Re: [v5 PATCH 4/7] mm: thp: kill transparent_hugepage_active()

[Zach O'Keefe <zokeefe@google.com>]

On 16 Jun 10:48, Yang Shi wrote:
> The transparent_hugepage_active() was introduced to show THP eligibility
> bit in smaps in proc, smaps is the only user.  But it actually does the
> similar check as hugepage_vma_check() which is used by khugepaged.  We
> definitely don't have to maintain two similar checks, so kill
> transparent_hugepage_active().
> 
> This patch also fixed the wrong behavior for VM_NO_KHUGEPAGED vmas.
> 
> Also move hugepage_vma_check() to huge_memory.c and huge_mm.h since it
> is not only for khugepaged anymore.
> 
> Reviewed-by: Zach O'Keefe <zokeefe@google.com>
> Signed-off-by: Yang Shi <shy828301@gmail.com>
> ---
>  fs/proc/task_mmu.c         |  2 +-
>  include/linux/huge_mm.h    | 16 +++++++-----
>  include/linux/khugepaged.h |  2 --
>  mm/huge_memory.c           | 50 +++++++++++++++++++++++++++++++-------
>  mm/khugepaged.c            | 48 +++---------------------------------
>  5 files changed, 56 insertions(+), 62 deletions(-)
> 
> diff --git a/fs/proc/task_mmu.c b/fs/proc/task_mmu.c
> index 37ccb5c9f4f8..39a40ec181e7 100644
> --- a/fs/proc/task_mmu.c
> +++ b/fs/proc/task_mmu.c
> @@ -863,7 +863,7 @@ static int show_smap(struct seq_file *m, void *v)
>  	__show_smap(m, &mss, false);
>  
>  	seq_printf(m, "THPeligible:    %d\n",
> -		   transparent_hugepage_active(vma));
> +		   hugepage_vma_check(vma, vma->vm_flags, true));
>  
>  	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 8a5a8bfce0f5..64487bcd0c7b 100644
> --- a/include/linux/huge_mm.h
> +++ b/include/linux/huge_mm.h
> @@ -202,7 +202,9 @@ static inline bool file_thp_enabled(struct vm_area_struct *vma)
>  	       !inode_is_open_for_write(inode) && S_ISREG(inode->i_mode);
>  }
>  
> -bool transparent_hugepage_active(struct vm_area_struct *vma);
> +bool hugepage_vma_check(struct vm_area_struct *vma,
> +			unsigned long vm_flags,
> +			bool smaps);
>  
>  #define transparent_hugepage_use_zero_page()				\
>  	(transparent_hugepage_flags &					\
> @@ -351,11 +353,6 @@ static inline bool __transparent_hugepage_enabled(struct vm_area_struct *vma)
>  	return false;
>  }
>  
> -static inline bool transparent_hugepage_active(struct vm_area_struct *vma)
> -{
> -	return false;
> -}
> -
>  static inline bool transhuge_vma_suitable(struct vm_area_struct *vma,
>  		unsigned long addr)
>  {
> @@ -368,6 +365,13 @@ static inline bool transhuge_vma_enabled(struct vm_area_struct *vma,
>  	return false;
>  }
>  
> +static inline bool hugepage_vma_check(struct vm_area_struct *vma,
> +				       unsigned long vm_flags,
> +				       bool smaps)
> +{
> +	return false;
> +}
> +
>  static inline void prep_transhuge_page(struct page *page) {}
>  
>  #define transparent_hugepage_flags 0UL
> diff --git a/include/linux/khugepaged.h b/include/linux/khugepaged.h
> index 31ca8a7f78f4..ea5fd4c398f7 100644
> --- a/include/linux/khugepaged.h
> +++ b/include/linux/khugepaged.h
> @@ -10,8 +10,6 @@ extern struct attribute_group khugepaged_attr_group;
>  extern int khugepaged_init(void);
>  extern void khugepaged_destroy(void);
>  extern int start_stop_khugepaged(void);
> -extern bool hugepage_vma_check(struct vm_area_struct *vma,
> -			       unsigned long vm_flags);
>  extern void __khugepaged_enter(struct mm_struct *mm);
>  extern void __khugepaged_exit(struct mm_struct *mm);
>  extern void khugepaged_enter_vma(struct vm_area_struct *vma,
> diff --git a/mm/huge_memory.c b/mm/huge_memory.c
> index b530462c4493..a28c6100b491 100644
> --- a/mm/huge_memory.c
> +++ b/mm/huge_memory.c
> @@ -69,21 +69,53 @@ static atomic_t huge_zero_refcount;
>  struct page *huge_zero_page __read_mostly;
>  unsigned long huge_zero_pfn __read_mostly = ~0UL;
>  
> -bool transparent_hugepage_active(struct vm_area_struct *vma)
> +bool hugepage_vma_check(struct vm_area_struct *vma,
> +			unsigned long vm_flags,
> +			bool smaps)
>  {
> -	/* 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, vm_flags))
> +		return false;
> +

During testing my work on top this patch, I found a small bug here.

Namely, transhuge_vma_enabled() will check vma->vm_mm->flags (to see if
MMF_DISABLE_THP is set); however, for vDSO vmas, vma->vm_mm is NULL.

Previously, transparent_hugepage_active() in smaps path would check
transhuge_vma_suitable() before checking these flags, which would fail for vDSO
vma since we'd take the !vma_is_anonymous() branch and find the vma (most
likely) wasn't suitably aligned (by chance ?).

Anyways, I think we need to check vma->vm_mm.

