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
