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

[Catalin Marinas <catalin.marinas@arm.com>]

On Tue, Sep 23, 2025 at 12:52:06PM +0100, Catalin Marinas wrote:
> 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.
[...]
> 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()
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():

diff --git a/arch/arm64/kernel/cpufeature.c b/arch/arm64/kernel/cpufeature.c
index 7b78c95a9017..e325ba34f45c 100644
--- a/arch/arm64/kernel/cpufeature.c
+++ b/arch/arm64/kernel/cpufeature.c
@@ -2419,17 +2419,21 @@ static void bti_enable(const struct arm64_cpu_capabilities *__unused)
 #ifdef CONFIG_ARM64_MTE
 static void cpu_enable_mte(struct arm64_cpu_capabilities const *cap)
 {
+	static bool cleared_zero_page = false;
+
 	sysreg_clear_set(sctlr_el1, 0, SCTLR_ELx_ATA | SCTLR_EL1_ATA0);
 
 	mte_cpu_setup();
 
 	/*
 	 * Clear the tags in the zero page. This needs to be done via the
-	 * linear map which has the Tagged attribute.
+	 * linear map which has the Tagged attribute. Since this page is
+	 * always mapped as pte_special(), set_pte_at() will not attempt to
+	 * clear the tags or set PG_mte_tagged.
 	 */
-	if (try_page_mte_tagging(ZERO_PAGE(0))) {
+	if (!cleared_zero_page) {
+		cleared_zero_page = true;
 		mte_clear_page_tags(lm_alias(empty_zero_page));
-		set_page_mte_tagged(ZERO_PAGE(0));
 	}
 
 	kasan_init_hw_tags_cpu();

-- 
Catalin