Re: [PATCH mm-unstable v14 08/16] khugepaged: generalize collapse_huge_page for mTHP collapse

[Lorenzo Stoakes]

On Wed, Feb 04, 2026 at 03:00:57PM -0700, Nico Pache wrote:
> On Tue, Feb 3, 2026 at 6:13 AM Lorenzo Stoakes
> <lorenzo.stoakes@xxxxxxxxxx> wrote:
> >
> > On Thu, Jan 22, 2026 at 12:28:33PM -0700, Nico Pache wrote:
> > > Pass an order and offset to collapse_huge_page to support collapsing anon
> > > memory to arbitrary orders within a PMD. order indicates what mTHP size we
> > > are attempting to collapse to, and offset indicates were in the PMD to
> > > start the collapse attempt.
> > >
> > > For non-PMD collapse we must leave the anon VMA write locked until after
> > > we collapse the mTHP-- in the PMD case all the pages are isolated, but in
> > > the mTHP case this is not true, and we must keep the lock to prevent
> > > changes to the VMA from occurring.
> > >
> > > Reviewed-by: Baolin Wang <baolin.wang@xxxxxxxxxxxxxxxxx>
> > > Tested-by: Baolin Wang <baolin.wang@xxxxxxxxxxxxxxxxx>
> > > Signed-off-by: Nico Pache <npache@xxxxxxxxxx>
> > > ---
> > >  mm/khugepaged.c | 111 +++++++++++++++++++++++++++++++-----------------
> > >  1 file changed, 71 insertions(+), 40 deletions(-)
> > >
> > > diff --git a/mm/khugepaged.c b/mm/khugepaged.c
> > > index 9b7e05827749..76cb17243793 100644
> > > --- a/mm/khugepaged.c
> > > +++ b/mm/khugepaged.c
> > > @@ -1151,44 +1151,54 @@ static enum scan_result alloc_charge_folio(struct folio **foliop, struct mm_stru
> > >       return SCAN_SUCCEED;
> > >  }
> > >
> > > -static enum scan_result collapse_huge_page(struct mm_struct *mm, unsigned long address,
> > > -             int referenced, int unmapped, struct collapse_control *cc)
> > > +static enum scan_result collapse_huge_page(struct mm_struct *mm, unsigned long start_addr,
> > > +             int referenced, int unmapped, struct collapse_control *cc,
> > > +             bool *mmap_locked, unsigned int order)
> > >  {
> > >       LIST_HEAD(compound_pagelist);
> > >       pmd_t *pmd, _pmd;
> > > -     pte_t *pte;
> > > +     pte_t *pte = NULL;
> > >       pgtable_t pgtable;
> > >       struct folio *folio;
> > >       spinlock_t *pmd_ptl, *pte_ptl;
> > >       enum scan_result result = SCAN_FAIL;
> > >       struct vm_area_struct *vma;
> > >       struct mmu_notifier_range range;
> > > +     bool anon_vma_locked = false;
> > > +     const unsigned long nr_pages = 1UL << order;
> > > +     const unsigned long pmd_address = start_addr & HPAGE_PMD_MASK;
> > >
> > > -     VM_BUG_ON(address & ~HPAGE_PMD_MASK);
> > > +     VM_WARN_ON_ONCE(pmd_address & ~HPAGE_PMD_MASK);
> > >
> > >       /*
> > >        * Before allocating the hugepage, release the mmap_lock read lock.
> > >        * The allocation can take potentially a long time if it involves
> > >        * sync compaction, and we do not need to hold the mmap_lock during
> > >        * that. We will recheck the vma after taking it again in write mode.
> > > +      * If collapsing mTHPs we may have already released the read_lock.
> > >        */
> > > -     mmap_read_unlock(mm);
> > > +     if (*mmap_locked) {
> > > +             mmap_read_unlock(mm);
> > > +             *mmap_locked = false;
> > > +     }
> > >
> > > -     result = alloc_charge_folio(&folio, mm, cc, HPAGE_PMD_ORDER);
> > > +     result = alloc_charge_folio(&folio, mm, cc, order);
> > >       if (result != SCAN_SUCCEED)
> > >               goto out_nolock;
> > >
> > >       mmap_read_lock(mm);
> > > -     result = hugepage_vma_revalidate(mm, address, true, &vma, cc,
> > > -                                      HPAGE_PMD_ORDER);
> > > +     *mmap_locked = true;
> > > +     result = hugepage_vma_revalidate(mm, pmd_address, true, &vma, cc, order);
> >
> > Why would we use the PMD address here rather than the actual start address?
>
> The revalidation relies on the pmd_addr not the start_addr. It (only)
> uses this to make sure the VMA is still at least PMD sized, and it
> uses the order to validate that the target order is allowed. I left a
> small comment about this in the revalidate function.

