[RFC 2/2] mm/mthp: relax anon mTHP PTE Mapping restrictions

[Lance Yang <ioworker0@gmail.com>]

Previously, mTHP could only be mapped to PTEs where all entries were none.
With this change, PTEs within the range mapping the demand-zero page can
now be treated as `pte_none` and remapped to a new mTHP, providing more
opportunities to take advantage of mTHP.

Signed-off-by: Mingzhe Yang <mingzhe.yang@ly.com>
Signed-off-by: Lance Yang <ioworker0@gmail.com>
---
 mm/memory.c | 11 +++++++++--
 1 file changed, 9 insertions(+), 2 deletions(-)

diff --git a/mm/memory.c b/mm/memory.c
index 4e148309b3e0..99ec75c6f0fe 100644
--- a/mm/memory.c
+++ b/mm/memory.c
@@ -4815,7 +4815,8 @@ static struct folio *alloc_anon_folio(struct vm_fault *vmf)
 	order = highest_order(orders);
 	while (orders) {
 		addr = ALIGN_DOWN(vmf->address, PAGE_SIZE << order);
-		if (pte_range_none(pte + pte_index(addr), 1 << order))
+		if (pte_range_none_or_zeropfn(pte + pte_index(addr), 1 << order,
+					      NULL))
 			break;
 		order = next_order(&orders, order);
 	}
@@ -4867,6 +4868,7 @@ static vm_fault_t do_anonymous_page(struct vm_fault *vmf)
 {
 	struct vm_area_struct *vma = vmf->vma;
 	unsigned long addr = vmf->address;
+	bool any_zeropfn = false;
 	struct folio *folio;
 	vm_fault_t ret = 0;
 	int nr_pages = 1;
@@ -4939,7 +4941,8 @@ static vm_fault_t do_anonymous_page(struct vm_fault *vmf)
 	if (nr_pages == 1 && vmf_pte_changed(vmf)) {
 		update_mmu_tlb(vma, addr, vmf->pte);
 		goto release;
-	} else if (nr_pages > 1 && !pte_range_none(vmf->pte, nr_pages)) {
+	} else if (nr_pages > 1 && !pte_range_none_or_zeropfn(
+					   vmf->pte, nr_pages, &any_zeropfn)) {
 		update_mmu_tlb_range(vma, addr, vmf->pte, nr_pages);
 		goto release;
 	}
@@ -4965,6 +4968,10 @@ static vm_fault_t do_anonymous_page(struct vm_fault *vmf)
 		entry = pte_mkuffd_wp(entry);
 	set_ptes(vma->vm_mm, addr, vmf->pte, entry, nr_pages);
 
+	/* At least one PTE was mapped to the zero page */
+	if (nr_pages > 1 && any_zeropfn)
+		flush_tlb_range(vma, addr, addr + (nr_pages * PAGE_SIZE));
+
 	/* No need to invalidate - it was non-present before */
 	update_mmu_cache_range(vmf, vma, addr, vmf->pte, nr_pages);
 unlock:
-- 
2.45.2