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

[Michal Hocko]

On Tue 23-04-19 17:22:36, Yang Shi wrote:
> 
> 
> 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:               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.
> > > 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().

Right. I have missed the earlier check. The main question remains
though.

-- 
Michal Hocko
SUSE Labs