On 9/25/18 11:50 PM, 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 disabled.

Userspace relies on the "nh" flag being emitted as part of /proc/pid/smaps
to determine if a vma has been disabled from being backed by hugepages.

Previous to this commit, prctl(PR_SET_THP_DISABLE, 1) would cause thp to
be disabled and emit "nh" as a flag for the corresponding vmas as part of
/proc/pid/smaps.  After the commit, thp is disabled by means of an mm
flag and "nh" is not emitted.

This causes smaps parsing libraries to assume a vma is enabled for thp
and ends up puzzling the user on why its memory is not backed by thp.

This also clears the "hg" flag to make the behavior of MADV_HUGEPAGE and
PR_SET_THP_DISABLE definitive.

Fixes: 1860033237d4 ("mm: make PR_SET_THP_DISABLE immediately active")
Signed-off-by: David Rientjes <rientjes@google.com>

Well, as Andrew said, we had the opportunity to provide a more complete
info to userspace e.g. with Michal's suggested /proc/pid/status
enhancement. If this is good enough for you (and nobody else cares) then
I won't block it either. It would be unfortunate though if we could not
revert this in case the MMF_DISABLE_THP querying is implemented later.
Hopefully the only consumers are internal tools such as yours, which can
be easily adapted...

Vlastimil