On Wed 26-09-18 08:12:47, Michal Hocko wrote:

On Tue 25-09-18 14:50:52, David Rientjes wrote:
[...]
Let's put my general disagreement with the approach asside for a while.
If this is really the best way forward the is the implementation really
correct?

quoted

+	/*
+	 * Disabling thp is possible through both MADV_NOHUGEPAGE and
+	 * PR_SET_THP_DISABLE.  Both historically used VM_NOHUGEPAGE.  Since
+	 * the introduction of MMF_DISABLE_THP, however, userspace needs the
+	 * ability to detect vmas where thp is not eligible in the same manner.
+	 */
+	if (vma->vm_mm && test_bit(MMF_DISABLE_THP, &vma->vm_mm->flags)) {
+		flags &= ~VM_HUGEPAGE;
+		flags |= VM_NOHUGEPAGE;
+	}

Do we want to report all vmas nh? Shouldn't we limit that to THP-able
mappings? It seems quite strange that an application started without
PR_SET_THP_DISABLE wouldn't report nh for most mappings while it would
otherwise. Also when can we have vma->vm_mm == NULL?

Hmm, after re-reading your documentation update to "A process mapping
may be advised to not be backed by transparent hugepages by either
madvise(MADV_NOHUGEPAGE) or prctl(PR_SET_THP_DISABLE)." the
implementation matches so scratch my comment.

As I've said, I am not happy about this approach but if there is a
general agreement this is really the best we can do I will not stand in
the way.
-- 
Michal Hocko
SUSE Labs