Re: [PATCH v12 mm-new 07/15] khugepaged: generalize collapse_huge_page for mTHP collapse

Lorenzo Stoakes
2025-11-06T18:15:40Z

On Wed, Oct 22, 2025 at 12:37:09PM -0600, 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

NIT but is this -- a typo?

> the mTHP case this is not true, and we must keep the lock to prevent
> changes to the VMA from occurring.
>
> Signed-off-by: Nico Pache <npache@redhat.com>
> ---
> mm/khugepaged.c | 108 ++++++++++++++++++++++++++++++------------------
> 1 file changed, 67 insertions(+), 41 deletions(-)
>
> diff --git a/mm/khugepaged.c b/mm/khugepaged.c
> index 286c3a7afdee..75e7ebdccc36 100644
> --- a/mm/khugepaged.c
> +++ b/mm/khugepaged.c
> @@ -1142,43 +1142,50 @@ static int alloc_charge_folio(struct folio **foliop, struct mm_struct *mm,
> return SCAN_SUCCEED;
> }
>
> -static int collapse_huge_page(struct mm_struct *mm, unsigned long address,
> - int referenced, int unmapped,
> - struct collapse_control *cc)
> +static int collapse_huge_page(struct mm_struct *mm, unsigned long pmd_address,

Presumably pmd_address is the PMD-aligned address?

> + int referenced, int unmapped, struct collapse_control *cc,
> + bool *mmap_locked, unsigned int order, unsigned long offset)

It'd be nice to pass through a helper struct at this point having so many params
but perhaps we can deal with that in a follow up series.

If PMD address is the PMD-aligned address, and mthp_address = pmd_address +
offset * PAGE_SIZE, couldn't we just pass in the mthp address and get the
PMD address by aligning down to PMD size and reduce the number of args by
1?

> {
> LIST_HEAD(compound_pagelist);
> pmd_t *pmd, _pmd;
> - pte_t *pte;
> + pte_t *pte = NULL, mthp_pte;

mthp_pte is only used in a single if () branch and can be declared there
AFAICT?

> pgtable_t pgtable;
> struct folio *folio;
> spinlock_t *pmd_ptl, *pte_ptl;
> int 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;
> + unsigned long mthp_address = pmd_address + offset * PAGE_SIZE;

Do we ever update this? If not we can const-ify.

>
> - VM_BUG_ON(address & ~HPAGE_PMD_MASK);
> + VM_BUG_ON(pmd_address & ~HPAGE_PMD_MASK);

NIT: Be nice to convert this to a VM_WARN_ON_ONCE(), as VM_BUG_ON() is not
right here.

>
> /*
> * 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);
> if (result != SCAN_SUCCEED) {
> mmap_read_unlock(mm);

I don't really love the semantics of 'sometimes we set *mmap_locked false
when we unlock, sometimes we rely on out_nolock doing it'.

Let's just set it false when we unlock and VM_WARN_ON_ONCE(*mmap_locked) in
out_nolock.

> 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);
> goto out_nolock;
> @@ -1190,13 +1197,14 @@ static int collapse_huge_page(struct mm_struct *mm, unsigned long address,
> * 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);
> + result = __collapse_huge_page_swapin(mm, vma, mthp_address, pmd,
> + referenced, order);
> if (result != SCAN_SUCCEED)
> 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
> @@ -1206,20 +1214,20 @@ static int collapse_huge_page(struct mm_struct *mm, unsigned long address,
> * 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, mthp_address,
> + mthp_address + (PAGE_SIZE << order));
> mmu_notifier_invalidate_range_start(&range);
>
> pmd_ptl = pmd_lock(mm, pmd); /* probably unnecessary */
> @@ -1231,24 +1239,21 @@ static int collapse_huge_page(struct mm_struct *mm, unsigned long address,
> * 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);

Not your fault but so hate this _p** convention. One for a follow up I
suppose.

