Re: [RFC 09/11] khugepaged: add mTHP support

Dev Jain
2025-01-10T09:21:11Z

On 09/01/25 5:01 am, Nico Pache wrote:
> Introduce the ability for khugepaged to collapse to different mTHP sizes.
> While scanning a PMD range for potential hugepage collapse, track pages
> in MIN_MTHP_ORDER chunks. Each bit represents a fully utilized region of
> order MIN_MTHP_ORDER ptes.
>
> With this bitmap we can determine which mTHP sizes would be the most
> efficient to collapse to if the PMD collapse is not suitible.
>
> Signed-off-by: Nico Pache <npache@redhat.com>
> ---
> mm/khugepaged.c | 111 +++++++++++++++++++++++++++++++++---------------
> 1 file changed, 77 insertions(+), 34 deletions(-)
>
> diff --git a/mm/khugepaged.c b/mm/khugepaged.c
> index de1dc6ea3c71..4d3c560f20b4 100644
> --- a/mm/khugepaged.c
> +++ b/mm/khugepaged.c
> @@ -1139,13 +1139,14 @@ static int collapse_huge_page(struct mm_struct *mm, unsigned long address,
> {
> LIST_HEAD(compound_pagelist);
> pmd_t *pmd, _pmd;
> - pte_t *pte;
> + pte_t *pte, mthp_pte;
> 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;
> + unsigned long _address = address + offset * PAGE_SIZE;
> VM_BUG_ON(address & ~HPAGE_PMD_MASK);
>
> /* if collapsing mTHPs we may have already released the read_lock, and
> @@ -1162,12 +1163,13 @@ static int collapse_huge_page(struct mm_struct *mm, unsigned long address,
> 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, address, true, &vma, cc, order);
> if (result != SCAN_SUCCEED) {
> mmap_read_unlock(mm);
> goto out_nolock;
> @@ -1185,13 +1187,14 @@ 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, _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
> @@ -1201,7 +1204,7 @@ 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, address, true, &vma, cc, order);
> if (result != SCAN_SUCCEED)
> goto out_up_write;
> /* check if the pmd is still valid */
> @@ -1212,11 +1215,12 @@ static int collapse_huge_page(struct mm_struct *mm, unsigned long address,
> vma_start_write(vma);
> anon_vma_lock_write(vma->anon_vma);
>
> - 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, _address,
> + _address + (PAGE_SIZE << order));

Since we are nuking the PMD for both cases, we do not need to change it
for order, this should remain address + HPAGE_PMD_SIZE.

> mmu_notifier_invalidate_range_start(&range);
>
> 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
> @@ -1230,10 +1234,10 @@ static int collapse_huge_page(struct mm_struct *mm, unsigned long address,
> 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, _address, &pte_ptl);
> if (pte) {
> - result = __collapse_huge_page_isolate(vma, address, pte, cc,
> - &compound_pagelist, HPAGE_PMD_ORDER);
> + result = __collapse_huge_page_isolate(vma, _address, pte, cc,
> + &compound_pagelist, order);
> spin_unlock(pte_ptl);
> } else {
> result = SCAN_PMD_NULL;
> @@ -1262,8 +1266,8 @@ static int collapse_huge_page(struct mm_struct *mm, unsigned long address,
> anon_vma_unlock_write(vma->anon_vma);
>
> result = __collapse_huge_page_copy(pte, folio, pmd, _pmd,
> - vma, address, pte_ptl,
> - &compound_pagelist, HPAGE_PMD_ORDER);
> + vma, _address, pte_ptl,
> + &compound_pagelist, order);
> pte_unmap(pte);
> if (unlikely(result != SCAN_SUCCEED))
> goto out_up_write;
> @@ -1274,20 +1278,37 @@ static int collapse_huge_page(struct mm_struct *mm, unsigned long address,
> * write.
> */
> __folio_mark_uptodate(folio);
> - pgtable = pmd_pgtable(_pmd);
> -
> - _pmd = mk_huge_pmd(&folio->page, vma->vm_page_prot);
> - _pmd = maybe_pmd_mkwrite(pmd_mkdirty(_pmd), vma);
> -
> - spin_lock(pmd_ptl);
> - BUG_ON(!pmd_none(*pmd));
> - folio_add_new_anon_rmap(folio, vma, address, RMAP_EXCLUSIVE);
> - folio_add_lru_vma(folio, vma);
> - pgtable_trans_huge_deposit(mm, pmd, pgtable);
> - set_pmd_at(mm, address, pmd, _pmd);
> - update_mmu_cache_pmd(vma, address, pmd);
> - deferred_split_folio(folio, false);
> - spin_unlock(pmd_ptl);
> + if (order == HPAGE_PMD_ORDER) {
> + pgtable = pmd_pgtable(_pmd);
> + _pmd = mk_huge_pmd(&folio->page, vma->vm_page_prot);
> + _pmd = maybe_pmd_mkwrite(pmd_mkdirty(_pmd), vma);
> +
> + spin_lock(pmd_ptl);
> + BUG_ON(!pmd_none(*pmd));
> + folio_add_new_anon_rmap(folio, vma, _address, RMAP_EXCLUSIVE);
> + folio_add_lru_vma(folio, vma);
> + pgtable_trans_huge_deposit(mm, pmd, pgtable);
> + set_pmd_at(mm, address, pmd, _pmd);
> + update_mmu_cache_pmd(vma, address, pmd);
> + deferred_split_folio(folio, false);
> + spin_unlock(pmd_ptl);
> + } else { //mTHP
> + mthp_pte = mk_pte(&folio->page, vma->vm_page_prot);
> + mthp_pte = maybe_mkwrite(pte_mkdirty(mthp_pte), vma);
> +
> + spin_lock(pmd_ptl);
> + folio_ref_add(folio, (1 << order) - 1);
> + folio_add_new_anon_rmap(folio, vma, _address, RMAP_EXCLUSIVE);
> + folio_add_lru_vma(folio, vma);
> + spin_lock(pte_ptl);
> + set_ptes(vma->vm_mm, _address, pte, mthp_pte, (1 << order));
> + update_mmu_cache_range(NULL, vma, _address, pte, (1 << order));
> + spin_unlock(pte_ptl);
> + smp_wmb(); /* make pte visible before pmd */
> + pmd_populate(mm, pmd, pmd_pgtable(_pmd));
> + deferred_split_folio(folio, false);
> + spin_unlock(pmd_ptl);
> + }

