Message-ID: <f95f7dc3201d62cdbe56ea397201974f5a586bb4.1761288179.git.lorenzo.stoakes@oracle.com>
Date: Fri, 24 Oct 2025 08:41:18 +0100
From: Lorenzo Stoakes <lorenzo.stoakes@...cle.com>
To: Andrew Morton <akpm@...ux-foundation.org>
Cc: Christian Borntraeger <borntraeger@...ux.ibm.com>,
        Janosch Frank <frankja@...ux.ibm.com>,
        Claudio Imbrenda <imbrenda@...ux.ibm.com>,
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        Baolin Wang <baolin.wang@...ux.alibaba.com>,
        "Liam R . Howlett" <Liam.Howlett@...cle.com>,
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        linux-mm@...ck.org
Subject: [RFC PATCH 02/12] mm: avoid unnecessary uses of is_swap_pte()

There's an established convention in the kernel that we treat PTEs as
containing swap entries (and the unfortunately named non-swap swap entries)
should they be neither empty (i.e. pte_none() evaluating true) nor present
(i.e. pte_present() evaluating true).

However, there is some inconsistency in how this is applied, as we also
have the is_swap_pte() helper which explicitly performs this check:

	/* check whether a pte points to a swap entry */
	static inline int is_swap_pte(pte_t pte)
	{
		return !pte_none(pte) && !pte_present(pte);
	}

As this represents a predicate, and it's logical to assume that in order to
establish that a PTE entry can correctly be manipulated as a swap/non-swap
entry, this predicate seems as if it must first be checked.

But we instead, we far more often utilise the established convention of
checking pte_none() / pte_present() before operating on entries as if they
were swap/non-swap.

This patch works towards correcting this inconsistency by removing all uses
of is_swap_pte() where we are already in a position where we perform
pte_none()/pte_present() checks anyway or otherwise it is clearly logical
to do so.

We also take advantage of the fact that pte_swp_uffd_wp() is only set on
swap entries.

Additionally, update comments referencing to is_swap_pte() and
non_swap_entry().

Signed-off-by: Lorenzo Stoakes <lorenzo.stoakes@...cle.com>
---
 fs/proc/task_mmu.c            | 49 ++++++++++++++++++++++++-----------
 include/linux/userfaultfd_k.h |  9 ++++---
 mm/hugetlb.c                  |  6 ++---
 mm/internal.h                 |  6 ++---
 mm/khugepaged.c               | 29 +++++++++++----------
 mm/migrate.c                  |  2 +-
 mm/mprotect.c                 | 43 ++++++++++++++----------------
 mm/mremap.c                   |  7 +++--
 mm/page_table_check.c         | 13 ++++++----
 mm/page_vma_mapped.c          | 27 ++++++++++---------
 10 files changed, 106 insertions(+), 85 deletions(-)

diff --git a/fs/proc/task_mmu.c b/fs/proc/task_mmu.c
index a7c8501266f4..5475acfa1a33 100644
--- a/fs/proc/task_mmu.c
+++ b/fs/proc/task_mmu.c
@@ -1017,7 +1017,9 @@ static void smaps_pte_entry(pte_t *pte, unsigned long addr,
 		young = pte_young(ptent);
 		dirty = pte_dirty(ptent);
 		present = true;
-	} else if (is_swap_pte(ptent)) {
+	} else if (pte_none(ptent)) {
+		smaps_pte_hole_lookup(addr, walk);
+	} else {
 		swp_entry_t swpent = pte_to_swp_entry(ptent);
 
 		if (!non_swap_entry(swpent)) {
@@ -1038,9 +1040,6 @@ static void smaps_pte_entry(pte_t *pte, unsigned long addr,
 				present = true;
 			page = pfn_swap_entry_to_page(swpent);
 		}
-	} else {
-		smaps_pte_hole_lookup(addr, walk);
-		return;
 	}
 
 	if (!page)
@@ -1611,6 +1610,9 @@ static inline void clear_soft_dirty(struct vm_area_struct *vma,
 	 */
 	pte_t ptent = ptep_get(pte);
 
+	if (pte_none(ptent))
+		return;
+
 	if (pte_present(ptent)) {
 		pte_t old_pte;
 
