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[v9,12/29] arm64: mte: Tags-aware aware memcmp_pages() implementation

Message ID 20200904103029.32083-13-catalin.marinas@arm.com (mailing list archive)
State New, archived
Headers show
Series arm64: Memory Tagging Extension user-space support | expand

Commit Message

Catalin Marinas Sept. 4, 2020, 10:30 a.m. UTC
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>

    - Remove page tag comparison. This is not very useful to detect
      identical pages as long as set_pte_at() can zero the tags on a page
      without copy-on-write if mapped with PROT_MTE. This can be improved
      if a real case appears but it's unlikely for heap pages to be
      identical across multiple processes.
    - Move the memcmp_pages() function to mte.c.

 arch/arm64/kernel/mte.c | 26 ++++++++++++++++++++++++++
 mm/util.c               |  2 +-
 2 files changed, 27 insertions(+), 1 deletion(-)
diff mbox series


diff --git a/arch/arm64/kernel/mte.c b/arch/arm64/kernel/mte.c
index 5bf9bbed5a25..5f54fd140610 100644
--- a/arch/arm64/kernel/mte.c
+++ b/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())
diff --git a/mm/util.c b/mm/util.c
index 5ef378a2a038..4e21fe7eae27 100644
--- a/mm/util.c
+++ b/mm/util.c
@@ -957,7 +957,7 @@  int get_cmdline(struct task_struct *task, char *buffer, int buflen)
 	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;