Re: [PATCH] mm: introduce MADV_CLR_HUGEPAGE

[Pavel Emelyanov]

On 05/24/2017 02:31 PM, Vlastimil Babka wrote:
> On 05/24/2017 12:39 PM, Mike Rapoport wrote:
>>> Hm so the prctl does:
>>>
>>>                 if (arg2)
>>>                         me->mm->def_flags |= VM_NOHUGEPAGE;
>>>                 else
>>>                         me->mm->def_flags &= ~VM_NOHUGEPAGE;
>>>
>>> That's rather lazy implementation IMHO. Could we change it so the flag
>>> is stored elsewhere in the mm, and the code that decides to (not) use
>>> THP will check both the per-vma flag and the per-mm flag?
>>
>> I afraid I don't understand how that can help.
>> What we need is an ability to temporarily disable collapse of the pages in
>> VMAs that do not have VM_*HUGEPAGE flags set and that after we re-enable
>> THP, the vma->vm_flags for those VMAs will remain intact.
> 
> That's what I'm saying - instead of implementing the prctl flag via
> mm->def_flags (which gets permanently propagated to newly created vma's
> but e.g. doesn't affect already existing ones), it would be setting a
> flag somewhere in mm, which khugepaged (and page faults) would check in
> addition to the per-vma flags.

I do not insist, but this would make existing paths (checking for flags) be 
2 times slower -- from now on these would need to check two bits (vma flags
and mm flags) which are 100% in different cache lines.

What Mike is proposing is the way to fine-tune the existing vma flags. This
would keep current paths as fast (or slow ;) ) as they are now. All the
complexity would go to rare cases when someone needs to turn thp off for a
while and then turn it back on.

-- Pavel