@@ -1620,7 +1622,7 @@ static inline void clear_soft_dirty(struct vm_area_struct *vma,
 		ptent = pte_wrprotect(old_pte);
 		ptent = pte_clear_soft_dirty(ptent);
 		ptep_modify_prot_commit(vma, addr, pte, old_pte, ptent);
-	} else if (is_swap_pte(ptent)) {
+	} else {
 		ptent = pte_swp_clear_soft_dirty(ptent);
 		set_pte_at(vma->vm_mm, addr, pte, ptent);
 	}
@@ -1923,6 +1925,9 @@ static pagemap_entry_t pte_to_pagemap_entry(struct pagemapread *pm,
 	struct page *page = NULL;
 	struct folio *folio;
 
+	if (pte_none(pte))
+		goto out;
+
 	if (pte_present(pte)) {
 		if (pm->show_pfn)
 			frame = pte_pfn(pte);
@@ -1932,8 +1937,9 @@ static pagemap_entry_t pte_to_pagemap_entry(struct pagemapread *pm,
 			flags |= PM_SOFT_DIRTY;
 		if (pte_uffd_wp(pte))
 			flags |= PM_UFFD_WP;
-	} else if (is_swap_pte(pte)) {
+	} else {
 		swp_entry_t entry;
+
 		if (pte_swp_soft_dirty(pte))
 			flags |= PM_SOFT_DIRTY;
 		if (pte_swp_uffd_wp(pte))
@@ -1941,6 +1947,7 @@ static pagemap_entry_t pte_to_pagemap_entry(struct pagemapread *pm,
 		entry = pte_to_swp_entry(pte);
 		if (pm->show_pfn) {
 			pgoff_t offset;
+
 			/*
 			 * For PFN swap offsets, keeping the offset field
 			 * to be PFN only to be compatible with old smaps.
@@ -1969,6 +1976,8 @@ static pagemap_entry_t pte_to_pagemap_entry(struct pagemapread *pm,
 		    __folio_page_mapped_exclusively(folio, page))
 			flags |= PM_MMAP_EXCLUSIVE;
 	}
+
+out:
 	if (vma->vm_flags & VM_SOFTDIRTY)
 		flags |= PM_SOFT_DIRTY;
 
@@ -2310,12 +2319,16 @@ static unsigned long pagemap_page_category(struct pagemap_scan_private *p,
 					   struct vm_area_struct *vma,
 					   unsigned long addr, pte_t pte)
 {
-	unsigned long categories = 0;
+	unsigned long categories;
+
+	if (pte_none(pte))
+		return 0;
 
 	if (pte_present(pte)) {
 		struct page *page;
 
-		categories |= PAGE_IS_PRESENT;
+		categories = PAGE_IS_PRESENT;
+
 		if (!pte_uffd_wp(pte))
 			categories |= PAGE_IS_WRITTEN;
 
@@ -2329,10 +2342,11 @@ static unsigned long pagemap_page_category(struct pagemap_scan_private *p,
 			categories |= PAGE_IS_PFNZERO;
 		if (pte_soft_dirty(pte))
 			categories |= PAGE_IS_SOFT_DIRTY;
-	} else if (is_swap_pte(pte)) {
+	} else {
 		swp_entry_t swp;
 
-		categories |= PAGE_IS_SWAPPED;
+		categories = PAGE_IS_SWAPPED;
+
 		if (!pte_swp_uffd_wp_any(pte))
 			categories |= PAGE_IS_WRITTEN;
 
@@ -2360,12 +2374,12 @@ static void make_uffd_wp_pte(struct vm_area_struct *vma,
 		old_pte = ptep_modify_prot_start(vma, addr, pte);
 		ptent = pte_mkuffd_wp(old_pte);
 		ptep_modify_prot_commit(vma, addr, pte, old_pte, ptent);
-	} else if (is_swap_pte(ptent)) {
-		ptent = pte_swp_mkuffd_wp(ptent);
-		set_pte_at(vma->vm_mm, addr, pte, ptent);
-	} else {
+	} else if (pte_none(ptent)) {
 		set_pte_at(vma->vm_mm, addr, pte,
 			   make_pte_marker(PTE_MARKER_UFFD_WP));
+	} else {
+		ptent = pte_swp_mkuffd_wp(ptent);
+		set_pte_at(vma->vm_mm, addr, pte, ptent);
 	}
 }
 
