[PATCH 2/2] mm: Allocate THP on hugezeropage wp-fault

[Dev Jain]

Introduce do_huge_zero_wp_pmd() to handle wp-fault on a hugezeropage and
replace it with a PMD-mapped THP. Change the helpers introduced in the
previous patch to flush TLB entry corresponding to the hugezeropage,
and preserve PMD uffd-wp marker. In case of failure, fallback to
splitting the PMD.

Signed-off-by: Dev Jain <dev.jain at arm.com>
---
 include/linux/huge_mm.h |  7 ++++
 mm/huge_memory.c        | 76 +++++++++++++++++++++++++++++++++++------
 mm/memory.c             |  5 +--
 3 files changed, 76 insertions(+), 12 deletions(-)

diff --git a/include/linux/huge_mm.h b/include/linux/huge_mm.h
index e25d9ebfdf89..375dba4fb130 100644
--- a/include/linux/huge_mm.h
+++ b/include/linux/huge_mm.h
@@ -9,6 +9,13 @@
 #include <linux/kobject.h>
 
 vm_fault_t do_huge_pmd_anonymous_page(struct vm_fault *vmf);
+vm_fault_t thp_fault_alloc(gfp_t gfp, int order, struct vm_area_struct *vma,
+			   unsigned long haddr, struct folio **foliop,
+			   unsigned long addr);
+void map_pmd_thp(struct folio *folio, struct vm_fault *vmf,
+		 struct vm_area_struct *vma, unsigned long haddr,
+		 pgtable_t pgtable)
+	__releases(vmf->ptl);
 int copy_huge_pmd(struct mm_struct *dst_mm, struct mm_struct *src_mm,
 		  pmd_t *dst_pmd, pmd_t *src_pmd, unsigned long addr,
 		  struct vm_area_struct *dst_vma, struct vm_area_struct *src_vma);
diff --git a/mm/huge_memory.c b/mm/huge_memory.c
index e5b568e2bb34..0f8b2e224795 100644
--- a/mm/huge_memory.c
+++ b/mm/huge_memory.c
@@ -943,9 +943,9 @@ unsigned long thp_get_unmapped_area(struct file *filp, unsigned long addr,
 }
 EXPORT_SYMBOL_GPL(thp_get_unmapped_area);
 
-static vm_fault_t thp_fault_alloc(gfp_t gfp, int order, struct vm_area_struct *vma,
-				  unsigned long haddr, struct folio **foliop,
-				  unsigned long addr)
+vm_fault_t thp_fault_alloc(gfp_t gfp, int order, struct vm_area_struct *vma,
+			   unsigned long haddr, struct folio **foliop,
+			   unsigned long addr)
 {
 	struct folio *folio = vma_alloc_folio(gfp, order, vma, haddr, true);
 
@@ -984,22 +984,30 @@ static void __thp_fault_success_stats(struct vm_area_struct *vma, int order)
 	count_memcg_event_mm(vma->vm_mm, THP_FAULT_ALLOC);
 }
 
-static void map_pmd_thp(struct folio *folio, struct vm_fault *vmf,
-			struct vm_area_struct *vma, unsigned long haddr,
-			pgtable_t pgtable)
+void map_pmd_thp(struct folio *folio, struct vm_fault *vmf,
+		 struct vm_area_struct *vma, unsigned long haddr,
+		 pgtable_t pgtable)
 	__releases(vmf->ptl)
 {
-	pmd_t entry;
+	pmd_t entry, old_pmd;
+	bool is_pmd_none = pmd_none(*vmf->pmd);
 
 	entry = mk_huge_pmd(&folio->page, vma->vm_page_prot);
 	entry = maybe_pmd_mkwrite(pmd_mkdirty(entry), vma);
 	folio_add_new_anon_rmap(folio, vma, haddr, RMAP_EXCLUSIVE);
 	folio_add_lru_vma(folio, vma);
-	pgtable_trans_huge_deposit(vma->vm_mm, vmf->pmd, pgtable);
+	if (!is_pmd_none) {
+		old_pmd = pmdp_huge_clear_flush(vma, haddr, vmf->pmd);
+		if (pmd_uffd_wp(old_pmd))
+			entry = pmd_mkuffd_wp(entry);
+	}
+	if (pgtable)
+		pgtable_trans_huge_deposit(vma->vm_mm, vmf->pmd, pgtable);
 	set_pmd_at(vma->vm_mm, haddr, vmf->pmd, entry);
 	update_mmu_cache_pmd(vma, vmf->address, vmf->pmd);
 	add_mm_counter(vma->vm_mm, MM_ANONPAGES, HPAGE_PMD_NR);
-	mm_inc_nr_ptes(vma->vm_mm);
+	if (is_pmd_none)
+		mm_inc_nr_ptes(vma->vm_mm);
 	spin_unlock(vmf->ptl);
 	__thp_fault_success_stats(vma, HPAGE_PMD_ORDER);
 }
@@ -1577,6 +1585,47 @@ void huge_pmd_set_accessed(struct vm_fault *vmf)
 	spin_unlock(vmf->ptl);
 }
 
