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

[Lance Yang]

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!

Cheers, Lance