@@ -2434,6 +2448,9 @@ static unsigned long pagemap_hugetlb_category(pte_t pte)
 {
 	unsigned long categories = PAGE_IS_HUGE;
 
+	if (pte_none(pte))
+		return categories;
+
 	/*
 	 * According to pagemap_hugetlb_range(), file-backed HugeTLB
 	 * page cannot be swapped. So PAGE_IS_FILE is not checked for
@@ -2441,6 +2458,7 @@ static unsigned long pagemap_hugetlb_category(pte_t pte)
 	 */
 	if (pte_present(pte)) {
 		categories |= PAGE_IS_PRESENT;
+
 		if (!huge_pte_uffd_wp(pte))
 			categories |= PAGE_IS_WRITTEN;
 		if (!PageAnon(pte_page(pte)))
@@ -2449,8 +2467,9 @@ static unsigned long pagemap_hugetlb_category(pte_t pte)
 			categories |= PAGE_IS_PFNZERO;
 		if (pte_soft_dirty(pte))
 			categories |= PAGE_IS_SOFT_DIRTY;
-	} else if (is_swap_pte(pte)) {
+	} else {
 		categories |= PAGE_IS_SWAPPED;
+
 		if (!pte_swp_uffd_wp_any(pte))
 			categories |= PAGE_IS_WRITTEN;
 		if (pte_swp_soft_dirty(pte))
diff --git a/include/linux/userfaultfd_k.h b/include/linux/userfaultfd_k.h
index 4c65adff2e7a..a362e1619b95 100644
--- a/include/linux/userfaultfd_k.h
+++ b/include/linux/userfaultfd_k.h
@@ -462,13 +462,14 @@ static inline bool pte_marker_uffd_wp(pte_t pte)
 static inline bool pte_swp_uffd_wp_any(pte_t pte)
 {
 #ifdef CONFIG_PTE_MARKER_UFFD_WP
-	if (!is_swap_pte(pte))
-		return false;
+	swp_entry_t entry;
 
+	if (pte_present(pte))
+		return false;
 	if (pte_swp_uffd_wp(pte))
 		return true;
-
-	if (pte_marker_uffd_wp(pte))
+	entry = pte_to_swp_entry(pte);
+	if (pte_marker_entry_uffd_wp(entry))
 		return true;
 #endif
 	return false;
diff --git a/mm/hugetlb.c b/mm/hugetlb.c
index 86e672fcb305..4510029761ed 100644
--- a/mm/hugetlb.c
+++ b/mm/hugetlb.c
@@ -5798,13 +5798,13 @@ static void move_huge_pte(struct vm_area_struct *vma, unsigned long old_addr,
 
 	pte = huge_ptep_get_and_clear(mm, old_addr, src_pte, sz);
 
-	if (need_clear_uffd_wp && pte_marker_uffd_wp(pte))
+	if (need_clear_uffd_wp && pte_marker_uffd_wp(pte)) {
 		huge_pte_clear(mm, new_addr, dst_pte, sz);
-	else {
+	} else {
 		if (need_clear_uffd_wp) {
 			if (pte_present(pte))
 				pte = huge_pte_clear_uffd_wp(pte);
-			else if (is_swap_pte(pte))
+			else
 				pte = pte_swp_clear_uffd_wp(pte);
 		}
 		set_huge_pte_at(mm, new_addr, dst_pte, pte, sz);
diff --git a/mm/internal.h b/mm/internal.h
index cbd3d897b16c..b855a4412878 100644
--- a/mm/internal.h
+++ b/mm/internal.h
@@ -325,8 +325,7 @@ unsigned int folio_pte_batch(struct folio *folio, pte_t *ptep, pte_t pte,
 /**
  * pte_move_swp_offset - Move the swap entry offset field of a swap pte
  *	 forward or backward by delta
- * @pte: The initial pte state; is_swap_pte(pte) must be true and
- *	 non_swap_entry() must be false.
+ * @pte: The initial pte state; must be a swap entry
  * @delta: The direction and the offset we are moving; forward if delta
  *	 is positive; backward if delta is negative
  *
@@ -352,8 +351,7 @@ static inline pte_t pte_move_swp_offset(pte_t pte, long delta)
 