You have done lock nesting here: lock(pmd_ptl) -> lock(pte_ptl) ->
unlock(pte_ptl) -> unlock(pmd_ptl). Anyways, you do not need to take
pmd_ptl when you are setting the ptes. I am almost done with my v2, and
according to me this function should look like this:

/* Similar to the PMD case except we have to batch set the PTEs */
static int vma_collapse_anon_folio(struct mm_struct *mm, unsigned long
address,
struct vm_area_struct *vma, struct collapse_control *cc, pmd_t *pmd,
struct folio *folio, int order)
{
LIST_HEAD(compound_pagelist);
spinlock_t *pmd_ptl, *pte_ptl;
int result = SCAN_FAIL;
struct mmu_notifier_range range;
pmd_t _pmd;
pte_t *pte;
pte_t entry;
int nr_pages = folio_nr_pages(folio);
unsigned long haddress = address & HPAGE_PMD_MASK;

VM_BUG_ON(address & ((1UL << order) - 1));;

mmap_read_unlock(mm);

mmap_write_lock(mm);
result = hugepage_vma_revalidate(mm, address, true, &vma, order, cc);
if (result != SCAN_SUCCEED)
goto out_up_write;
result = check_pmd_still_valid(mm, address, pmd);
if (result != SCAN_SUCCEED)
goto out_up_write;

vma_start_write(vma);
anon_vma_lock_write(vma->anon_vma);

mmu_notifier_range_init(&range, MMU_NOTIFY_CLEAR, 0, mm, haddress,
haddress + HPAGE_PMD_SIZE);
mmu_notifier_invalidate_range_start(&range);

pmd_ptl = pmd_lock(mm, pmd);
_pmd = pmdp_collapse_flush(vma, haddress, 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);
if (pte) {
result = __collapse_huge_page_isolate(vma, address, pte, cc,
&compound_pagelist, order);
spin_unlock(pte_ptl);
} else {
result = SCAN_PMD_NULL;
}

if (unlikely(result != SCAN_SUCCEED)) {
if (pte)
pte_unmap(pte);
spin_lock(pmd_ptl);
BUG_ON(!pmd_none(*pmd));
pmd_populate(mm, pmd, pmd_pgtable(_pmd));
spin_unlock(pmd_ptl);
anon_vma_unlock_write(vma->anon_vma);
goto out_up_write;
}

anon_vma_unlock_write(vma->anon_vma);

__folio_mark_uptodate(folio);
entry = mk_pte(&folio->page, vma->vm_page_prot);
entry = maybe_mkwrite(pte_mkdirty(entry), vma);

result = __collapse_huge_page_copy(pte, folio, pmd, *pmd,
vma, address, pte_ptl,
&compound_pagelist, order);
pte_unmap(pte);
if (unlikely(result != SCAN_SUCCEED))
goto out_up_write;

folio_ref_add(folio, nr_pages - 1);
folio_add_new_anon_rmap(folio, vma, address, RMAP_EXCLUSIVE);
folio_add_lru_vma(folio, vma);
spin_lock(pte_ptl);
set_ptes(mm, address, pte, entry, nr_pages);
spin_unlock(pte_ptl);
spin_lock(pmd_ptl);

/* See pmd_install() */
smp_wmb();
pmd_populate(mm, pmd, pmd_pgtable(_pmd));
update_mmu_cache_pmd(vma, haddress, pmd);
spin_unlock(pmd_ptl);

result = SCAN_SUCCEED;
out_up_write:
mmap_write_unlock(mm);
return result;
}

The difference being, I take the pte_ptl, set the ptes, drop the
pte_ptl, then take pmd_ptl, do pmd_populate(). Now, instead of
update_mmu_cache_range() in the mTHP case, we still need to do
update_mmu_cache_pmd() since we are repopulating the PMD. And, IIUC
update_mmu_cache_pmd() is a superset of update_mmu_cache_range(), so we
can drop the latter altogether.

