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

On Mon, Jun 1, 2026 at 9:00 AM Nico Pache [off-list ref] wrote:

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

quoted

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:

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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 :)

I've implemented something slightly different actually and I *think* its better!

} else {
       /* this is map_anon_folio_pte_nopf with no mmu update */
        __map_anon_folio_pte_nopf(folio, pte, vma, start_addr,
                      /*uffd_wp=*/ false);
       smp_wmb();
        pmd_populate(mm, pmd, pmd_pgtable(_pmd));
        /*
         * Some architectures (e.g. MIPS) walk the live page table in
         * their implementation. update_mmu_cache_range() must be called
         * with a valid page table hierarchy and the PTE lock held.
         * Acquire it nested inside pmd_ptl when they are distinct locks.
         */
        if (pte_ptl != pmd_ptl)
            spin_lock_nested(pte_ptl, SINGLE_DEPTH_NESTING);
        update_mmu_cache_range(NULL, vma, start_addr, pte, nr_pages);
        if (pte_ptl != pmd_ptl)
            spin_unlock(pte_ptl);
    }
spin_unlock(pmd_ptl);

The logic here is that when the PMD becomes visible, PTEs are already
populated (no possibility of spurious faults on local CPU)

the SMP_WMB makes sure of the above

And the pmd is installed with the pte and pmd lock both held through
the mmu_cache update.

This follows the conventions used in pmd_install() and clears the
potential for local CPU faults hitting cleared PTE entries.

I think both approaches are correct but this prevents any possibility
of my first point. although mmap_write_lock prevents this too.

Let me know what you think. I can revert to your implementation but
this is what I tested.

Cheers,
-- Nico

Or an even crazier idea... what if we ensure MIPS checks for PMD_none
before walking a PTE table?

-- Nico

quoted

--
Cheers,

David