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

[David Hildenbrand]

On 30.04.21 03:57, Miaohe Lin wrote:
> 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.

Oh, sorry, I missed the hugepage_vma_check() user. That's unfortunate.

> > >    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?

transparent_hugepage_enabled() vs. transhuge_vma_enabled() is really 
sub-optimal naming. I guess "transparent_hugepage_active()" would have 
been clearer (enabled + suitable + applicable). Cannot really give a 
good suggestion here on how to name transhuge_vma_enabled() differently.


We now have

transparent_hugepage_enabled()
-> transhuge_vma_enabled()
-> __transparent_hugepage_enabled() -> transhuge_vma_enabled()
-> shmem_huge_enabled() -> transhuge_vma_enabled()

That looks sub-optimal as well. Maybe we should have a

static inline bool file_thp_enabled(struct vma *vma)
{
	return transhuge_vma_enabled() &&
	       IS_ENABLED(CONFIG_READ_ONLY_THP_FOR_FS) &&
	       !inode_is_open_for_write(vma->vm_file->f_inode) &&
	       (vma->vm_flags & VM_EXEC))
}

and in transparent_hugepage_enabled() only do a

if (vma->vm_file)
	return file_thp_enabled(vma);


Or move the transhuge_vma_enabled() check completely to 
transparent_hugepage_enabled() if possible.

--
Thanks,

David / dhildenb