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

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

On Tue, Jun 02, 2026 at 09:30:06AM -0600, Nico Pache wrote:
>On Mon, Jun 1, 2026 at 4:48 AM Lance Yang <lance.yang@linux.dev> wrote:
>>
>>
>>
>> On 2026/6/1 18:23, David Hildenbrand (Arm) wrote:
>> > On 6/1/26 11:08, Lance Yang wrote:
>> >>
>> >>
>> >> On 2026/6/1 14:54, David Hildenbrand (Arm) wrote:
>> >>> On 6/1/26 05:28, Lance Yang wrote:
>> >>>>
>> >>>>
>> >>>> Ah, fair point.
>> >>>>
>> >>>> I was mostly worried about arch hooks that walk vma->vm_mm again, rather
>> >>>> than only using the pte pointer passed in. For example, mips does:
>> >>>
>> >>> Right, a re-walk would be the real problem.
>> >>>
>> >>>>
>> >>>>     update_mmu_cache_range()
>> >>>>       -> __update_tlb()
>> >>>>         -> pgd_offset(vma->vm_mm, address)
>> >>>>         -> pte_offset_map(...)
>> >>>>
>> >>>> and __update_tlb() has this assumption:
>> >>>>
>> >>>>          /*
>> >>>>           * update_mmu_cache() is called between pte_offset_map_lock()
>> >>>>           * and pte_unmap_unlock(), so we can assume that ptep is not
>> >>>>           * NULL here: and what should be done below if it were NULL?
>> >>>>           */
>> >>>>
>> >>>> So if khugepaged happens to run with current->active_mm == vma->vm_mm
>> >>>> here, could __update_tlb() hit the none PMD, get NULL from
>> >>>> pte_offset_map(), and then dereference it?
>> >>>
>> >>> Likely yes -- that MIPS code is horrible. And the comment in MIPS code
>> >>> even spells that out. :(
>> >>>
>> >>> Do you know about other code like that, or is MIPS the only one doing a
>> >>> re-walk and crossing fingers?
>> >>>
>> >>>>
>> >>>> Just wanted to raise it since some arch code may still have assumptions
>> >>>> like this, and the always-enable-mTHP work is getting closer ...
>> >>>
>> >>> Right. I assume set_pte_at() couldn't trigger something similar (re-walk) in
>> >>> arch code,
>> >>> because we simply provide the ptep. update_mmu_cache_range() only consumes the
>> >>> pte.
>> >>>
>> >>>>
>> >>>> Probably very very very hard to hit, though :)
>> >>>
>> >>> Delaying update_mmu_cache_range() is nasty, as we'd have to make sure that
>> >>> nobody can interfere in the meantime ... and the PMD lock will not be sufficient.
>> >>>
>> >>> Maybe we could reinstall the page table with the cleared (none) entries while
>> >>> still holding the PTL?
>> >>>
>> >>> Thinking out loud:
>> >>>
>> >>> diff --git a/mm/khugepaged.c b/mm/khugepaged.c
>> >>> index 5ba298d420b7..e39b750b1e6f 100644
>> >>> --- a/mm/khugepaged.c
>> >>> +++ b/mm/khugepaged.c
>> >>> @@ -1413,13 +1413,17 @@ static enum scan_result collapse_huge_page(struct
>> >>> mm_struct *mm, unsigned long s
>> >>>                   map_anon_folio_pmd_nopf(folio, pmd, vma, pmd_addr);
>> >>>           } else {
>> >>>                   /*
>> >>> -                * set_ptes is called in map_anon_folio_pte_nopf with the
>> >>> -                * pmd_ptl lock still held; this is safe as the PMD is expected
>> >>> -                * to be none. The pmd entry is then repopulated below.
>> >>> +                * Re-insert the page table with the cleared entries, but
>> >>> +                * hold the PTL, such that no one can mess with the re-installed
>> >>> +                * page table until we updated the temporarily-cleared entries
>> >>> +                * through map_anon_folio_pte_nopf().
>> >>>                    */
>> >>> -               map_anon_folio_pte_nopf(folio, pte, vma, start_addr, /
>> >>> *uffd_wp=*/ false);
>> >>> -               smp_wmb(); /* make PTEs visible before PMD. See pmd_install() */
>> >>
>> >> 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!
>
>are we sure? that folio_mark_uptodate is done before the PTEs are
>reinstalled. Then we reinstall the PMD right after. Currently
>separated by the smp_wmb().

Reinstalling the PMD first makes the PTE table reachable again, right?

So before pmd_populate(), we only need to order the old PTE clears before
the PTE table is reachable again; __folio_mark_uptodate() already has the
smp_wmb() for that :)

The new PTEs are filled later under the PTL.

Hopefully I didn't miss soemthing :)

>I was copying this from other THP code that performs similar PTE/PMD juggling.
>
>I can remove it, but I'd rather air on the side of caution with this.

Cheers, Lance