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

Nico Pache
2025-11-07T19:33:41Z

On Thu, Nov 6, 2025 at 11:15 AM Lorenzo Stoakes
<lorenzo.stoakes@oracle.com> wrote:
>
> 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?

no its an em dash. I can replace it with a period if you'd like, but
both work in this context.

>
> > 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?

Yeah that seems like a good idea. Thanks

>
> > {
> > 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?

ack!

>
> > 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.

ack!

>
> >
> > - 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.

Ok that sounds like a good idea! thanks

>
> > 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?

Correct, I dont think there are any issues here. The checks for pte
and anon_vma_locked in out_up_write should keep everything in order.

>
> > 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.

ack

>
> > + 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?

I dont believe so because those checks are coming from fault handling.
Here we are doing almost the same thing that the PMD case was doing
with some influence from do_anonymous_page()

>
> > +
> > + spin_lock(pmd_ptl);
>
> We're duplicating this in both branches, why not do outside if/else?

ack

>
> > + 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 :)

We remove the PMD entry to ensure no GUP-fast call can operate on this PMD.

pmd_ptl = pmd_lock(mm, pmd); /* probably unnecessary */
/*
* This removes any huge TLB entry from the CPU so we won't allow
* huge and small TLB entries for the same virtual address to
* avoid the risk of CPU bugs in that area.
*
* Parallel GUP-fast is fine since GUP-fast will back off when
* it detects PMD is changed.
*/
_pmd = pmdp_collapse_flush(vma, pmd_address, pmd);

pmdp_collapse_flush removes the PMD
pmd = pmdp_huge_get_and_clear(vma->vm_mm, address, pmdp);

In the PMD case we install a new PMD, in the mTHP case (and in the
failure cases), we reinstall the same PMD once we are done/exit.

>
> > + 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?

Both folios are initiated with a single ref. PMDs only need 1 ref,
while mTHPs need a ref for each PTE; hence why we increment by
nr_pages - 1.

>
> > + 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).

Yes, I tried to do something similar to the new
map_anon_folio_pmd_nopf, but it proved to be harder than expected. The
other cases that do similar operations all differ slightly so unifying
is going to be tricky/require more testing.

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

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

The locking seems to differ which may make that tricky.

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

If I understand correctly, we already have accounted for the ptes when
we alloc'd them and their parent PMD. Since we are operating on an
already allocated PMD, I dont think we need to handle accounting for
PMD or mTHP collapse. Ill send some time confirming this before
posting.

>
>
> > + pmd_populate(mm, pmd, pmd_pgtable(_pmd));
>
> Why are we referencing pmd in PMD branch and _pmd here?

I explained it a little more above, but we are reinstalling the
original PMD entry, which was removed for gup race reasons.

>
> > + spin_unlock(pmd_ptl);
>
> The PMD case does this stuff in map_anon_pmd_nopf(), could we add one for
> mTHP?

Yes but I believe we should clean it up after. Unifying most of the
callers proved tricky.

>
> 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.

ack

Thanks for the reviews!
-- Nico

>
> > 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
> >
>