> +	if (vm_flags & VM_NO_KHUGEPAGED)
> +		return false;
> +
> +	/* Don't run khugepaged against DAX vma */
> +	if (vma_is_dax(vma))
> +		return false;
>  
> -	if (!transhuge_vma_suitable(vma, addr))
> +	/* Check alignment for file vma and size for both file and anon vma */
> +	if (!transhuge_vma_suitable(vma, (vma->vm_end - HPAGE_PMD_SIZE)))
>  		return false;
> -	if (vma_is_anonymous(vma))
> -		return __transparent_hugepage_enabled(vma);
> -	if (vma_is_shmem(vma))
> +
> +	/* Enabled via shmem mount options or sysfs settings. */
> +	if (shmem_file(vma->vm_file))
>  		return shmem_huge_enabled(vma);
> -	if (transhuge_vma_enabled(vma, vma->vm_flags) && file_thp_enabled(vma))
> +
> +	if (!khugepaged_enabled())
> +		return false;
> +
> +	/* THP settings require madvise. */
> +	if (!(vm_flags & VM_HUGEPAGE) && !khugepaged_always())
> +		return false;
> +
> +	/* Only regular file is valid */
> +	if (file_thp_enabled(vma))
>  		return true;
>  
> -	return false;
> +	if (!vma_is_anonymous(vma))
> +		return false;
> +
> +	if (vma_is_temporary_stack(vma))
> +		return false;
> +
> +	/*
> +	 * THPeligible bit of smaps should show 1 for proper VMAs even
> +	 * though anon_vma is not initialized yet.
> +	 */
> +	if (!vma->anon_vma)
> +		return smaps;
> +
> +	return true;
>  }
>  
>  static bool get_huge_zero_page(void)
> diff --git a/mm/khugepaged.c b/mm/khugepaged.c
> index 5baa394e34c8..3afd87f8c0b1 100644
> --- a/mm/khugepaged.c
> +++ b/mm/khugepaged.c
> @@ -437,46 +437,6 @@ static inline int khugepaged_test_exit(struct mm_struct *mm)
>  	return atomic_read(&mm->mm_users) == 0;
>  }
>  
> -bool hugepage_vma_check(struct vm_area_struct *vma,
> -			unsigned long vm_flags)
> -{
> -	if (!transhuge_vma_enabled(vma, vm_flags))
> -		return false;
> -
> -	if (vm_flags & VM_NO_KHUGEPAGED)
> -		return false;
> -
> -	/* Don't run khugepaged against DAX vma */
> -	if (vma_is_dax(vma))
> -		return false;
> -
> -	/* Check alignment for file vma and size for both file and anon vma */
> -	if (!transhuge_vma_suitable(vma, (vma->vm_end - HPAGE_PMD_SIZE)))
> -		return false;
> -
> -	/* Enabled via shmem mount options or sysfs settings. */
> -	if (shmem_file(vma->vm_file))
> -		return shmem_huge_enabled(vma);
> -
> -	if (!khugepaged_enabled())
> -		return false;
> -
> -	/* THP settings require madvise. */
> -	if (!(vm_flags & VM_HUGEPAGE) && !khugepaged_always())
> -		return false;
> -
> -	/* Only regular file is valid */
> -	if (file_thp_enabled(vma))
> -		return true;
> -
> -	if (!vma->anon_vma || !vma_is_anonymous(vma))
> -		return false;
> -	if (vma_is_temporary_stack(vma))
> -		return false;
> -
> -	return true;
> -}
> -
>  void __khugepaged_enter(struct mm_struct *mm)
>  {
>  	struct mm_slot *mm_slot;
> @@ -513,7 +473,7 @@ void khugepaged_enter_vma(struct vm_area_struct *vma,
>  {
>  	if (!test_bit(MMF_VM_HUGEPAGE, &vma->vm_mm->flags) &&
>  	    khugepaged_enabled()) {
> -		if (hugepage_vma_check(vma, vm_flags))
> +		if (hugepage_vma_check(vma, vm_flags, false))
>  			__khugepaged_enter(vma->vm_mm);
>  	}
>  }
> @@ -958,7 +918,7 @@ static int hugepage_vma_revalidate(struct mm_struct *mm, unsigned long address,
>  
>  	if (!transhuge_vma_suitable(vma, address))
>  		return SCAN_ADDRESS_RANGE;
> -	if (!hugepage_vma_check(vma, vma->vm_flags))
> +	if (!hugepage_vma_check(vma, vma->vm_flags, false))
>  		return SCAN_VMA_CHECK;
>  	/*
>  	 * Anon VMA expected, the address may be unmapped then
> @@ -1448,7 +1408,7 @@ void collapse_pte_mapped_thp(struct mm_struct *mm, unsigned long addr)
>  	 * the valid THP. Add extra VM_HUGEPAGE so hugepage_vma_check()
>  	 * will not fail the vma for missing VM_HUGEPAGE
>  	 */
> -	if (!hugepage_vma_check(vma, vma->vm_flags | VM_HUGEPAGE))
> +	if (!hugepage_vma_check(vma, vma->vm_flags | VM_HUGEPAGE, false))
>  		return;
>  
>  	/* Keep pmd pgtable for uffd-wp; see comment in retract_page_tables() */
> @@ -2143,7 +2103,7 @@ static unsigned int khugepaged_scan_mm_slot(unsigned int pages,
>  			progress++;
>  			break;
>  		}
> -		if (!hugepage_vma_check(vma, vma->vm_flags)) {
> +		if (!hugepage_vma_check(vma, vma->vm_flags, false)) {
>  skip:
>  			progress++;
>  			continue;
> -- 
> 2.26.3
>