Re: [PATCH 1/1] mm/thp: fix MTE tag mismatch when replacing zero-filled subpages

On 23.09.25 18:14, Catalin Marinas wrote:

On Tue, Sep 23, 2025 at 12:52:06PM +0100, Catalin Marinas wrote:

quoted

I just realised that on arm64 with MTE we won't get any merging with the
zero page even if the user page isn't mapped with PROT_MTE. In
cpu_enable_mte() we zero the tags in the zero page and set
PG_mte_tagged. The reason is that we want to use the zero page with
PROT_MTE mappings (until tag setting causes CoW). Hmm, the arm64
memcmp_pages() messed up KSM merging with the zero page even before this
patch.

[...]

quoted

diff --git a/arch/arm64/kernel/mte.c b/arch/arm64/kernel/mte.c
index e5e773844889..72a1dfc54659 100644
--- a/arch/arm64/kernel/mte.c
+++ b/arch/arm64/kernel/mte.c

@@ -73,6 +73,8 @@ int memcmp_pages(struct page *page1, struct page *page2)
  {
  	char *addr1, *addr2;
  	int ret;
+	bool page1_tagged = page_mte_tagged(page1) && !is_zero_page(page1);
+	bool page2_tagged = page_mte_tagged(page2) && !is_zero_page(page2);
  
  	addr1 = page_address(page1);
  	addr2 = page_address(page2);

@@ -83,11 +85,10 @@ int memcmp_pages(struct page *page1, struct page *page2)
  
  	/*
  	 * 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_ptes() may zero or change the tags of the
-	 * other page via mte_sync_tags().
+	 * tagged, return non-zero to avoid KSM merging. Ignore the zero page
+	 * since it is always tagged with the tags cleared.
  	 */
-	if (page_mte_tagged(page1) || page_mte_tagged(page2))
+	if (page1_tagged || page2_tagged)
  		return addr1 != addr2;
  
  	return ret;

Unrelated to this discussion, I got an internal report that Linux hangs
during boot with CONFIG_DEFERRED_STRUCT_PAGE_INIT because
try_page_mte_tagging() locks up on uninitialised page flags.

Since we (always?) map the zero page as pte_special(), set_pte_at()

Yes. (if pte_special is implemented by the arch of course)

won't check if the tags have to be initialised, so we can skip the
PG_mte_tagged altogether. We actually had this code for some time until
we introduced the pte_special() check in set_pte_at().

So alternative patch that also fixes the deferred struct page init (on
the assumptions that the zero page is always mapped as pte_special():

LGTM!

-- 
Cheers

David / dhildenb