linux-kernel - Re: [v3 PATCH 2/7] mm: thp: introduce transhuge_vma_size

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Date:   Thu, 9 Jun 2022 15:21:04 -0700
From:   "Zach O'Keefe" <zokeefe@...gle.com>
To:     Yang Shi <shy828301@...il.com>
Cc:     vbabka@...e.cz, kirill.shutemov@...ux.intel.com,
        willy@...radead.org, akpm@...ux-foundation.org, linux-mm@...ck.org,
        linux-kernel@...r.kernel.org
Subject: Re: [v3 PATCH 2/7] mm: thp: introduce transhuge_vma_size_ok() helper

On Mon, Jun 6, 2022 at 2:44 PM Yang Shi <shy828301@...il.com> wrote:
>
> There are couple of places that check whether the vma size is ok for
> THP or not, they are open coded and duplicate, introduce
> transhuge_vma_size_ok() helper to do the job.
>
> Signed-off-by: Yang Shi <shy828301@...il.com>
> ---
>  include/linux/huge_mm.h | 17 +++++++++++++++++
>  mm/huge_memory.c        |  5 +----
>  mm/khugepaged.c         | 12 ++++++------
>  3 files changed, 24 insertions(+), 10 deletions(-)
>
> diff --git a/include/linux/huge_mm.h b/include/linux/huge_mm.h
> index 648cb3ce7099..a8f61db47f2a 100644
> --- a/include/linux/huge_mm.h
> +++ b/include/linux/huge_mm.h
> @@ -116,6 +116,18 @@ extern struct kobj_attribute shmem_enabled_attr;
>
>  extern unsigned long transparent_hugepage_flags;
>
> +/*
> + * The vma size has to be large enough to hold an aligned HPAGE_PMD_SIZE area.
> + */
> +static inline bool transhuge_vma_size_ok(struct vm_area_struct *vma)
> +{
> +       if (round_up(vma->vm_start, HPAGE_PMD_SIZE) <
> +           (vma->vm_end & HPAGE_PMD_MASK))
> +               return true;
> +
> +       return false;
> +}

First time coming across round_up() - thanks for that - but for
symmetry, maybe also use round_down() for the end? No strong opinion -
just a suggestion given I've just discovered it.

>  static inline bool transhuge_vma_suitable(struct vm_area_struct *vma,
>                 unsigned long addr)
>  {
> @@ -345,6 +357,11 @@ static inline bool transparent_hugepage_active(struct vm_area_struct *vma)
>         return false;
>  }
>
> +static inline bool transhuge_vma_size_ok(struct vm_area_struct *vma)
> +{
> +       return false;
> +}
> +
>  static inline bool transhuge_vma_suitable(struct vm_area_struct *vma,
>                 unsigned long addr)
>  {
> diff --git a/mm/huge_memory.c b/mm/huge_memory.c
> index 48182c8fe151..36ada544e494 100644
> --- a/mm/huge_memory.c
> +++ b/mm/huge_memory.c
> @@ -71,10 +71,7 @@ unsigned long huge_zero_pfn __read_mostly = ~0UL;
>
>  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;
> -
> -       if (!transhuge_vma_suitable(vma, addr))
> +       if (!transhuge_vma_size_ok(vma))
>                 return false;
>         if (vma_is_anonymous(vma))
>                 return __transparent_hugepage_enabled(vma);

Do we need a check for vma->vm_pgoff alignment here, after
!vma_is_anonymous(), and now that we don't call
transhuge_vma_suitable()?

> diff --git a/mm/khugepaged.c b/mm/khugepaged.c
> index 84b9cf4b9be9..d0f8020164fc 100644
> --- a/mm/khugepaged.c
> +++ b/mm/khugepaged.c
> @@ -454,6 +454,9 @@ bool hugepage_vma_check(struct vm_area_struct *vma,
>                                 vma->vm_pgoff, HPAGE_PMD_NR))
>                 return false;
>
> +       if (!transhuge_vma_size_ok(vma))
> +               return false;
> +
>         /* Enabled via shmem mount options or sysfs settings. */
>         if (shmem_file(vma->vm_file))
>                 return shmem_huge_enabled(vma);
> @@ -512,9 +515,7 @@ void khugepaged_enter_vma(struct vm_area_struct *vma,
>                           unsigned long vm_flags)
>  {
>         if (!test_bit(MMF_VM_HUGEPAGE, &vma->vm_mm->flags) &&
> -           khugepaged_enabled() &&
> -           (((vma->vm_start + ~HPAGE_PMD_MASK) & HPAGE_PMD_MASK) <
> -            (vma->vm_end & HPAGE_PMD_MASK))) {
> +           khugepaged_enabled()) {
>                 if (hugepage_vma_check(vma, vm_flags))
>                         __khugepaged_enter(vma->vm_mm);
>         }
> @@ -2142,10 +2143,9 @@ static unsigned int khugepaged_scan_mm_slot(unsigned int pages,
>                         progress++;
>                         continue;
>                 }
> -               hstart = (vma->vm_start + ~HPAGE_PMD_MASK) & HPAGE_PMD_MASK;
> +
> +               hstart = round_up(vma->vm_start, HPAGE_PMD_SIZE);
>                 hend = vma->vm_end & HPAGE_PMD_MASK;
> -               if (hstart >= hend)
> -                       goto skip;
>                 if (khugepaged_scan.address > hend)
>                         goto skip;
>                 if (khugepaged_scan.address < hstart)

Likewise, could do round_down() here (just a suggestion)

> --
> 2.26.3
>
>