Re: [PATCH] mm: make PR_SET_THP_DISABLE immediately active

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

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?

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