Re: [PATCH v2 0/7] Optimize mprotect for large folios

[Dev Jain <dev.jain@arm.com>]

On 30/04/25 11:52 am, Lance Yang wrote:
> 
> 
> On 2025/4/30 13:42, Dev Jain wrote:
>>
>>
>> On 29/04/25 4:11 pm, Lorenzo Stoakes wrote:
>>> FWIW can confirm the same thing. Lance's fixes sort most of it out, 
>>> but I also
>>> get this error:
> 
> Good catch!
> 
>>>
>>> mm/mprotect.c: In function ‘can_change_ptes_writable’:
>>> mm/mprotect.c:46:22: error: unused variable ‘page’ [-Werror=unused- 
>>> variable]
>>>     46 |         struct page *page;
>>>        |                      ^~~~
>>>
>>> So you also need to remove this unused variable at the stop of
>>> can_change_ptes_writable().
>>
>> Strange that my build didn't catch this.
> 
> Well, to catch unused variable warnings with GCC, enable stricter
> checks by passing -Wunused-variable via KCFLAGS, and use -Werror=unused- 
> variable
> to force the build to fail if any variable is declared but unused:
> 
> make -j$(nproc) KCFLAGS="-Wunused-variable -Werror=unused-variable"

I thought this was the default, but thanks Lance!

> 
> Thanks,
> Lance
> 
> 
>>
>>>
>>> Cheers, Lorenzo
>>>
>>> On Tue, Apr 29, 2025 at 02:32:59PM +0530, Dev Jain wrote:
>>>>
>>>>
>>>> On 29/04/25 12:36 pm, Lance Yang wrote:
>>>>> Hey Dev,
>>>>>
>>>>> Hmm... I also hit the same compilation errors:
>>>>>
>>>>> In file included from ./include/linux/kasan.h:37,
>>>>>                    from ./include/linux/slab.h:260,
>>>>>                    from ./include/linux/crypto.h:19,
>>>>>                    from arch/x86/kernel/asm-offsets.c:9:
>>>>> ./include/linux/pgtable.h: In function ‘modify_prot_start_ptes’:
>>>>> ./include/linux/pgtable.h:905:15: error: implicit declaration of
>>>>> function ‘ptep_modify_prot_start’
>>>>> [-Werror=implicit-function-declaration]
>>>>>     905 |         pte = ptep_modify_prot_start(vma, addr, ptep);
>>>>>         |               ^~~~~~~~~~~~~~~~~~~~~~
>>>>> ./include/linux/pgtable.h:905:15: error: incompatible types when
>>>>> assigning to type ‘pte_t’ from type ‘int’
>>>>> ./include/linux/pgtable.h:909:27: error: incompatible types when
>>>>> assigning to type ‘pte_t’ from type ‘int’
>>>>>     909 |                 tmp_pte = ptep_modify_prot_start(vma, 
>>>>> addr, ptep);
>>>>>         |                           ^~~~~~~~~~~~~~~~~~~~~~
>>>>> ./include/linux/pgtable.h: In function ‘modify_prot_commit_ptes’:
>>>>> ./include/linux/pgtable.h:925:17: error: implicit declaration of
>>>>> function ‘ptep_modify_prot_commit’
>>>>> [-Werror=implicit-function-declaration]
>>>>>     925 |                 ptep_modify_prot_commit(vma, addr, ptep,
>>>>> old_pte, pte);
>>>>>         |                 ^~~~~~~~~~~~~~~~~~~~~~~
>>>>> ./include/linux/pgtable.h: At top level:
>>>>> ./include/linux/pgtable.h:1360:21: error: conflicting types for
>>>>> ‘ptep_modify_prot_start’; have ‘pte_t(struct vm_area_struct *, long
>>>>> unsigned int,  pte_t *)’
>>>>>    1360 | static inline pte_t ptep_modify_prot_start(struct
>>>>> vm_area_struct *vma,
>>>>>         |                     ^~~~~~~~~~~~~~~~~~~~~~
>>>>> ./include/linux/pgtable.h:905:15: note: previous implicit 
>>>>> declaration of
>>>>> ‘ptep_modify_prot_start’ with type ‘int()’
>>>>>     905 |         pte = ptep_modify_prot_start(vma, addr, ptep);
>>>>>         |               ^~~~~~~~~~~~~~~~~~~~~~
>>>>> ./include/linux/pgtable.h:1371:20: warning: conflicting types for
>>>>> ‘ptep_modify_prot_commit’; have ‘void(struct vm_area_struct *, long
>>>>> unsigned int,  pte_t *, pte_t,  pte_t)’
>>>>>    1371 | static inline void ptep_modify_prot_commit(struct
>>>>> vm_area_struct *vma,
>>>>>         |                    ^~~~~~~~~~~~~~~~~~~~~~~
>>>>> ./include/linux/pgtable.h:1371:20: error: static declaration of
>>>>> ‘ptep_modify_prot_commit’ follows non-static declaration
>>>>> ./include/linux/pgtable.h:925:17: note: previous implicit 
>>>>> declaration of
>>>>> ‘ptep_modify_prot_commit’ with type ‘void(struct vm_area_struct *, 
>>>>> long
>>>>> unsigned int,  pte_t *, pte_t,  pte_t)’
>>>>>     925 |                 ptep_modify_prot_commit(vma, addr, ptep,
>>>>> old_pte, pte);
>>>>>         |                 ^~~~~~~~~~~~~~~~~~~~~~~
>>>>>     CC /home/runner/work/mm-test-robot/mm-test-robot/linux/tools/ 
>>>>> objtool/
>>>>> libstring.o
>>>>>     CC /home/runner/work/mm-test-robot/mm-test-robot/linux/tools/ 
>>>>> objtool/
>>>>> libctype.o
>>>>>     CC /home/runner/work/mm-test-robot/mm-test-robot/linux/tools/ 
>>>>> objtool/
>>>>> str_error_r.o
>>>>>     CC /home/runner/work/mm-test-robot/mm-test-robot/linux/tools/ 
>>>>> objtool/
>>>>> librbtree.o
>>>>> cc1: some warnings being treated as errors
>>>>> make[2]: *** [scripts/Makefile.build:98: arch/x86/kernel/asm- 
>>>>> offsets.s]
>>>>> Error 1
>>>>> make[1]: *** [/home/runner/work/mm-test-robot/mm-test-robot/linux/
>>>>> Makefile:1280: prepare0] Error 2
>>>>> make[1]: *** Waiting for unfinished jobs....
>>>>>     LD /home/runner/work/mm-test-robot/mm-test-robot/linux/tools/ 
>>>>> objtool/
>>>>> objtool-in.o
>>>>>     LINK /home/runner/work/mm-test-robot/mm-test-robot/linux/tools/
>>>>> objtool/objtool
>>>>> make: *** [Makefile:248: __sub-make] Error 2
>>>>>
>>>>> Well, modify_prot_start_ptes() calls ptep_modify_prot_start(), but x86
>>>>> does not define __HAVE_ARCH_PTEP_MODIFY_PROT_TRANSACTION. To avoid
>>>>> implicit declaration errors, the architecture-independent
>>>>> ptep_modify_prot_start() must be defined before 
>>>>> modify_prot_start_ptes().