 /**
  * pte_next_swp_offset - Increment the swap entry offset field of a swap pte.
- * @pte: The initial pte state; is_swap_pte(pte) must be true and
- *	 non_swap_entry() must be false.
+ * @pte: The initial pte state; must be a swap entry.
  *
  * Increments the swap offset, while maintaining all other fields, including
  * swap type, and any swp pte bits. The resulting pte is returned.
diff --git a/mm/khugepaged.c b/mm/khugepaged.c
index 5b7276bc14b1..2079f270196f 100644
--- a/mm/khugepaged.c
+++ b/mm/khugepaged.c
@@ -1019,7 +1019,8 @@ static int __collapse_huge_page_swapin(struct mm_struct *mm,
 		}
 
 		vmf.orig_pte = ptep_get_lockless(pte);
-		if (!is_swap_pte(vmf.orig_pte))
+		if (pte_none(vmf.orig_pte) ||
+		    pte_present(vmf.orig_pte))
 			continue;
 
 		vmf.pte = pte;
@@ -1276,7 +1277,19 @@ static int hpage_collapse_scan_pmd(struct mm_struct *mm,
 	for (addr = start_addr, _pte = pte; _pte < pte + HPAGE_PMD_NR;
 	     _pte++, addr += PAGE_SIZE) {
 		pte_t pteval = ptep_get(_pte);
-		if (is_swap_pte(pteval)) {
+		if (pte_none_or_zero(pteval)) {
+			++none_or_zero;
+			if (!userfaultfd_armed(vma) &&
+			    (!cc->is_khugepaged ||
+			     none_or_zero <= khugepaged_max_ptes_none)) {
+				continue;
+			} else {
+				result = SCAN_EXCEED_NONE_PTE;
+				count_vm_event(THP_SCAN_EXCEED_NONE_PTE);
+				goto out_unmap;
+			}
+		}
+		if (!pte_present(pteval)) {
 			++unmapped;
 			if (!cc->is_khugepaged ||
 			    unmapped <= khugepaged_max_ptes_swap) {
@@ -1296,18 +1309,6 @@ static int hpage_collapse_scan_pmd(struct mm_struct *mm,
 				goto out_unmap;
 			}
 		}
-		if (pte_none_or_zero(pteval)) {
-			++none_or_zero;
-			if (!userfaultfd_armed(vma) &&
-			    (!cc->is_khugepaged ||
-			     none_or_zero <= khugepaged_max_ptes_none)) {
-				continue;
-			} else {
-				result = SCAN_EXCEED_NONE_PTE;
-				count_vm_event(THP_SCAN_EXCEED_NONE_PTE);
-				goto out_unmap;
-			}
-		}
 		if (pte_uffd_wp(pteval)) {
 			/*
 			 * Don't collapse the page if any of the small
diff --git a/mm/migrate.c b/mm/migrate.c
index 4324fc01bfce..69d8b4a9db25 100644
--- a/mm/migrate.c
+++ b/mm/migrate.c
@@ -492,7 +492,7 @@ void migration_entry_wait(struct mm_struct *mm, pmd_t *pmd,
 	pte = ptep_get(ptep);
 	pte_unmap(ptep);
 
-	if (!is_swap_pte(pte))
+	if (pte_none(pte) || pte_present(pte))
 		goto out;
 
