[PATCH mm-unstable v15 05/13] mm/khugepaged: generalize collapse_huge_page for mTHP collapse

Nico Pache
2026-02-26T03:24:43Z

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.

Also convert these BUG_ON's to WARN_ON_ONCE's as these conditions, while
unexpected, should not bring down the system.

Reviewed-by: Baolin Wang <baolin.wang@linux.alibaba.com>
Tested-by: Baolin Wang <baolin.wang@linux.alibaba.com>
Signed-off-by: Nico Pache <npache@redhat.com>
---
mm/khugepaged.c | 102 +++++++++++++++++++++++++++++-------------------
1 file changed, 62 insertions(+), 40 deletions(-)

diff --git a/mm/khugepaged.c b/mm/khugepaged.c
index 99f78f0e44c6..fb3ba8fe5a6c 100644
--- a/mm/khugepaged.c
+++ b/mm/khugepaged.c
@@ -1150,44 +1150,53 @@ 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 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);
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;
}

@@ -1197,13 +1206,16 @@ 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
@@ -1213,20 +1225,20 @@ 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 */
@@ -1238,24 +1250,21 @@ 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));
/*
@@ -1265,21 +1274,21 @@ 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;

@@ -1289,20 +1298,34 @@ 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 */
+ spin_lock(pmd_ptl);
+ WARN_ON_ONCE(!pmd_none(*pmd));
+ map_anon_folio_pte_nopf(folio, pte, vma, start_addr, /*uffd_wp=*/ false);
+ smp_wmb(); /* make PTEs visible before PMD. See pmd_install() */
+ pmd_populate(mm, pmd, pmd_pgtable(_pmd));
+ }
spin_unlock(pmd_ptl);

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);
+ *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);
@@ -1483,9 +1506,8 @@ 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,
--
2.53.0