>>>>>
>>>>> With the changes below, things work correctly now ;)
>>>>
>>>> Ah thanks! My bad :(
>>>>
>>>>>
>>>>> diff --git a/include/linux/pgtable.h b/include/linux/pgtable.h
>>>>> index 10cdb87ccecf..d9d6c49bb914 100644
>>>>> --- a/include/linux/pgtable.h
>>>>> +++ b/include/linux/pgtable.h
>>>>> @@ -895,44 +895,6 @@ static inline void wrprotect_ptes(struct 
>>>>> mm_struct
>>>>> *mm, unsigned long addr,
>>>>>    }
>>>>>    #endif
>>>>>
>>>>> -/* See the comment for ptep_modify_prot_start */
>>>>> -#ifndef modify_prot_start_ptes
>>>>> -static inline pte_t modify_prot_start_ptes(struct vm_area_struct 
>>>>> *vma,
>>>>> -        unsigned long addr, pte_t *ptep, unsigned int nr)
>>>>> -{
>>>>> -    pte_t pte, tmp_pte;
>>>>> -
>>>>> -    pte = ptep_modify_prot_start(vma, addr, ptep);
>>>>> -    while (--nr) {
>>>>> -        ptep++;
>>>>> -        addr += PAGE_SIZE;
>>>>> -        tmp_pte = ptep_modify_prot_start(vma, addr, ptep);
>>>>> -        if (pte_dirty(tmp_pte))
>>>>> -            pte = pte_mkdirty(pte);
>>>>> -        if (pte_young(tmp_pte))
>>>>> -            pte = pte_mkyoung(pte);
>>>>> -    }
>>>>> -    return pte;
>>>>> -}
>>>>> -#endif
>>>>> -
>>>>> -/* See the comment for ptep_modify_prot_commit */
>>>>> -#ifndef modify_prot_commit_ptes
>>>>> -static inline void modify_prot_commit_ptes(struct vm_area_struct 
>>>>> *vma,
>>>>> unsigned long addr,
>>>>> -        pte_t *ptep, pte_t old_pte, pte_t pte, unsigned int nr)
>>>>> -{
>>>>> -    for (;;) {
>>>>> -        ptep_modify_prot_commit(vma, addr, ptep, old_pte, pte);
>>>>> -        if (--nr == 0)
>>>>> -            break;
>>>>> -        ptep++;
>>>>> -        addr += PAGE_SIZE;
>>>>> -        old_pte = pte_next_pfn(old_pte);
>>>>> -        pte = pte_next_pfn(pte);
>>>>> -    }
>>>>> -}
>>>>> -#endif
>>>>> -
>>>>>    /*
>>>>>     * On some architectures hardware does not set page access bit when
>>>>> accessing
>>>>>     * memory page, it is responsibility of software setting this 
>>>>> bit. It
>>>>> brings
>>>>> @@ -1375,6 +1337,45 @@ static inline void 
>>>>> ptep_modify_prot_commit(struct
>>>>> vm_area_struct *vma,
>>>>>        __ptep_modify_prot_commit(vma, addr, ptep, pte);
>>>>>    }
>>>>>    #endif /* __HAVE_ARCH_PTEP_MODIFY_PROT_TRANSACTION */
>>>>> +
>>>>> +/* See the comment for ptep_modify_prot_start */
>>>>> +#ifndef modify_prot_start_ptes
>>>>> +static inline pte_t modify_prot_start_ptes(struct vm_area_struct 
>>>>> *vma,
>>>>> +        unsigned long addr, pte_t *ptep, unsigned int nr)
>>>>> +{
>>>>> +    pte_t pte, tmp_pte;
>>>>> +
>>>>> +    pte = ptep_modify_prot_start(vma, addr, ptep);
>>>>> +    while (--nr) {
>>>>> +        ptep++;
>>>>> +        addr += PAGE_SIZE;
>>>>> +        tmp_pte = ptep_modify_prot_start(vma, addr, ptep);
>>>>> +        if (pte_dirty(tmp_pte))
>>>>> +            pte = pte_mkdirty(pte);
>>>>> +        if (pte_young(tmp_pte))
>>>>> +            pte = pte_mkyoung(pte);
>>>>> +    }
>>>>> +    return pte;
>>>>> +}
>>>>> +#endif
>>>>> +
>>>>> +/* See the comment for ptep_modify_prot_commit */
>>>>> +#ifndef modify_prot_commit_ptes
>>>>> +static inline void modify_prot_commit_ptes(struct vm_area_struct 
>>>>> *vma,
>>>>> unsigned long addr,
>>>>> +        pte_t *ptep, pte_t old_pte, pte_t pte, unsigned int nr)
>>>>> +{
>>>>> +    for (;;) {
>>>>> +        ptep_modify_prot_commit(vma, addr, ptep, old_pte, pte);
>>>>> +        if (--nr == 0)
>>>>> +            break;
>>>>> +        ptep++;
>>>>> +        addr += PAGE_SIZE;
>>>>> +        old_pte = pte_next_pfn(old_pte);
>>>>> +        pte = pte_next_pfn(pte);
>>>>> +    }
>>>>> +}
>>>>> +#endif
>>>>> +
>>>>>    #endif /* CONFIG_MMU */
>>>>>
>>>>>    /*
>>>>> -- 
>>>>>
>>>>> Thanks,
>>>>> Lance
>>>>>
>>>>> On 2025/4/29 13:23, Dev Jain wrote:
>>>>>> This patchset optimizes the mprotect() system call for large folios
>>>>>> by PTE-batching.