Yeah having these different addresses is a bit icky, easy to make mistakes here.

Oh how we need to refactor all of these...

>
> >
> > Also please add /*expect_anon=*/ before the 'true' because it's hard to
> > understand what that references.
>
> ack
>
> >
> > >       if (result != SCAN_SUCCEED) {
> > >               mmap_read_unlock(mm);
> > > +             *mmap_locked = false;
> > >               goto out_nolock;
> > >       }
> > >
> > > -     result = find_pmd_or_thp_or_none(mm, address, &pmd);
> > > +     result = find_pmd_or_thp_or_none(mm, pmd_address, &pmd);
> > >       if (result != SCAN_SUCCEED) {
> > >               mmap_read_unlock(mm);
> > > +             *mmap_locked = false;
> > >               goto out_nolock;
> > >       }
> > >
> > > @@ -1198,13 +1208,16 @@ static enum scan_result collapse_huge_page(struct mm_struct *mm, unsigned long a
> > >                * released when it fails. So we jump out_nolock directly in
> > >                * that case.  Continuing to collapse causes inconsistency.
> > >                */
> > > -             result = __collapse_huge_page_swapin(mm, vma, address, pmd,
> > > -                                                  referenced, HPAGE_PMD_ORDER);
> > > -             if (result != SCAN_SUCCEED)
> > > +             result = __collapse_huge_page_swapin(mm, vma, start_addr, pmd,
> > > +                                                  referenced, order);
> > > +             if (result != SCAN_SUCCEED) {
> > > +                     *mmap_locked = false;
> > >                       goto out_nolock;
> > > +             }
> > >       }
> > >
> > >       mmap_read_unlock(mm);
> > > +     *mmap_locked = false;
> > >       /*
> > >        * Prevent all access to pagetables with the exception of
> > >        * gup_fast later handled by the ptep_clear_flush and the VM
> > > @@ -1214,20 +1227,20 @@ static enum scan_result collapse_huge_page(struct mm_struct *mm, unsigned long a
> > >        * mmap_lock.
> > >        */
> > >       mmap_write_lock(mm);
> > > -     result = hugepage_vma_revalidate(mm, address, true, &vma, cc,
> > > -                                      HPAGE_PMD_ORDER);
> > > +     result = hugepage_vma_revalidate(mm, pmd_address, true, &vma, cc, order);
> > >       if (result != SCAN_SUCCEED)
> > >               goto out_up_write;
> > >       /* check if the pmd is still valid */
> > >       vma_start_write(vma);
> > > -     result = check_pmd_still_valid(mm, address, pmd);
> > > +     result = check_pmd_still_valid(mm, pmd_address, pmd);
> > >       if (result != SCAN_SUCCEED)
> > >               goto out_up_write;
> > >
> > >       anon_vma_lock_write(vma->anon_vma);
> > > +     anon_vma_locked = true;
> > >
> > > -     mmu_notifier_range_init(&range, MMU_NOTIFY_CLEAR, 0, mm, address,
> > > -                             address + HPAGE_PMD_SIZE);
> > > +     mmu_notifier_range_init(&range, MMU_NOTIFY_CLEAR, 0, mm, start_addr,
> > > +                             start_addr + (PAGE_SIZE << order));
> > >       mmu_notifier_invalidate_range_start(&range);
> > >
> > >       pmd_ptl = pmd_lock(mm, pmd); /* probably unnecessary */
> > > @@ -1239,24 +1252,21 @@ static enum scan_result collapse_huge_page(struct mm_struct *mm, unsigned long a
> > >        * Parallel GUP-fast is fine since GUP-fast will back off when
> > >        * it detects PMD is changed.
> > >        */
> > > -     _pmd = pmdp_collapse_flush(vma, address, pmd);
> > > +     _pmd = pmdp_collapse_flush(vma, pmd_address, pmd);
> > >       spin_unlock(pmd_ptl);
> > >       mmu_notifier_invalidate_range_end(&range);
> > >       tlb_remove_table_sync_one();
> > >
> > > -     pte = pte_offset_map_lock(mm, &_pmd, address, &pte_ptl);
> > > +     pte = pte_offset_map_lock(mm, &_pmd, start_addr, &pte_ptl);
> > >       if (pte) {
> > > -             result = __collapse_huge_page_isolate(vma, address, pte, cc,
> > > -                                                   HPAGE_PMD_ORDER,
> > > -                                                   &compound_pagelist);
