Re: [v2 PATCH 1/2] hugetlb: arm64: add mte support

[Yang Shi <yang@os.amperecomputing.com>]

On 9/2/24 7:33 AM, David Hildenbrand wrote:
> On 21.08.24 20:47, Yang Shi wrote:
>> Enable MTE support for hugetlb.
>>
>> The MTE page flags will be set on the head page only.  When copying
>> hugetlb folio, the tags for all tail pages will be copied when copying
>> head page.
>>
>> When freeing hugetlb folio, the MTE flags will be cleared.
>>
>> Signed-off-by: Yang Shi <yang@os.amperecomputing.com>
>> ---
>>   arch/arm64/include/asm/hugetlb.h | 11 ++++++++++-
>>   arch/arm64/include/asm/mman.h    |  3 ++-
>>   arch/arm64/kernel/hibernate.c    |  7 +++++++
>>   arch/arm64/kernel/mte.c          | 25 +++++++++++++++++++++++--
>>   arch/arm64/kvm/guest.c           | 16 +++++++++++++---
>>   arch/arm64/kvm/mmu.c             | 11 +++++++++++
>>   arch/arm64/mm/copypage.c         | 25 +++++++++++++++++++++++--
>>   fs/hugetlbfs/inode.c             |  2 +-
>>   8 files changed, 90 insertions(+), 10 deletions(-)
>>
>> v2: * Reimplemented the patch to fix the comments from Catalin.
>>      * Added test cases (patch #2) per Catalin.
>>
>> diff --git a/arch/arm64/include/asm/hugetlb.h 
>> b/arch/arm64/include/asm/hugetlb.h
>> index 293f880865e8..00a1f75d40ee 100644
>> --- a/arch/arm64/include/asm/hugetlb.h
>> +++ b/arch/arm64/include/asm/hugetlb.h
>> @@ -11,6 +11,7 @@
>>   #define __ASM_HUGETLB_H
>>     #include <asm/cacheflush.h>
>> +#include <asm/mte.h>
>>   #include <asm/page.h>
>>     #ifdef CONFIG_ARCH_ENABLE_HUGEPAGE_MIGRATION
>> @@ -20,7 +21,15 @@ extern bool 
>> arch_hugetlb_migration_supported(struct hstate *h);
>>     static inline void arch_clear_hugetlb_flags(struct folio *folio)
>>   {
>> -    clear_bit(PG_dcache_clean, &folio->flags);
>> +    const unsigned long clear_flags = BIT(PG_dcache_clean) |
>> +        BIT(PG_mte_tagged) | BIT(PG_mte_lock);
>> +
>> +    if (!system_supports_mte()) {
>> +        clear_bit(PG_dcache_clean, &folio->flags);
>> +        return;
>> +    }
>> +
>> +    folio->flags &= ~clear_flags;
>>   }
>>   #define arch_clear_hugetlb_flags arch_clear_hugetlb_flags
>>   diff --git a/arch/arm64/include/asm/mman.h 
>> b/arch/arm64/include/asm/mman.h
>> index 5966ee4a6154..304dfc499e68 100644
>> --- a/arch/arm64/include/asm/mman.h
>> +++ b/arch/arm64/include/asm/mman.h
>> @@ -28,7 +28,8 @@ static inline unsigned long 
>> arch_calc_vm_flag_bits(unsigned long flags)
>>        * backed by tags-capable memory. The vm_flags may be 
>> overridden by a
>>        * filesystem supporting MTE (RAM-based).
>>        */
>> -    if (system_supports_mte() && (flags & MAP_ANONYMOUS))
>> +    if (system_supports_mte() &&
>> +        (flags & (MAP_ANONYMOUS | MAP_HUGETLB)))
>>           return VM_MTE_ALLOWED;
>>         return 0;
>> diff --git a/arch/arm64/kernel/hibernate.c 
>> b/arch/arm64/kernel/hibernate.c
>> index 02870beb271e..722e76f29141 100644
>> --- a/arch/arm64/kernel/hibernate.c
>> +++ b/arch/arm64/kernel/hibernate.c
>> @@ -266,10 +266,17 @@ static int swsusp_mte_save_tags(void)
>>           max_zone_pfn = zone_end_pfn(zone);
>>           for (pfn = zone->zone_start_pfn; pfn < max_zone_pfn; pfn++) {
>>               struct page *page = pfn_to_online_page(pfn);
>> +            struct folio *folio;
>>                 if (!page)
>>                   continue;
>>   +            folio = page_folio(page);
>> +
>> +            if (folio_test_hugetlb(folio) &&
>> +                !page_mte_tagged(&folio->page))
>> +                continue;
>
> Can we have folio_test_mte_tagged() whereby you make sure that only 
> folio_test_hugetlb() uses it for now (VM_WARN_ON_ONCE) and then make 
> sure that nobody uses page_mte_tagged() on hugetlb folios 
> (VM_WARN_ON_ONCE)?


IIUC, you mean something like the below?

bool folio_test_mte_tagged(struct folio *folio)
{
     VM_WARN_ON_ONCE(!folio_test_hugetlb(folio));

     return test_bit(PG_mte_tagged, &folio->page->flags);
}

bool page_mte_tagged(struct page *page)
{
     struct folio *folio = page_folio(page);

     VM_WARN_ON_ONCE(folio_test_hugetlb(folio));

     return test_bit(PG_mte_tagged, &pge->flags);
}

>
> Same for folio_set_mte_tagged() and other functions. We could slap a 
> "hugetlb" into the function names, but maybe in the future we'll be 
> able to use a single flag per folio (I know, it's complicated ...).

A single flag per folio should be the future direction, but I haven't 
done the research so can't tell how complicated it will be.