[RFC 09/11] khugepaged: add mTHP support

[Nico Pache <npache@redhat.com>]

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));
 	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);
+	}
 
 	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,
-- 
2.47.1