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

On Mon, Jun 1, 2026 at 5:14 AM David Hildenbrand (Arm) [off-list ref] wrote:

On 6/1/26 12:47, Lance Yang wrote:

quoted

On 2026/6/1 18:23, David Hildenbrand (Arm) wrote:

quoted

On 6/1/26 11:08, Lance Yang wrote:

quoted

One small thing, I think we should probably keep the smp_wmb(), and just
move it before the earlier pmd_populate().

IIUC, the ordering we want is still:

   clear old PTEs
   smp_wmb()
   pmd_populate()

so another CPU cannot walk through the re-installed PMD and still observe
the old PTEs, right?

There is a smp_wmb() in __folio_mark_uptodate(), that should be sufficient?

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?

-- Nico

--
Cheers,

David