Re: [PATCH v4] mm/huge_memory.c: add missing read-only THP checking in transparent_hugepage_enabled()

[Miaohe Lin <linmiaohe@huawei.com>]

On 2022/2/25 5:56, Yang Shi wrote:
> On Thu, Feb 24, 2022 at 1:29 PM Yang Shi <shy828301@gmail.com> wrote:
>>
>> On Thu, Feb 24, 2022 at 10:51 AM Vlastimil Babka <vbabka@suse.cz> wrote:
>>>
>>> On 5/14/21 11:30, 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. We rename
>>>> transparent_hugepage_enabled to transparent_hugepage_active to make
>>>> the code easier to follow as suggested by David Hildenbrand.
>>>>
>>>> Fixes: 99cb0dbd47a1 ("mm,thp: add read-only THP support for (non-shmem) FS")
>>>> Reviewed-by: Yang Shi <shy828301@gmail.com>
>>>> Signed-off-by: Miaohe Lin <linmiaohe@huawei.com>
>>>
>>> FYI, I stumbled upon needing this for my reproducer for the read()
>>> corruption [1] to work, and I think it's still not ideal. I also needed
>>> madvise(MADV_HUGEPAGE) for the executable file mapping to make sure
>>> khugepaged acts on my process. Yeah, commit 99cb0dbd47a1 suggests madvise.
>>> Yet, khugepaged will happily collapse file mappings even without madvise
>>> anyway. However, it might not know about the mm at all unless
>>> __khugepaged_enter() has been applied at least on one of its vma's.
>>> madvise() is one way to do it for executable file mappings, but it can also
>>> happen through e.g. do_huge_pmd_anonymous_page() on another mapping, which
>>> has nothing to do with the file mapping.
>>> So what I'm trying to say is that we are somewhat inconsistent - the rules
>>> to consider a vma in khugepaged seem to be clear and result of admin
>>> configuration and madvise, but the rules to consider a mm for khugepaged
>>> (which is the only way to collapse file mappings) are not completely
>>> identical and there might be "random luck" involved.
>>
>> Yes, khugepaged_enter() is not called for file vma explicitly. My wild
>> guess is THP for readonly fs was assumed to be collapsed by explicit
>> demand from the users, for example, madvise.
>>
>> To achieve more consistent behavior we could call khugepaged_enter()
>> for file fault just like what huge anonymous fault does. Of course we
>> need to fix khugepaged_enter() to do the proper check as well.
> 
> Or we could follow shmem's implementation to call khugepaged_enter()
> in .mmap method for each filesystem so that khugepaged_enter() doesn't
> have to be called for each filemap_fault(). Luckily most filesystems
> use generic_file_mmap().
> 

I think I get the point. We need to fix the inconsistent behavior between file
vma and anon vma where khugepaged_enter() is only called for anon vma explicitly.
Am I supposed to do this or Yang Shi would kindly help do this ?

Many thanks both of you. :)

