Re: [v2 PATCH] mm: thp: fix false negative of shmem vma's THP eligibility

[Yang Shi]

On 4/23/19 11:34 AM, Yang Shi wrote:
> On 4/23/19 10:52 AM, Michal Hocko wrote:
> > On Wed 24-04-19 00:43:01, 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:
> > >
> > > 7fc92ec00000-7fc92f000000 rw-s 00000000 00:14 27764 /dev/shm/test
> > > Size:ÂÂÂÂÂÂÂÂÂÂÂÂÂÂ 4096 kB
> > > ...
> > > [snip]
> > > ...
> > > ShmemPmdMapped:ÂÂÂÂ 4096 kB
> > > ...
> > > [snip]
> > > ...
> > > THPeligible:ÂÂÂ 0
> > >
> > > And, /proc/meminfo does show THP allocated and PMD mapped too:
> > >
> > > ShmemHugePages:ÂÂÂÂ 4096 kB
> > > ShmemPmdMapped:ÂÂÂÂ 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.
> > Kirill, can we get a confirmation that this is really intended behavior
> > rather than an omission please? Is this documented? What is a global
> > knob to simply disable THP system wise?
> >
> > I have to say that the THP tuning API is one giant mess :/
> >
> > Btw. this patch also seem to fix khugepaged behavior because it 
> > previously
> > ignored both VM_NOHUGEPAGE and MMF_DISABLE_THP.

Second look shows this is not ignored. hugepage_vma_check() would check 
this for both anonymous vma and shmem vma before scanning. It is called 
before shmem_huge_enabled().

> Aha, I didn't notice this. It looks we need separate the patch to fix 
> that khugepaged problem for both 5.1-rc and LTS.
>
> > > 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>
> > > ---
> > > v2: Check VM_NOHUGEPAGE per Michal Hocko
> > >
> > > Â mm/huge_memory.c | 4 ++--
> > > Â mm/shmem.cÂÂÂÂÂÂ | 3 +++
> > > Â 2 files changed, 5 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)
> > > Â {
> > > ÂÂÂÂÂ if (vma_is_anonymous(vma))
> > > ÂÂÂÂÂÂÂÂÂ return __transparent_hugepage_enabled(vma);
> > > -ÂÂÂ if (vma_is_shmem(vma) && shmem_huge_enabled(vma))
> > > -ÂÂÂÂÂÂÂ return __transparent_hugepage_enabled(vma);
> > > +ÂÂÂ if (vma_is_shmem(vma))
> > > +ÂÂÂÂÂÂÂ return shmem_huge_enabled(vma);
> > > Â ÂÂÂÂÂ return false;
> > > Â }
> > > diff --git a/mm/shmem.c b/mm/shmem.c
> > > index 2275a0f..6f09a31 100644
> > > --- a/mm/shmem.c
> > > +++ b/mm/shmem.c
> > > @@ -3873,6 +3873,9 @@ bool shmem_huge_enabled(struct vm_area_struct 
> > > *vma)
> > > ÂÂÂÂÂ loff_t i_size;
> > > ÂÂÂÂÂ pgoff_t off;
> > > Â +ÂÂÂ if ((vma->vm_flags & VM_NOHUGEPAGE) ||
> > > +ÂÂÂÂÂÂÂ test_bit(MMF_DISABLE_THP, &vma->vm_mm->flags))
> > > +ÂÂÂÂÂÂÂ return false;
> > > ÂÂÂÂÂ if (shmem_huge == SHMEM_HUGE_FORCE)
> > > ÂÂÂÂÂÂÂÂÂ return true;
> > > ÂÂÂÂÂ if (shmem_huge == SHMEM_HUGE_DENY)
> > > --
> > > 1.8.3.1