> > > +             result = __collapse_huge_page_isolate(vma, start_addr, pte, cc,
> > > +                                                   order, &compound_pagelist);
> > >               spin_unlock(pte_ptl);
> > >       } else {
> > >               result = SCAN_NO_PTE_TABLE;
> > >       }
> > >
> > >       if (unlikely(result != SCAN_SUCCEED)) {
> > > -             if (pte)
> > > -                     pte_unmap(pte);
> > >               spin_lock(pmd_ptl);
> > >               BUG_ON(!pmd_none(*pmd));
> > >               /*
> > > @@ -1266,21 +1276,21 @@ static enum scan_result collapse_huge_page(struct mm_struct *mm, unsigned long a
> > >                */
> > >               pmd_populate(mm, pmd, pmd_pgtable(_pmd));
> > >               spin_unlock(pmd_ptl);
> > > -             anon_vma_unlock_write(vma->anon_vma);
> > >               goto out_up_write;
> > >       }
> > >
> > >       /*
> > > -      * All pages are isolated and locked so anon_vma rmap
> > > -      * can't run anymore.
> > > +      * For PMD collapse all pages are isolated and locked so anon_vma
> > > +      * rmap can't run anymore. For mTHP collapse we must hold the lock
> > >        */
> > > -     anon_vma_unlock_write(vma->anon_vma);
> > > +     if (is_pmd_order(order)) {
> > > +             anon_vma_unlock_write(vma->anon_vma);
> > > +             anon_vma_locked = false;
> > > +     }
> > >
> > >       result = __collapse_huge_page_copy(pte, folio, pmd, _pmd,
> > > -                                        vma, address, pte_ptl,
> > > -                                        HPAGE_PMD_ORDER,
> > > -                                        &compound_pagelist);
> > > -     pte_unmap(pte);
> > > +                                        vma, start_addr, pte_ptl,
> > > +                                        order, &compound_pagelist);
> > >       if (unlikely(result != SCAN_SUCCEED))
> > >               goto out_up_write;
> > >
> > > @@ -1290,20 +1300,42 @@ static enum scan_result collapse_huge_page(struct mm_struct *mm, unsigned long a
> > >        * write.
> > >        */
> > >       __folio_mark_uptodate(folio);
> > > -     pgtable = pmd_pgtable(_pmd);
> > > +     if (is_pmd_order(order)) { /* PMD collapse */
> > > +             pgtable = pmd_pgtable(_pmd);
> > >
> > > -     spin_lock(pmd_ptl);
> > > -     BUG_ON(!pmd_none(*pmd));
> > > -     pgtable_trans_huge_deposit(mm, pmd, pgtable);
> > > -     map_anon_folio_pmd_nopf(folio, pmd, vma, address);
> > > +             spin_lock(pmd_ptl);
> > > +             WARN_ON_ONCE(!pmd_none(*pmd));
> > > +             pgtable_trans_huge_deposit(mm, pmd, pgtable);
> > > +             map_anon_folio_pmd_nopf(folio, pmd, vma, pmd_address);
> > > +     } else { /* mTHP collapse */
> > > +             pte_t mthp_pte = mk_pte(folio_page(folio, 0), vma->vm_page_prot);
> > > +
> > > +             mthp_pte = maybe_mkwrite(pte_mkdirty(mthp_pte), vma);
> > > +             spin_lock(pmd_ptl);
> > > +             WARN_ON_ONCE(!pmd_none(*pmd));
> > > +             folio_ref_add(folio, nr_pages - 1);
> > > +             folio_add_new_anon_rmap(folio, vma, start_addr, RMAP_EXCLUSIVE);
> > > +             folio_add_lru_vma(folio, vma);
> > > +             set_ptes(vma->vm_mm, start_addr, pte, mthp_pte, nr_pages);
> > > +             update_mmu_cache_range(NULL, vma, start_addr, pte, nr_pages);
> > > +
> > > +             smp_wmb(); /* make PTEs visible before PMD. See pmd_install() */
> > > +             pmd_populate(mm, pmd, pmd_pgtable(_pmd));
> >
> > I seriously hate this being open-coded, can we separate it out into another
> > function?
>
> Yeah I think we've discussed this before. I started to generalize
> this, and apply it to other parts of the kernel that maintain a
> similar pattern, but each potential user of the helper was slightly
> different in its approach and I was unable to find a quick solution to
> make it apply to all. I think it will require a lot more thought to
> cleanly refactor this. I figured I could leave this to the later
> cleanup work, or I could just create a static function just for
> khugepaged for now?