>>
>>>
>>> [1] https://lore.kernel.org/all/df3b5d1c-a36b-2c73-3e27-99e74983de3a@suse.cz/
>>>
>>>> ---
>>>> v3->v4:
>>>>   collect Reviewed-by tag
>>>>   define transhuge_vma_enabled next to transhuge_vma_suitable
>>>> ---
>>>>  fs/proc/task_mmu.c      |  2 +-
>>>>  include/linux/huge_mm.h | 57 +++++++++++++++++++++++++----------------
>>>>  mm/huge_memory.c        | 11 +++++++-
>>>>  mm/khugepaged.c         |  4 +--
>>>>  mm/shmem.c              |  3 +--
>>>>  5 files changed, 48 insertions(+), 29 deletions(-)
>>>>
>>>> diff --git a/fs/proc/task_mmu.c b/fs/proc/task_mmu.c
>>>> index fc9784544b24..7389df326edd 100644
>>>> --- a/fs/proc/task_mmu.c
>>>> +++ b/fs/proc/task_mmu.c
>>>> @@ -832,7 +832,7 @@ static int show_smap(struct seq_file *m, void *v)
>>>>       __show_smap(m, &mss, false);
>>>>
>>>>       seq_printf(m, "THPeligible:    %d\n",
>>>> -                transparent_hugepage_enabled(vma));
>>>> +                transparent_hugepage_active(vma));
>>>>
>>>>       if (arch_pkeys_enabled())
>>>>               seq_printf(m, "ProtectionKey:  %8u\n", vma_pkey(vma));
>>>> diff --git a/include/linux/huge_mm.h b/include/linux/huge_mm.h
>>>> index 0a526f211fec..7b7f7b52ccb8 100644
>>>> --- a/include/linux/huge_mm.h
>>>> +++ b/include/linux/huge_mm.h
>>>> @@ -115,9 +115,34 @@ extern struct kobj_attribute shmem_enabled_attr;
>>>>
>>>>  extern unsigned long transparent_hugepage_flags;
>>>>
>>>> +static inline bool transhuge_vma_suitable(struct vm_area_struct *vma,
>>>> +             unsigned long haddr)
>>>> +{
>>>> +     /* Don't have to check pgoff for anonymous vma */
>>>> +     if (!vma_is_anonymous(vma)) {
>>>> +             if (!IS_ALIGNED((vma->vm_start >> PAGE_SHIFT) - vma->vm_pgoff,
>>>> +                             HPAGE_PMD_NR))
>>>> +                     return false;
>>>> +     }
>>>> +
>>>> +     if (haddr < vma->vm_start || haddr + HPAGE_PMD_SIZE > vma->vm_end)
>>>> +             return false;
>>>> +     return true;
>>>> +}
>>>> +
>>>> +static inline bool transhuge_vma_enabled(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))
>>>> +             return false;
>>>> +     return true;
>>>> +}
>>>> +
>>>>  /*
>>>>   * to be used on vmas which are known to support THP.
>>>> - * Use transparent_hugepage_enabled otherwise
>>>> + * Use transparent_hugepage_active otherwise
>>>>   */
>>>>  static inline bool __transparent_hugepage_enabled(struct vm_area_struct *vma)
>>>>  {
>>>> @@ -128,15 +153,12 @@ 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;
>>>>
>>>> @@ -150,22 +172,7 @@ static inline bool __transparent_hugepage_enabled(struct vm_area_struct *vma)
>>>>       return false;
>>>>  }
>>>>
>>>> -bool transparent_hugepage_enabled(struct vm_area_struct *vma);
>>>> -
>>>> -static inline bool transhuge_vma_suitable(struct vm_area_struct *vma,
>>>> -             unsigned long haddr)
>>>> -{
>>>> -     /* Don't have to check pgoff for anonymous vma */
>>>> -     if (!vma_is_anonymous(vma)) {
>>>> -             if (!IS_ALIGNED((vma->vm_start >> PAGE_SHIFT) - vma->vm_pgoff,
>>>> -                             HPAGE_PMD_NR))
>>>> -                     return false;
>>>> -     }
>>>> -
>>>> -     if (haddr < vma->vm_start || haddr + HPAGE_PMD_SIZE > vma->vm_end)
>>>> -             return false;
>>>> -     return true;
>>>> -}
>>>> +bool transparent_hugepage_active(struct vm_area_struct *vma);
>>>>
>>>>  #define transparent_hugepage_use_zero_page()                         \
>>>>       (transparent_hugepage_flags &                                   \
>>>> @@ -351,7 +358,7 @@ static inline bool __transparent_hugepage_enabled(struct vm_area_struct *vma)
>>>>       return false;
>>>>  }
>>>>
>>>> -static inline bool transparent_hugepage_enabled(struct vm_area_struct *vma)
>>>> +static inline bool transparent_hugepage_active(struct vm_area_struct *vma)
>>>>  {
>>>>       return false;
>>>>  }
>>>> @@ -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..4f37867eed12 100644
>>>> --- a/mm/huge_memory.c
>>>> +++ b/mm/huge_memory.c
>>>> @@ -63,7 +63,14 @@ static struct shrinker deferred_split_shrinker;
>>>>  static atomic_t huge_zero_refcount;
>>>>  struct page *huge_zero_page __read_mostly;
>>>>
>>>> -bool transparent_hugepage_enabled(struct vm_area_struct *vma)
>>>> +static inline bool file_thp_enabled(struct vm_area_struct *vma)
>>>> +{
>>>> +     return transhuge_vma_enabled(vma, vma->vm_flags) && vma->vm_file &&
>>>> +            !inode_is_open_for_write(vma->vm_file->f_inode) &&
>>>> +            (vma->vm_flags & VM_EXEC);
>>>> +}
>>>> +
>>>> +bool transparent_hugepage_active(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;
>>>> @@ -74,6 +81,8 @@ bool transparent_hugepage_enabled(struct vm_area_struct *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))
>>>> +             return file_thp_enabled(vma);
>>>>
>>>>       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;
>>>
> .
>