Re: [PATCH] mm: make PR_SET_THP_DISABLE immediately active

[Andrew Morton <akpm@linux-foundation.org>]

On Fri, 2 Jun 2017 22:31:47 +0200 Vlastimil Babka <vbabka@suse.cz> wrote:

> On 06/02/2017 09:50 PM, Andrew Morton wrote:
> > On Fri,  2 Jun 2017 18:03:22 +0300 "Mike Rapoport" <rppt@linux.vnet.ibm.com> wrote:
> > 
> >> PR_SET_THP_DISABLE has a rather subtle semantic. It doesn't affect any
> >> existing mapping because it only updated mm->def_flags which is a template
> >> for new mappings. The mappings created after prctl(PR_SET_THP_DISABLE) have
> >> VM_NOHUGEPAGE flag set.  This can be quite surprising for all those
> >> applications which do not do prctl(); fork() & exec() and want to control
> >> their own THP behavior.
> >>
> >> Another usecase when the immediate semantic of the prctl might be useful is
> >> a combination of pre- and post-copy migration of containers with CRIU.  In
> >> this case CRIU populates a part of a memory region with data that was saved
> >> during the pre-copy stage. Afterwards, the region is registered with
> >> userfaultfd and CRIU expects to get page faults for the parts of the region
> >> that were not yet populated. However, khugepaged collapses the pages and
> >> the expected page faults do not occur.
> >>
> >> In more general case, the prctl(PR_SET_THP_DISABLE) could be used as a
> >> temporary mechanism for enabling/disabling THP process wide.
> >>
> >> Implementation wise, a new MMF_DISABLE_THP flag is added. This flag is
> >> tested when decision whether to use huge pages is taken either during page
> >> fault of at the time of THP collapse.
> >>
> >> It should be noted, that the new implementation makes PR_SET_THP_DISABLE
> >> master override to any per-VMA setting, which was not the case previously.
> >>
> >> Fixes: a0715cc22601 ("mm, thp: add VM_INIT_DEF_MASK and PRCTL_THP_DISABLE")
> > 
> > "Fixes" is a bit strong.  I'd say "alters".  And significantly altering
> > the runtime behaviour of a three-year-old interface is rather a worry,
> > no?
> > 
> > Perhaps we should be adding new prctl modes to select this new
> > behaviour and leave the existing PR_SET_THP_DISABLE behaviour as-is?
> 
> I think we can reasonably assume that most users of the prctl do just
> the fork() & exec() thing, so they will be unaffected.

That sounds optimistic.  Perhaps people are using the current behaviour
to set on particular mapping to MMF_DISABLE_THP, with

	prctl(PR_SET_THP_DISABLE)
	mmap()
	prctl(PR_CLR_THP_DISABLE)

?

Seems a reasonable thing to do.  But who knows - people do all sorts of
inventive things.

> And as usual, if
> somebody does complain in the end, we revert and try the other way?

But by then it's too late - the new behaviour will be out in the field.

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