Re: [PATCH] mm/huge_memory: restrict __GFP_ZEROTAGS to HW tagging architectures

["David Hildenbrand (Red Hat)" <davidhildenbrandkernel@gmail.com>]

On 10.11.25 10:48, Jan Polensky wrote:
> On Mon, Nov 10, 2025 at 10:09:31AM +0100, David Hildenbrand (Red Hat) wrote:
>> On 09.11.25 01:36, Jan Polensky wrote:
>>> The previous change added __GFP_ZEROTAGS when allocating the huge zero
>>> folio to ensure tag initialization for arm64 with MTE enabled. However,
>>> on s390 this flag is unnecessary and triggers a regression
>>> (observed as a crash during repeated 'dnf makecache').
>>>
>>> Restrict the use of __GFP_ZEROTAGS to architectures that support
>>> hardware memory tagging (currently arm64 with MTE or KASAN HW tags).
>>> This avoids unintended side effects on other platforms.
>>>
>>> Fixes: 1579227fe0f0 ("mm/huge_memory: initialise the tags of the huge zero folio")
>>> Link: https://lore.kernel.org/r/20251031170133.280742-1-catalin.marinas@arm.com
>>> Signed-off-by: Jan Polensky <japo@linux.ibm.com>
>>> ---
>>>    mm/huge_memory.c | 9 +++++----
>>>    1 file changed, 5 insertions(+), 4 deletions(-)
>>>
>>> diff --git a/mm/huge_memory.c b/mm/huge_memory.c
>>> index aae283b00857..0c1794656d7a 100644
>>> --- a/mm/huge_memory.c
>>> +++ b/mm/huge_memory.c
>>> @@ -209,14 +209,15 @@ unsigned long __thp_vma_allowable_orders(struct vm_area_struct *vma,
>>>
>>>    static bool get_huge_zero_folio(void)
>>>    {
>>> +	gfp_t gfp = (GFP_TRANSHUGE | __GFP_ZERO) & ~__GFP_MOVABLE;
>>>    	struct folio *zero_folio;
>>>    retry:
>>>    	if (likely(atomic_inc_not_zero(&huge_zero_refcount)))
>>>    		return true;
>>> -
>>> -	zero_folio = folio_alloc((GFP_TRANSHUGE | __GFP_ZERO | __GFP_ZEROTAGS) &
>>> -				 ~__GFP_MOVABLE,
>>> -			HPAGE_PMD_ORDER);
>>> +#if IS_ENABLED(CONFIG_KASAN_HW_TAGS) || IS_ENABLED(CONFIG_ARM64_MTE)
>>> +	gfp |= __GFP_ZEROTAGS;
>>> +#endif
>>
>> That looks like the wrong approach. If an architecture does not support
>> __GFP_ZEROTAGS it should not trigger anything. __GFP_ZEROTAGS should be ignored.
>>
>> I think the problem is that post_alloc_hook() does
>>
>> if (zero_tags) {
>> 	/* Initialize both memory and memory tags. */
>> 	for (i = 0; i != 1 << order; ++i)
>> 		tag_clear_highpage(page + i);
>>
>> 	/* Take note that memory was initialized by the loop above. */
>> 	init = false;
>> }
>>
>> And tag_clear_highpage() is a NOP on other architectures.
>>
>> Gah.
>>
>> I wonder if the following would work:
>>
>>
>> diff --git a/include/linux/gfp_types.h b/include/linux/gfp_types.h
>> index 65db9349f9053..56b82e116cb79 100644
>> --- a/include/linux/gfp_types.h
>> +++ b/include/linux/gfp_types.h
>> @@ -47,7 +47,9 @@ enum {
>>          ___GFP_HARDWALL_BIT,
>>          ___GFP_THISNODE_BIT,
>>          ___GFP_ACCOUNT_BIT,
>> +#ifdef __HAVE_ARCH_TAG_CLEAR_HIGHPAGE
>>          ___GFP_ZEROTAGS_BIT,
>> +#endif
>>   #ifdef CONFIG_KASAN_HW_TAGS
>>          ___GFP_SKIP_ZERO_BIT,
>>          ___GFP_SKIP_KASAN_BIT,
>> @@ -85,7 +87,11 @@ enum {
>>   #define ___GFP_HARDWALL                BIT(___GFP_HARDWALL_BIT)
>>   #define ___GFP_THISNODE                BIT(___GFP_THISNODE_BIT)
>>   #define ___GFP_ACCOUNT         BIT(___GFP_ACCOUNT_BIT)
>> +#ifdef __HAVE_ARCH_TAG_CLEAR_HIGHPAGE
>>   #define ___GFP_ZEROTAGS                BIT(___GFP_ZEROTAGS_BIT)
>> +#else
>> +#define ___GFP_ZEROTAGS                0
>> +#endif
>>   #ifdef CONFIG_KASAN_HW_TAGS
>>   #define ___GFP_SKIP_ZERO       BIT(___GFP_SKIP_ZERO_BIT)
>>   #define ___GFP_SKIP_KASAN      BIT(___GFP_SKIP_KASAN_BIT)
>>
>>
>> Likely we'd have to make __HAVE_ARCH_TAG_CLEAR_HIGHPAGE a proper
>> kconfig option.
>>
>>
>> Then we could turn the default implementation of
>> tag_clear_highpage() into a BUILD_BUG.
>>
> I'd like to suggest to keep the enum untouched and only use the second
> part of your suggestion.

Why? We also do that for CONFIG_KASAN_HW_TAGS, CONFIG_LOCKDEP and 
CONFIG_SLAB_OBJ_EXT.

> Which works by the way for our arch (s390).
> 
>   include/linux/gfp_types.h | 4 ++++
>   1 file changed, 4 insertions(+)
> 
> diff --git a/include/linux/gfp_types.h b/include/linux/gfp_types.h
> index 65db9349f905..c12d8a601bb3 100644
> --- a/include/linux/gfp_types.h
> +++ b/include/linux/gfp_types.h
> @@ -85,7 +85,11 @@ enum {
>   #define ___GFP_HARDWALL        BIT(___GFP_HARDWALL_BIT)
>   #define ___GFP_THISNODE        BIT(___GFP_THISNODE_BIT)
>   #define ___GFP_ACCOUNT     BIT(___GFP_ACCOUNT_BIT)
> +#ifdef __HAVE_ARCH_TAG_CLEAR_HIGHPAGE
>   #define ___GFP_ZEROTAGS        BIT(___GFP_ZEROTAGS_BIT)
> +#else
> +#define ___GFP_ZEROTAGS        0
> +#endif
>   #ifdef CONFIG_KASAN_HW_TAGS
>   #define ___GFP_SKIP_ZERO   BIT(___GFP_SKIP_ZERO_BIT)
>   #define ___GFP_SKIP_KASAN  BIT(___GFP_SKIP_KASAN_BIT)
> 
> This solution would be sufficient from my side, and I would appreciate a
> quick application if there are no objections.

As raised, to be sure that __HAVE_ARCH_TAG_CLEAR_HIGHPAGE is always seen 
early in that file, it should likely become a CONFIG_ thing.