Re: [patch v2] mm, thp: always specify ineligible vmas as nh in smaps

[Michal Hocko]

On Mon 24-09-18 21:56:03, Michal Hocko wrote:
> On Mon 24-09-18 12:30:07, David Rientjes wrote:
> > Commit 1860033237d4 ("mm: make PR_SET_THP_DISABLE immediately active")
> > introduced a regression in that userspace cannot always determine the set
> > of vmas where thp is ineligible.
> > 
> > Userspace relies on the "nh" flag being emitted as part of /proc/pid/smaps
> > to determine if a vma is eligible to be backed by hugepages.
> 
> I was under impression that nh resp hg flags only tell about the madvise
> status. How do you exactly use these flags in an application?
> 
> Your eligible rules as defined here:
> 
> > + [*] A process mapping is eligible to be backed by transparent hugepages (thp)
> > +     depending on system-wide settings and the mapping itself.  See
> > +     Documentation/admin-guide/mm/transhuge.rst for default behavior.  If a
> > +     mapping has a flag of "nh", it is not eligible to be backed by hugepages
> > +     in any condition, either because of prctl(PR_SET_THP_DISABLE) or
> > +     madvise(MADV_NOHUGEPAGE).  PR_SET_THP_DISABLE takes precedence over any
> > +     MADV_HUGEPAGE.
> 
> doesn't seem to match the reality. I do not see all the file backed
> mappings to be nh marked. So is this really about eligibility rather
> than the madvise status? Maybe it is just the above documentation that
> needs to be updated.
> 
> That being said, I do not object to the patch, I am just trying to
> understand what is the intended usage for the flag that does try to say
> more than the madvise status.

And moreover, how is the PR_SET_THP_DISABLE any different from the
global THP disabled case. Do we want to set all vmas to nh as well?
-- 
Michal Hocko
SUSE Labs