Re: [PATCH] mm: make PR_SET_THP_DISABLE immediately active

[Vlastimil Babka]

On 06/02/2017 09:50 PM, Andrew Morton wrote:
> On Fri,  2 Jun 2017 18:03:22 +0300 "Mike Rapoport" <rppt@xxxxxxxxxxxxxxxxxx> 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. And as usual, if
somebody does complain in the end, we revert and try the other way?