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

[Yang Shi]

On 4/22/19 11:50 PM, Michal Hocko wrote:
> On Tue 23-04-19 06:24:53, 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.
> Hmm, I was under impression that thw global sysfs is not anonymous
> memory specific and it overrides whatever sysfs comes with. Isn't
> ignoring the global setting a bug in the shmemfs allocation paths?
> Kirill what is the actual semantic here?

I tried 4.9, 4.14, 4.20 and 5.1-rc5, all behaves in the same way. So, 
I'm supposed "enabled" is for anonymous THP only.

> > 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:
> >
> > 7fc92ec00000-7fc92f000000 rw-s 00000000 00:14 27764 /dev/shm/test
> > Size:               4096 kB
> > ...
> > [snip]
> > ...
> > ShmemPmdMapped:     4096 kB
> > ...
> > [snip]
> > ...
> > THPeligible:    0
> >
> > And, /proc/meminfo does show THP allocated and PMD mapped too:
> >
> > ShmemHugePages:     4096 kB
> > ShmemPmdMapped:     4096 kB
> >
> > This doesn't make too much sense.  The anonymous THP flag should not
> > intervene shmem THP.  Calling shmem_huge_enabled() with checking
> > MMF_DISABLE_THP sounds good enough.  And, we could skip stack and
> > dax vma check since we already checked if the vma is shmem already.
> Even if I am wrong about the /sys/kernel/mm/transparent_hugepage/enabled
> being the global setting for _all_ THP then this patch is not sufficient
> because it doesn't reflect VM_NOHUGEPAGE.

Aha, yes, thanks for catching this. Will fix in v2.

> > Fixes: 7635d9cbe832 ("mm, thp, proc: report THP eligibility for each vma")
> > Cc: Michal Hocko <mhocko@xxxxxxxx>
> > Cc: Vlastimil Babka <vbabka@xxxxxxx>
> > Cc: David Rientjes <rientjes@xxxxxxxxxx>
> > Cc: Kirill A. Shutemov <kirill@xxxxxxxxxxxxx>
> > Signed-off-by: Yang Shi <yang.shi@xxxxxxxxxxxxxxxxx>
> > ---
> >   mm/huge_memory.c | 4 ++--
> >   mm/shmem.c       | 2 ++
> >   2 files changed, 4 insertions(+), 2 deletions(-)
> >
> > diff --git a/mm/huge_memory.c b/mm/huge_memory.c
> > index 165ea46..5881e82 100644
> > --- a/mm/huge_memory.c
> > +++ b/mm/huge_memory.c
> > @@ -67,8 +67,8 @@ bool transparent_hugepage_enabled(struct vm_area_struct *vma)
> >   {
> >   	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 2275a0f..be15e9b 100644
> > --- a/mm/shmem.c
> > +++ b/mm/shmem.c
> > @@ -3873,6 +3873,8 @@ bool shmem_huge_enabled(struct vm_area_struct *vma)
> >   	loff_t i_size;
> >   	pgoff_t off;
> >   
> > +	if (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)
> > --
> > 1.8.3.1