In the following two cases, the PTE page cannot be empty and cannot be
reclaimed:

1. an uffd-wp pte was re-installed
2. should_zap_folio() return false

Let's expose this information to the caller through is_pt_unreclaimable,
so that subsequent commits can use this information in zap_pte_range() to
detect whether the PTE page can be reclaimed.

Signed-off-by: Qi Zheng <zhengqi.arch@bytedance.com>
---
 mm/memory.c | 25 +++++++++++++++----------
 1 file changed, 15 insertions(+), 10 deletions(-)

diff --git a/mm/memory.c b/mm/memory.c
index 534d9d52b5ebe..cc89ede8ce2ab 100644
--- a/mm/memory.c
+++ b/mm/memory.c
@@ -1501,7 +1501,7 @@ static __always_inline void zap_present_folio_ptes(struct mmu_gather *tlb,
 		struct vm_area_struct *vma, struct folio *folio,
 		struct page *page, pte_t *pte, pte_t ptent, unsigned int nr,
 		unsigned long addr, struct zap_details *details, int *rss,
-		bool *force_flush, bool *force_break)
+		bool *force_flush, bool *force_break, bool *is_pt_unreclaimable)
 {
 	struct mm_struct *mm = tlb->mm;
 	bool delay_rmap = false;
@@ -1527,8 +1527,8 @@ static __always_inline void zap_present_folio_ptes(struct mmu_gather *tlb,
 	arch_check_zapped_pte(vma, ptent);
 	tlb_remove_tlb_entries(tlb, pte, nr, addr);
 	if (unlikely(userfaultfd_pte_wp(vma, ptent)))
-		zap_install_uffd_wp_if_needed(vma, addr, pte, nr, details,
-					      ptent);
+		*is_pt_unreclaimable =
+			zap_install_uffd_wp_if_needed(vma, addr, pte, nr, details, ptent);
 
 	if (!delay_rmap) {
 		folio_remove_rmap_ptes(folio, page, nr, vma);
@@ -1552,7 +1552,7 @@ static inline int zap_present_ptes(struct mmu_gather *tlb,
 		struct vm_area_struct *vma, pte_t *pte, pte_t ptent,
 		unsigned int max_nr, unsigned long addr,
 		struct zap_details *details, int *rss, bool *force_flush,
-		bool *force_break)
+		bool *force_break, bool *is_pt_unreclaimable)
 {
 	const fpb_t fpb_flags = FPB_IGNORE_DIRTY | FPB_IGNORE_SOFT_DIRTY;
 	struct mm_struct *mm = tlb->mm;
@@ -1567,15 +1567,18 @@ static inline int zap_present_ptes(struct mmu_gather *tlb,
 		arch_check_zapped_pte(vma, ptent);
 		tlb_remove_tlb_entry(tlb, pte, addr);
 		if (userfaultfd_pte_wp(vma, ptent))
-			zap_install_uffd_wp_if_needed(vma, addr, pte, 1,
-						      details, ptent);
+			*is_pt_unreclaimable =
+				zap_install_uffd_wp_if_needed(vma, addr, pte, 1,
+							      details, ptent);
 		ksm_might_unmap_zero_page(mm, ptent);
 		return 1;
 	}
 
 	folio = page_folio(page);
-	if (unlikely(!should_zap_folio(details, folio)))
+	if (unlikely(!should_zap_folio(details, folio))) {
+		*is_pt_unreclaimable = true;
 		return 1;
+	}
 
 	/*
 	 * Make sure that the common "small folio" case is as fast as possible
@@ -1587,11 +1590,12 @@ static inline int zap_present_ptes(struct mmu_gather *tlb,
 
 		zap_present_folio_ptes(tlb, vma, folio, page, pte, ptent, nr,
 				       addr, details, rss, force_flush,
-				       force_break);
+				       force_break, is_pt_unreclaimable);
 		return nr;
 	}
 	zap_present_folio_ptes(tlb, vma, folio, page, pte, ptent, 1, addr,
-			       details, rss, force_flush, force_break);
+			       details, rss, force_flush, force_break,
+			       is_pt_unreclaimable);
 	return 1;
 }
 
@@ -1622,6 +1626,7 @@ static unsigned long zap_pte_range(struct mmu_gather *tlb,
 		pte_t ptent = ptep_get(pte);
 		struct folio *folio;
 		struct page *page;
+		bool is_pt_unreclaimable = false;
 		int max_nr;
 
 		nr = 1;
@@ -1635,7 +1640,7 @@ static unsigned long zap_pte_range(struct mmu_gather *tlb,
 			max_nr = (end - addr) / PAGE_SIZE;
 			nr = zap_present_ptes(tlb, vma, pte, ptent, max_nr,
 					      addr, details, rss, &force_flush,
-					      &force_break);
+					      &force_break, &is_pt_unreclaimable);
 			if (unlikely(force_break)) {
 				addr += nr * PAGE_SIZE;
 				break;
-- 
2.20.1