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

[Lance Yang <lance.yang@linux.dev>]

On 2025/9/24 00:14, Catalin Marinas wrote:
> 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():

I can confirm that this alternative patch also works correctly; my tests
for MTE all pass ;)

This looks like a better fix since it solves the boot hang issue too.

> 
> 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();
>