On Wed 31-05-17 16:18:09, Andrea Arcangeli wrote:

On Wed, May 31, 2017 at 03:39:22PM +0300, Mike Rapoport wrote:

quoted

For the CRIU usecase, disabling THP for a while and re-enabling it
back will do the trick, provided VMAs flags are not affected, like
in the patch you've sent.  Moreover, we may even get away with

Are you going to check uname -r to know when the kABI changed in your
favor (so CRIU cannot ever work with enterprise backports unless you
expand the uname -r coverage), or how do you know the patch is
applied?

I would assume such a patch would be backported to stable trees because
to me it sounds like the current semantic is simply broken and needs
fixing anyway but it shouldn't be much different from any other bugs.

This is far from ideal from the "guarantee POV" of course.

Optimistically assuming people is going to run new CRIU code only on
new kernels looks very risky, it would leads to silent random memory
corruption, so I doubt you can get away without a uname -r check.

This is fairly simple change too, its main cons is that it adds a
branch to the page fault fast path, the old behavior of the prctl and
the new madvise were both zero cost.

Still if the prctl is preferred despite the added branch, to avoid
uname -r clashes, to me it sounds better to add a new prctl ID and
keep the old one too. The old one could be implemented the same way as
the new one if you want to save a few bytes of .text. But the old one
should probably do a printk_once to print a deprecation warning so the
old ID with weaker (zero runtime cost) semantics can be removed later.

this would be an option as well although it adds to the mess...

-- 
Michal Hocko
SUSE Labs