> 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, mthp_address, &pte_ptl);
> if (pte) {
> - result = __collapse_huge_page_isolate(vma, address, pte, cc,
> - HPAGE_PMD_ORDER,
> - &compound_pagelist);
> + result = __collapse_huge_page_isolate(vma, mthp_address, pte, cc,
> + order, &compound_pagelist);
> spin_unlock(pte_ptl);
> } else {
> result = SCAN_PMD_NULL;
> }
>
> if (unlikely(result != SCAN_SUCCEED)) {
> - if (pte)
> - pte_unmap(pte);

OK I guess we drop this because it's handled in out_up_write. I assume no
issue keeping PTE mapped here?

> spin_lock(pmd_ptl);
> BUG_ON(!pmd_none(*pmd));
> /*
> @@ -1258,21 +1263,21 @@ static int collapse_huge_page(struct mm_struct *mm, unsigned long address,
> */
> 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 (order == HPAGE_PMD_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, mthp_address, pte_ptl,
> + order, &compound_pagelist);

Looking through __collapse_huge_page_copy() there doesn't seem to be any
issue with holding anon lock here.

> if (unlikely(result != SCAN_SUCCEED))
> goto out_up_write;
>
> @@ -1282,20 +1287,42 @@ static int collapse_huge_page(struct mm_struct *mm, unsigned long address,
> * write.
> */
> __folio_mark_uptodate(folio);
> - pgtable = pmd_pgtable(_pmd);
> + if (order == HPAGE_PMD_ORDER) {
> + 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_unlock(pmd_ptl);
> + 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);
> + spin_unlock(pmd_ptl);
> + } else { /* mTHP collapse */

As per above, let's just declare mthp_pte here.

> + mthp_pte = mk_pte(folio_page(folio, 0), vma->vm_page_prot);

Hm, so we make a PTE that references the first page of the folio? I guess
the folio will be an mTHP folio so we're just creating essentially a

> + mthp_pte = maybe_mkwrite(pte_mkdirty(mthp_pte), vma);

In set_pte_range() we have a whole host of other checks like dirty,
uffd_wp, etc. I wonder if we need to consider those?

> +
> + spin_lock(pmd_ptl);

We're duplicating this in both branches, why not do outside if/else?

> + WARN_ON_ONCE(!pmd_none(*pmd));

Hmm so the PMD entry will still always be empty on mTHP collapse? Surely we
could be collapsing more than one mTHP into an existing PTE table no? I may
be missing something here/confused :)

> + folio_ref_add(folio, nr_pages - 1);

If we're setting the refcount here, where is the ref count being set in the
PMD path?

> + folio_add_new_anon_rmap(folio, vma, mthp_address, RMAP_EXCLUSIVE);
> + folio_add_lru_vma(folio, vma);
> + set_ptes(vma->vm_mm, mthp_address, pte, mthp_pte, nr_pages);
> + update_mmu_cache_range(NULL, vma, mthp_address, pte, nr_pages);

Prior to this change the only user of this are functions in memory.c, I
do wonder if this is the wrong abstraction here.

But maybe that's _yet another_ thing for a follow up (the THP code is a
mess).

> +
> + smp_wmb(); /* make PTEs visible before PMD. See pmd_install() */

Feels like we could avoid open-coding this by just using pmd_install()?

Also are we therefore missing a mm_inc_nr_ptes() invocation here, or do we
update mm->pgtables_bytes elsewhere?

> + pmd_populate(mm, pmd, pmd_pgtable(_pmd));

Why are we referencing pmd in PMD branch and _pmd here?

> + spin_unlock(pmd_ptl);

The PMD case does this stuff in map_anon_pmd_nopf(), could we add one for
mTHP?

This function is already horribly overwrought (not your fault) so I'd like
to avoid adding open-coded blocks as much as possible.

> + }
>
> folio = NULL;
>
> result = SCAN_SUCCEED;
> out_up_write:
> + if (anon_vma_locked)
> + anon_vma_unlock_write(vma->anon_vma);
> + if (pte)
> + pte_unmap(pte);
> mmap_write_unlock(mm);
> out_nolock:
> + *mmap_locked = false;

See above comment about setting this prior to jumping to out_nolock.

> if (folio)
> folio_put(folio);
> trace_mm_collapse_huge_page(mm, result == SCAN_SUCCEED, result);
> @@ -1463,9 +1490,8 @@ static int 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, 0);
> }
> out:
> trace_mm_khugepaged_scan_pmd(mm, folio, referenced,
> --
> 2.51.0
>