Re: [PATCH v2 3/5] mm/huge_memory.c: add missing read-only THP checking in transparent_hugepage_enabled()

[Miaohe Lin]

On 2021/4/29 22:57, David Hildenbrand wrote:
> On 29.04.21 15:26, Miaohe Lin wrote:
>> Since commit 99cb0dbd47a1 ("mm,thp: add read-only THP support for
>> (non-shmem) FS"), read-only THP file mapping is supported. But it
>> forgot to add checking for it in transparent_hugepage_enabled().
>> To fix it, we add checking for read-only THP file mapping and also
>> introduce helper transhuge_vma_enabled() to check whether thp is
>> enabled for specified vma to reduce duplicated code.
>>
>> Fixes: 99cb0dbd47a1 ("mm,thp: add read-only THP support for (non-shmem) FS")
>> Signed-off-by: Miaohe Lin <linmiaohe@xxxxxxxxxx>
>> ---
>>   include/linux/huge_mm.h | 21 +++++++++++++++++----
>>   mm/huge_memory.c        |  6 ++++++
>>   mm/khugepaged.c         |  4 +---
>>   mm/shmem.c              |  3 +--
>>   4 files changed, 25 insertions(+), 9 deletions(-)
>>
>> diff --git a/include/linux/huge_mm.h b/include/linux/huge_mm.h
>> index 0a526f211fec..f460b74619fc 100644
>> --- a/include/linux/huge_mm.h
>> +++ b/include/linux/huge_mm.h
>> @@ -115,6 +115,16 @@ extern struct kobj_attribute shmem_enabled_attr;
>>     extern unsigned long transparent_hugepage_flags;
>>   +static inline bool transhuge_vma_enabled(struct vm_area_struct *vma,
>> +                      unsigned long vm_flags)
> 
> You're passing the vma already, why do you pass vma->vm_flags separately? It's sufficient to pass in the vma only.
> 

Many thanks for comment! IMO, vm_flags may not always equal to vma->vm_flags. When hugepage_vma_check()
is called from collapse_pte_mapped_thp, vma_flags = vma->vm_flags | VM_HUGEPAGE. So I think we should
pass vm_flags here.

>> +{
>> +    /* Explicitly disabled through madvise. */
>> +    if ((vm_flags & VM_NOHUGEPAGE) ||
>> +        test_bit(MMF_DISABLE_THP, &vma->vm_mm->flags))
>> +        return false;
>> +    return true;
>> +}
>> +
>>   /*
>>    * to be used on vmas which are known to support THP.
>>    * Use transparent_hugepage_enabled otherwise
>> @@ -128,15 +138,12 @@ static inline bool __transparent_hugepage_enabled(struct vm_area_struct *vma)
>>       if (transparent_hugepage_flags & (1 << TRANSPARENT_HUGEPAGE_NEVER_DAX))
>>           return false;
>>   -    if (vma->vm_flags & VM_NOHUGEPAGE)
>> +    if (!transhuge_vma_enabled(vma, vma->vm_flags))
>>           return false;
>>         if (vma_is_temporary_stack(vma))
>>           return false;
>>   -    if (test_bit(MMF_DISABLE_THP, &vma->vm_mm->flags))
>> -        return false;
>> -
>>       if (transparent_hugepage_flags & (1 << TRANSPARENT_HUGEPAGE_FLAG))
>>           return true;
>>   @@ -362,6 +369,12 @@ static inline bool transhuge_vma_suitable(struct vm_area_struct *vma,
>>       return false;
>>   }
>>   +static inline bool transhuge_vma_enabled(struct vm_area_struct *vma,
>> +                      unsigned long vm_flags)
>> +{
>> +    return false;
>> +}
>> +
>>   static inline void prep_transhuge_page(struct page *page) {}
>>     static inline bool is_transparent_hugepage(struct page *page)
>> diff --git a/mm/huge_memory.c b/mm/huge_memory.c
>> index 76ca1eb2a223..e24a96de2e37 100644
>> --- a/mm/huge_memory.c
>> +++ b/mm/huge_memory.c
>> @@ -68,12 +68,18 @@ bool transparent_hugepage_enabled(struct vm_area_struct *vma)
>>       /* The addr is used to check if the vma size fits */
>>       unsigned long addr = (vma->vm_end & HPAGE_PMD_MASK) - HPAGE_PMD_SIZE;
>>   +    if (!transhuge_vma_enabled(vma, vma->vm_flags))
>> +        return false;
>>       if (!transhuge_vma_suitable(vma, addr))
>>           return false;
>>       if (vma_is_anonymous(vma))
>>           return __transparent_hugepage_enabled(vma);
>>       if (vma_is_shmem(vma))
>>           return shmem_huge_enabled(vma);
>> +    if (IS_ENABLED(CONFIG_READ_ONLY_THP_FOR_FS) && vma->vm_file &&
>> +        !inode_is_open_for_write(vma->vm_file->f_inode) &&
>> +        (vma->vm_flags & VM_EXEC))
>> +        return true;
> 
> Nit: I'm really wondering why we have 3 different functions that sound like they are doing the same thing
> 
> transparent_hugepage_enabled(vma)
> transhuge_vma_enabled()
> transhuge_vma_suitable()
> 
> Which check belongs where? Does it really have to be that complicated?
> 

IMO, transhuge_vma_suitable() checks whether pgoff , vm_start and vm_end is possible for thp.
transhuge_vma_enabled() checks whether thp is explicitly disabled through madvise.
And transparent_hugepage_enabled() use these helpers to get the conclusion whether thp is
enabled for specified vma.

Any suggestions?
Thanks again!

>>         return false;
>>   }
>> diff --git a/mm/khugepaged.c b/mm/khugepaged.c
>> index 6c0185fdd815..d97b20fad6e8 100644
>> --- a/mm/khugepaged.c
>> +++ b/mm/khugepaged.c
>> @@ -442,9 +442,7 @@ static inline int khugepaged_test_exit(struct mm_struct *mm)
>>   static bool hugepage_vma_check(struct vm_area_struct *vma,
>>                      unsigned long vm_flags)
>>   {
>> -    /* Explicitly disabled through madvise. */
>> -    if ((vm_flags & VM_NOHUGEPAGE) ||
>> -        test_bit(MMF_DISABLE_THP, &vma->vm_mm->flags))
>> +    if (!transhuge_vma_enabled(vma, vm_flags))
>>           return false;
>>         /* Enabled via shmem mount options or sysfs settings. */
>> diff --git a/mm/shmem.c b/mm/shmem.c
>> index a08cedefbfaa..1dcbec313c70 100644
>> --- a/mm/shmem.c
>> +++ b/mm/shmem.c
>> @@ -4032,8 +4032,7 @@ bool shmem_huge_enabled(struct vm_area_struct *vma)
>>       loff_t i_size;
>>       pgoff_t off;
>>   -    if ((vma->vm_flags & VM_NOHUGEPAGE) ||
>> -        test_bit(MMF_DISABLE_THP, &vma->vm_mm->flags))
>> +    if (!transhuge_vma_enabled(vma, vma->vm_flags))
>>           return false;
>>       if (shmem_huge == SHMEM_HUGE_FORCE)
>>           return true;
>>
> 
>