>
> folio = NULL;
>
> @@ -1367,21 +1388,26 @@ static int khugepaged_scan_pmd(struct mm_struct *mm,
> {
> pmd_t *pmd;
> pte_t *pte, *_pte;
> + int i;
> int result = SCAN_FAIL, referenced = 0;
> int none_or_zero = 0, shared = 0;
> struct page *page = NULL;
> struct folio *folio = NULL;
> unsigned long _address;
> + unsigned long enabled_orders;
> spinlock_t *ptl;
> int node = NUMA_NO_NODE, unmapped = 0;
> bool writable = false;
> -
> + bool all_valid = true;
> + unsigned long tva_flags = cc->is_khugepaged ? TVA_ENFORCE_SYSFS : 0;
> VM_BUG_ON(address & ~HPAGE_PMD_MASK);
>
> result = find_pmd_or_thp_or_none(mm, address, &pmd);
> if (result != SCAN_SUCCEED)
> goto out;
>
> + bitmap_zero(cc->mthp_bitmap, 1 << (HPAGE_PMD_ORDER - MIN_MTHP_ORDER));
> + bitmap_zero(cc->mthp_bitmap_temp, 1 << (HPAGE_PMD_ORDER - MIN_MTHP_ORDER));
> memset(cc->node_load, 0, sizeof(cc->node_load));
> nodes_clear(cc->alloc_nmask);
> pte = pte_offset_map_lock(mm, pmd, address, &ptl);
> @@ -1390,8 +1416,12 @@ static int khugepaged_scan_pmd(struct mm_struct *mm,
> goto out;
> }
>
> - for (_address = address, _pte = pte; _pte < pte + HPAGE_PMD_NR;
> - _pte++, _address += PAGE_SIZE) {
> + for (i = 0; i < HPAGE_PMD_NR; i++) {
> + if (i % MIN_MTHP_NR == 0)
> + all_valid = true;
> +
> + _pte = pte + i;
> + _address = address + i * PAGE_SIZE;
> pte_t pteval = ptep_get(_pte);
> if (is_swap_pte(pteval)) {
> ++unmapped;
> @@ -1414,6 +1444,7 @@ static int khugepaged_scan_pmd(struct mm_struct *mm,
> }
> }
> if (pte_none(pteval) || is_zero_pfn(pte_pfn(pteval))) {
> + all_valid = false;
> ++none_or_zero;
> if (!userfaultfd_armed(vma) &&
> (!cc->is_khugepaged ||
> @@ -1514,7 +1545,15 @@ static int khugepaged_scan_pmd(struct mm_struct *mm,
> folio_test_referenced(folio) || mmu_notifier_test_young(vma->vm_mm,
> address)))
> referenced++;
> +
> + /*
> + * we are reading in MIN_MTHP_NR page chunks. if there are no empty
> + * pages keep track of it in the bitmap for mTHP collapsing.
> + */
> + if (all_valid && (i + 1) % MIN_MTHP_NR == 0)
> + bitmap_set(cc->mthp_bitmap, i / MIN_MTHP_NR, 1);
> }
> +
> if (!writable) {
> result = SCAN_PAGE_RO;
> } else if (cc->is_khugepaged &&
> @@ -1527,10 +1566,12 @@ static int khugepaged_scan_pmd(struct mm_struct *mm,
> out_unmap:
> pte_unmap_unlock(pte, ptl);
> if (result == SCAN_SUCCEED) {
> - result = collapse_huge_page(mm, address, referenced,
> - unmapped, cc, mmap_locked, HPAGE_PMD_ORDER, 0);
> - /* collapse_huge_page will return with the mmap_lock released */
> - *mmap_locked = false;
> + enabled_orders = thp_vma_allowable_orders(vma, vma->vm_flags,
> + tva_flags, THP_ORDERS_ALL_ANON);
> + result = khugepaged_scan_bitmap(mm, address, referenced, unmapped, cc,
> + mmap_locked, enabled_orders);
> + if (result > 0)
> + result = SCAN_SUCCEED;
> }
> out:
> trace_mm_khugepaged_scan_pmd(mm, &folio->page, writable, referenced,
> @@ -2477,11 +2518,13 @@ static int khugepaged_collapse_single_pmd(unsigned long addr, struct mm_struct *
> fput(file);
> if (result == SCAN_PTE_MAPPED_HUGEPAGE) {
> mmap_read_lock(mm);
> + *mmap_locked = true;
> if (khugepaged_test_exit_or_disable(mm))
> goto end;
> result = collapse_pte_mapped_thp(mm, addr,
> !cc->is_khugepaged);
> mmap_read_unlock(mm);
> + *mmap_locked = false;
> }
> } else {
> result = khugepaged_scan_pmd(mm, vma, addr,