Re: [PATCH mm-unstable v18 06/14] mm/khugepaged: generalize collapse_huge_page for mTHP collapse

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

On 2026/6/1 23:05, David Hildenbrand (Arm) wrote:
> On 6/1/26 17:00, Nico Pache wrote:
>> On Mon, Jun 1, 2026 at 5:14 AM David Hildenbrand (Arm) <david@kernel.org> wrote:
>>>
>>> On 6/1/26 12:47, Lance Yang wrote:
>>>>
>>>>
>>>>
>>>> Ah, cool! __folio_mark_uptodate() already does the job :P
>>>>
>>>> So yeah, no extra smp_wmb() needed here!
>>>
>>> Yeah. BTW, I think we'd need a spin_lock_nested(), so @Nico, treat my code as a
>>> draft.
>>
>> Okay, I read the above and did some investigating.
>>
>> I will try to implement and verify the changes you suggested :)
>>
>> Or an even crazier idea... what if we ensure MIPS checks for PMD_none
>> before walking a PTE table?
> 
> But how would they update the cache then correctly?
> 
> I'm too non-MIPS to know the answer :)

Right, that's my concern as well ...

If MIPS sees pmd_none(), it has no PTE table to walk, so it also has
no way to do the cache update it wanted to do, I guess :)

But, the PTE table is not really gone there. khugepaged only cleared
the PMD temporarily while still using the old PTE table through _pmd.

So I'd go with David's suggestion:

"
Best to make sure the page table is already installed when updating
the entries.
"

Cheers, Lance