Re: [v3 PATCH 2/2] mm: thp: fix false negative of shmem vma's THP eligibility

[Yang Shi]

On 6/19/19 5:12 AM, Vlastimil Babka wrote:
> On 6/13/19 6:44 AM, Yang Shi wrote:
> > The commit 7635d9cbe832 ("mm, thp, proc: report THP eligibility for each
> > vma") introduced THPeligible bit for processes' smaps. But, when checking
> > the eligibility for shmem vma, __transparent_hugepage_enabled() is
> > called to override the result from shmem_huge_enabled().  It may result
> > in the anonymous vma's THP flag override shmem's.  For example, running a
> > simple test which create THP for shmem, but with anonymous THP disabled,
> > when reading the process's smaps, it may show:
> ...
>
> > diff --git a/fs/proc/task_mmu.c b/fs/proc/task_mmu.c
> > index 01d4eb0..6a13882 100644
> > --- a/fs/proc/task_mmu.c
> > +++ b/fs/proc/task_mmu.c
> > @@ -796,7 +796,8 @@ static int show_smap(struct seq_file *m, void *v)
> >   
> >   	__show_smap(m, &mss);
> >   
> > -	seq_printf(m, "THPeligible:    %d\n", transparent_hugepage_enabled(vma));
> > +	seq_printf(m, "THPeligible:		%d\n",
> > +		   transparent_hugepage_enabled(vma));
> >   
> >   	if (arch_pkeys_enabled())
> >   		seq_printf(m, "ProtectionKey:  %8u\n", vma_pkey(vma));
> > diff --git a/mm/huge_memory.c b/mm/huge_memory.c
> > index 4bc2552..36f0225 100644
> > --- a/mm/huge_memory.c
> > +++ b/mm/huge_memory.c
> > @@ -65,10 +65,15 @@
> >   
> >   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_suitable(vma, addr))
> > +		return false;
> Sorry for replying rather late, and not in the v2 thread, but unlike
> Hugh I'm not convinced that we should include vma size/alignment in the
> test for reporting THPeligible, which was supposed to reflect
> administrative settings and madvise hints. I guess it's mostly a matter
> of personal feeling. But one objective distinction is that the admin
> settings and madvise do have an exact binary result for the whole VMA,
> while this check is more fuzzy - only part of the VMA's span might be
> properly sized+aligned, and THPeligible will be 1 for the whole VMA.

I think THPeligible is used to tell us if the vma is suitable for 
allocating THP. Both anonymous and shmem THP checks vma size/alignment 
to decide to or not to allocate THP.

And, if vma size/alignment is not checked, THPeligible may show "true" 
for even 4K mapping. This doesn't make too much sense either.

> >   	if (vma_is_anonymous(vma))
> >   		return __transparent_hugepage_enabled(vma);
> > -	if (vma_is_shmem(vma) && shmem_huge_enabled(vma))
> > -		return __transparent_hugepage_enabled(vma);
> > +	if (vma_is_shmem(vma))
> > +		return shmem_huge_enabled(vma);
> >   
> >   	return false;
> >   }
> > diff --git a/mm/shmem.c b/mm/shmem.c
> > index 1bb3b8d..a807712 100644
> > --- a/mm/shmem.c
> > +++ b/mm/shmem.c
> > @@ -3872,6 +3872,9 @@ 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))
> > +		return false;
> >   	if (shmem_huge == SHMEM_HUGE_FORCE)
> >   		return true;
> >   	if (shmem_huge == SHMEM_HUGE_DENY)