Re: [PATCH mm-new 1/3] mm/khugepaged: skip unsuitable VMAs earlier in khugepaged_scan_mm_slot()

[Lance Yang <lance.yang@linux.dev>]

Hi Hugh,

Thanks for taking a look and for raising this important point!

On 2025/9/16 13:32, Hugh Dickins wrote:
> On Sun, 14 Sep 2025, Lance Yang wrote:
> 
>> From: Lance Yang <lance.yang@linux.dev>
>>
>> Let's skip unsuitable VMAs early in the khugepaged scan; specifically,
>> mlocked VMAs should not be touched.
> 
> Why?  That's a change in behaviour, isn't it?
> 
> I'm aware that hugepage collapse on an mlocked VMA can insert a fault
> latency, not universally welcome; but I've not seen discussion, let
> alone agreement, that current behaviour should be changed.
> Somewhere in yet-to-be-read mail?  Please give us a link.
> 
> Hugh

You're right, this is indeed a change in behaviour. But it's specifically
for khugepaged.

Users of mlock() expect low and predictable latency. THP collapse is a
heavy operation that introduces exactly the kind of unpredictable delays
they want to avoid. It has to unmap PTEs, copy data from the small folios
to a new THP, and then remap the THP back to the PMD ;)

IMO, that change is acceptable because THP is generally transparent to
users, and khugepaged does not guarantee when THP collapse or split will
happen.

Well, we don't have a discussion on that, just something I noticed.

Thanks,
Lance