>>>>>>
>>>>>> We use the following test cases to measure performance, 
>>>>>> mprotect()'ing
>>>>>> the mapped memory to read-only then read-write 40 times:
>>>>>>
>>>>>> Test case 1: Mapping 1G of memory, touching it to get PMD-THPs, then
>>>>>> pte-mapping those THPs
>>>>>> Test case 2: Mapping 1G of memory with 64K mTHPs
>>>>>> Test case 3: Mapping 1G of memory with 4K pages
>>>>>>
>>>>>> Average execution time on arm64, Apple M3:
>>>>>> Before the patchset:
>>>>>> T1: 7.9 seconds   T2: 7.9 seconds   T3: 4.2 seconds
>>>>>>
>>>>>> After the patchset:
>>>>>> T1: 2.1 seconds   T2: 2.2 seconds   T3: 4.2 seconds
>>>>>>
>>>>>> Observing T1/T2 and T3 before the patchset, we also remove the 
>>>>>> regression
>>>>>> introduced by ptep_get() on a contpte block. And, for large folios 
>>>>>> we get
>>>>>> an almost 74% performance improvement.
>>>>>>
>>>>>> v1->v2:
>>>>>>    - Rebase onto mm-unstable (6ebffe676fcf: util_macros.h: make the
>>>>>> header more resilient)
>>>>>>    - Abridge the anon-exclusive condition (Lance Yang)
>>>>>>
>>>>>> Dev Jain (7):
>>>>>>     mm: Refactor code in mprotect
>>>>>>     mm: Optimize mprotect() by batch-skipping PTEs
>>>>>>     mm: Add batched versions of ptep_modify_prot_start/commit
>>>>>>     arm64: Add batched version of ptep_modify_prot_start
>>>>>>     arm64: Add batched version of ptep_modify_prot_commit
>>>>>>     mm: Batch around can_change_pte_writable()
>>>>>>     mm: Optimize mprotect() through PTE-batching
>>>>>>
>>>>>>    arch/arm64/include/asm/pgtable.h |  10 ++
>>>>>>    arch/arm64/mm/mmu.c              |  21 +++-
>>>>>>    include/linux/mm.h               |   4 +-
>>>>>>    include/linux/pgtable.h          |  42 ++++++++
>>>>>>    mm/gup.c                         |   2 +-
>>>>>>    mm/huge_memory.c                 |   4 +-
>>>>>>    mm/memory.c                      |   6 +-
>>>>>>    mm/mprotect.c                    | 165 +++++++++++++++++++ 
>>>>>> +-----------
>>>>>>    mm/pgtable-generic.c             |  16 ++-
>>>>>>    9 files changed, 198 insertions(+), 72 deletions(-)
>>>>>>
>>>>>
>>>>
>>
>