Yeah let's at least separate it out for this logic anyway.

Really we should have done the refactoring in advance of these changes, but that
ship has sailed :)

>
> >
> > > +     }
> > >       spin_unlock(pmd_ptl);
> > >
> > >       folio = NULL;
> > >
> > >       result = SCAN_SUCCEED;
> > >  out_up_write:
> > > +     if (anon_vma_locked)
> > > +             anon_vma_unlock_write(vma->anon_vma);
> >
> > Thanks it's much better tracking this specifically.
> >
> > The whole damn thing needs refactoring (by this I mean - khugepaged and really
> > THP in general to be clear :) but it's not your fault.
>
> Yeah it has not been the prettiest code to try and understand/work on!

:)

>
> >
> > Could I ask though whether you might help out with some cleanups after this
> > lands :)
> >
> > I feel like we all need to do our bit to pay down some technical debt!
>
>
> Yes ofc! I had already planned on doing so. I have some in mind, and I
> believe others have already tackled some. After this land, let's
> discuss further plans (discussion thread or THP meeting).

Yeah, I'll get that TODO list discussed in meeting shared soon...

>
> Cheers,
> -- Nico
>
> >
> > > +     if (pte)
> > > +             pte_unmap(pte);
> > >       mmap_write_unlock(mm);
> > > +     *mmap_locked = false;
> > >  out_nolock:
> > > +     WARN_ON_ONCE(*mmap_locked);
> > >       if (folio)
> > >               folio_put(folio);
> > >       trace_mm_collapse_huge_page(mm, result == SCAN_SUCCEED, result);
> > > @@ -1471,9 +1503,8 @@ static enum scan_result collapse_scan_pmd(struct mm_struct *mm,
> > >       pte_unmap_unlock(pte, ptl);
> > >       if (result == SCAN_SUCCEED) {
> > >               result = collapse_huge_page(mm, start_addr, referenced,
> > > -                                         unmapped, cc);
> > > -             /* collapse_huge_page will return with the mmap_lock released */
> > > -             *mmap_locked = false;
> > > +                                         unmapped, cc, mmap_locked,
> > > +                                         HPAGE_PMD_ORDER);
> > >       }
> > >  out:
> > >       trace_mm_khugepaged_scan_pmd(mm, folio, referenced,
> > > --
> > > 2.52.0
> > >
> >
> > Cheers, Lorenzo
> >
>

Cheers, Lorenzo