+static vm_fault_t do_huge_zero_wp_pmd_locked(struct vm_fault *vmf,
+					     unsigned long haddr)
+{
+	struct vm_area_struct *vma = vmf->vma;
+	gfp_t gfp = vma_thp_gfp_mask(vma);
+	struct folio *folio = NULL;
+	vm_fault_t ret;
+
+	ret = thp_fault_alloc(gfp, HPAGE_PMD_ORDER, vma, haddr, &folio,
+			      vmf->address);
+	if (ret)
+		goto unlock;
+	ret = check_stable_address_space(vma->vm_mm);
+	if (ret)
+		goto unlock;
+	map_pmd_thp(folio, vmf, vma, haddr, NULL);
+	return 0;
+
+unlock:
+	spin_unlock(vmf->ptl);
+	return ret;
+}
+
+static vm_fault_t do_huge_zero_wp_pmd(struct vm_fault *vmf, unsigned long haddr)
+{
+	struct vm_area_struct *vma = vmf->vma;
+	struct mmu_notifier_range range;
+	vm_fault_t ret = 0;
+
+	mmu_notifier_range_init(&range, MMU_NOTIFY_CLEAR, 0, vma->vm_mm, haddr,
+				haddr + HPAGE_PMD_SIZE);
+	mmu_notifier_invalidate_range_start(&range);
+	vmf->ptl = pmd_lock(vma->vm_mm, vmf->pmd);
+	if (likely(pmd_same(pmdp_get(vmf->pmd), vmf->orig_pmd)))
+		ret = do_huge_zero_wp_pmd_locked(vmf, haddr);
+	else
+		spin_unlock(vmf->ptl);
+	mmu_notifier_invalidate_range_end(&range);
+	return ret;
+}
+
 vm_fault_t do_huge_pmd_wp_page(struct vm_fault *vmf)
 {
 	const bool unshare = vmf->flags & FAULT_FLAG_UNSHARE;
@@ -1589,8 +1638,15 @@ vm_fault_t do_huge_pmd_wp_page(struct vm_fault *vmf)
 	vmf->ptl = pmd_lockptr(vma->vm_mm, vmf->pmd);
 	VM_BUG_ON_VMA(!vma->anon_vma, vma);
 
-	if (is_huge_zero_pmd(orig_pmd))
+	if (is_huge_zero_pmd(orig_pmd)) {
+		vm_fault_t ret = do_huge_zero_wp_pmd(vmf, haddr);
+
+		if (!(ret & VM_FAULT_FALLBACK))
+			return ret;
+
+		/* Fallback to splitting PMD if THP cannot be allocated */
 		goto fallback;
+	}
 
 	spin_lock(vmf->ptl);
 
diff --git a/mm/memory.c b/mm/memory.c
index 3c01d68065be..c081a25f5173 100644
--- a/mm/memory.c
+++ b/mm/memory.c
@@ -5409,9 +5409,10 @@ static inline vm_fault_t wp_huge_pmd(struct vm_fault *vmf)
 	if (vma_is_anonymous(vma)) {
 		if (likely(!unshare) &&
 		    userfaultfd_huge_pmd_wp(vma, vmf->orig_pmd)) {
-			if (userfaultfd_wp_async(vmf->vma))
+			if (!userfaultfd_wp_async(vmf->vma))
+				return handle_userfault(vmf, VM_UFFD_WP);
+			if (!is_huge_zero_pmd(vmf->orig_pmd))
 				goto split;
-			return handle_userfault(vmf, VM_UFFD_WP);
 		}
 		return do_huge_pmd_wp_page(vmf);
 	}
-- 
2.30.2