add an
arm64-specific implementation which returns non-zero if any of the two
pages contain valid MTE tags (PG_mte_tagged set). There isn't much
benefit in comparing the tags of two pages since these are normally used
for heap allocations and likely to differ anyway.

Co-developed-by: Vincenzo Frascino <vincenzo.frascino@arm.com>
Signed-off-by: Vincenzo Frascino <vincenzo.frascino@arm.com>
Signed-off-by: Catalin Marinas <catalin.marinas@arm.com>
Cc: Will Deacon <will@kernel.org>

arch/arm64/kernel/mte.c

@@ -5,6 +5,7 @@
 
 #include <linux/bitops.h>
 #include <linux/mm.h>
+#include <linux/string.h>
 #include <linux/thread_info.h>
 
 #include <asm/cpufeature.h>
@@ -23,6 +24,31 @@ void mte_sync_tags(pte_t *ptep, pte_t pte)
 	}
 }
 
+int memcmp_pages(struct page *page1, struct page *page2)
+{
+	char *addr1, *addr2;
+	int ret;
+
+	addr1 = page_address(page1);
+	addr2 = page_address(page2);
+	ret = memcmp(addr1, addr2, PAGE_SIZE);
+
+	if (!system_supports_mte() || ret)
+		return ret;
+
+	/*
+	 * If the page content is identical but at least one of the pages is
+	 * tagged, return non-zero to avoid KSM merging. If only one of the
+	 * pages is tagged, set_pte_at() may zero or change the tags of the
+	 * other page via mte_sync_tags().
+	 */
+	if (test_bit(PG_mte_tagged, &page1->flags) ||
+	    test_bit(PG_mte_tagged, &page2->flags))
+		return addr1 != addr2;
+
+	return ret;
+}
+
 void flush_mte_state(void)
 {
 	if (!system_supports_mte())

mm/util.c

@@ -957,7 +957,7 @@ out:
 	return res;
 }
 
-int memcmp_pages(struct page *page1, struct page *page2)
+int __weak memcmp_pages(struct page *page1, struct page *page2)
 {
 	char *addr1, *addr2;
 	int ret;