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

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Message-ID: <cd172334-d1d1-4ace-b4b5-a7dd42c6e58f@redhat.com>
Date: Tue, 3 Sep 2024 23:35:36 +0200
From: David Hildenbrand <david@...hat.com>
To: Yang Shi <yang@...amperecomputing.com>, catalin.marinas@....com,
 will@...nel.org, muchun.song@...ux.dev, akpm@...ux-foundation.org
Cc: linux-arm-kernel@...ts.infradead.org, linux-mm@...ck.org,
 linux-kernel@...r.kernel.org
Subject: Re: [v2 PATCH 1/2] hugetlb: arm64: add mte support

On 03.09.24 18:46, Yang Shi wrote:
> 
> 
> 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@...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);

folio->flags

> }
> 
> bool page_mte_tagged(struct page *page)
> {
>       struct folio *folio = page_folio(page);
> 
>       VM_WARN_ON_ONCE(folio_test_hugetlb(folio));

Yes, but better as

VM_WARN_ON_ONCE(folio_test_hugetlb(page_folio(page)));

> 
>       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.

There were some discussions on it, and it's tricky. So maybe we should 
really just have folio_test_hugetlb_mte_tagged() etc. for now

-- 
Cheers,

David / dhildenb