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

[Yang Shi]

On Tue, Apr 27, 2021 at 7:06 PM Miaohe Lin <linmiaohe@xxxxxxxxxx> wrote:
>
> On 2021/4/28 5:03, Yang Shi wrote:
> > On Tue, Apr 27, 2021 at 6:32 AM Miaohe Lin <linmiaohe@xxxxxxxxxx> 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().
> >>
> >> Fixes: 99cb0dbd47a1 ("mm,thp: add read-only THP support for (non-shmem) FS")
> >> Signed-off-by: Miaohe Lin <linmiaohe@xxxxxxxxxx>
> >> ---
> >>  mm/huge_memory.c | 3 +++
> >>  1 file changed, 3 insertions(+)
> >>
> >> diff --git a/mm/huge_memory.c b/mm/huge_memory.c
> >> index 76ca1eb2a223..aa22a0ae9894 100644
> >> --- a/mm/huge_memory.c
> >> +++ b/mm/huge_memory.c
> >> @@ -74,6 +74,9 @@ 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) && vma->vm_file &&
> >> +           (vma->vm_flags & VM_DENYWRITE))
> >> +               return true;
> >
>
> Many thanks for your quick respond and Reviewed-by tag!
>
> > I don't think this change is correct. This function is used to
> > indicate if allocating THP is eligible for the VMAs or not showed by
> > smap. And currently readonly FS THP is collapsed by khugepaged only.
> >
> > So, you need check if the vma is suitable for khugepaged. Take a look
> > at what hugepage_vma_check() does.
> >
> > And, the new patch
> > (https://lore.kernel.org/linux-mm/20210406000930.3455850-1-cfijalkovich@xxxxxxxxxx/)
> > relax the constraints for readonly FS THP, it might be already in -mm
> > tree, so you need adopt the new condition as well.
> >
>
> Many thanks for your comment. I referred to what hugepage_vma_check() does about
> Read-only file mappings when I came up this patch. But it seems I am miss something.

Yes, you need do the below check for readonly FS THP too:

if ((vm_flags & VM_NOHUGEPAGE) ||
    test_bit(MMF_DISABLE_THP, &vma->vm_mm->flags))
return false;

This check is done separately for anonymous and shmem. It seems not
perfect to do it three times in a row. So I'd suggest extracting the
check into a common helper then call it at the top of
transparent_hugepage_enabled() .

The helper also could replace the same check in
__transparent_hugepage_enabled() and shmem_huge_enabled().

> Take the new patch into account, the check for READ_ONLY_THP now should be:
>
> diff --git a/mm/huge_memory.c b/mm/huge_memory.c
> index 76ca1eb2a223..a46a558233b4 100644
> --- a/mm/huge_memory.c
> +++ b/mm/huge_memory.c
> @@ -74,6 +74,10 @@ 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) && vma->vm_file &&
> +           !inode_is_open_for_write(vma->vm_file->f_inode) &&
> +           (vma->vm_flags & VM_EXEC))
> +               return true;
>
>         return false;
>  }
>
> Am I miss something about checking for READ_ONLY_THP case? Or READ_ONLY_THP case is ok
> but other case is missed? Could you please explain this more detailed for me?
>
> Many thanks!
>
> >>
> >>         return false;
> >>  }
> >
> >> --
> >> 2.23.0
> >>
> >>
> >
> > .
> >
>