 	entry = pte_to_swp_entry(pte);
diff --git a/mm/mprotect.c b/mm/mprotect.c
index c090bc063a31..e25ac9835cc2 100644
--- a/mm/mprotect.c
+++ b/mm/mprotect.c
@@ -345,7 +345,26 @@ static long change_pte_range(struct mmu_gather *tlb,
 				prot_commit_flush_ptes(vma, addr, pte, oldpte, ptent,
 					nr_ptes, /* idx = */ 0, /* set_write = */ false, tlb);
 			pages += nr_ptes;
-		} else if (is_swap_pte(oldpte)) {
+		} else if (pte_none(oldpte)) {
+			/*
+			 * Nobody plays with any none ptes besides
+			 * userfaultfd when applying the protections.
+			 */
+			if (likely(!uffd_wp))
+				continue;
+
+			if (userfaultfd_wp_use_markers(vma)) {
+				/*
+				 * For file-backed mem, we need to be able to
+				 * wr-protect a none pte, because even if the
+				 * pte is none, the page/swap cache could
+				 * exist.  Doing that by install a marker.
+				 */
+				set_pte_at(vma->vm_mm, addr, pte,
+					   make_pte_marker(PTE_MARKER_UFFD_WP));
+				pages++;
+			}
+		} else  {
 			swp_entry_t entry = pte_to_swp_entry(oldpte);
 			pte_t newpte;
 
@@ -406,28 +425,6 @@ static long change_pte_range(struct mmu_gather *tlb,
 				set_pte_at(vma->vm_mm, addr, pte, newpte);
 				pages++;
 			}
-		} else {
-			/* It must be an none page, or what else?.. */
-			WARN_ON_ONCE(!pte_none(oldpte));
-
-			/*
-			 * Nobody plays with any none ptes besides
-			 * userfaultfd when applying the protections.
-			 */
-			if (likely(!uffd_wp))
-				continue;
-
-			if (userfaultfd_wp_use_markers(vma)) {
-				/*
-				 * For file-backed mem, we need to be able to
-				 * wr-protect a none pte, because even if the
-				 * pte is none, the page/swap cache could
-				 * exist.  Doing that by install a marker.
-				 */
-				set_pte_at(vma->vm_mm, addr, pte,
-					   make_pte_marker(PTE_MARKER_UFFD_WP));
-				pages++;
-			}
 		}
 	} while (pte += nr_ptes, addr += nr_ptes * PAGE_SIZE, addr != end);
 	arch_leave_lazy_mmu_mode();
diff --git a/mm/mremap.c b/mm/mremap.c
index bd7314898ec5..f01c74add990 100644
--- a/mm/mremap.c
+++ b/mm/mremap.c
@@ -158,6 +158,9 @@ static void drop_rmap_locks(struct vm_area_struct *vma)
 
 static pte_t move_soft_dirty_pte(pte_t pte)
 {
+	if (pte_none(pte))
+		return pte;
+
 	/*
 	 * Set soft dirty bit so we can notice
 	 * in userspace the ptes were moved.
@@ -165,7 +168,7 @@ static pte_t move_soft_dirty_pte(pte_t pte)
 #ifdef CONFIG_MEM_SOFT_DIRTY
 	if (pte_present(pte))
 		pte = pte_mksoft_dirty(pte);
-	else if (is_swap_pte(pte))
+	else
 		pte = pte_swp_mksoft_dirty(pte);
 #endif
 	return pte;
@@ -294,7 +297,7 @@ static int move_ptes(struct pagetable_move_control *pmc,
 			if (need_clear_uffd_wp) {
 				if (pte_present(pte))
 					pte = pte_clear_uffd_wp(pte);
-				else if (is_swap_pte(pte))
+				else
 					pte = pte_swp_clear_uffd_wp(pte);
 			}
 			set_ptes(mm, new_addr, new_ptep, pte, nr_ptes);
diff --git a/mm/page_table_check.c b/mm/page_table_check.c
index 4eeca782b888..43f75d2f7c36 100644
--- a/mm/page_table_check.c
+++ b/mm/page_table_check.c
@@ -185,12 +185,15 @@ static inline bool swap_cached_writable(swp_entry_t entry)
 	       is_writable_migration_entry(entry);
 }
 
-static inline void page_table_check_pte_flags(pte_t pte)
+static void page_table_check_pte_flags(pte_t pte)
 {
-	if (pte_present(pte) && pte_uffd_wp(pte))
-		WARN_ON_ONCE(pte_write(pte));
-	else if (is_swap_pte(pte) && pte_swp_uffd_wp(pte))
-		WARN_ON_ONCE(swap_cached_writable(pte_to_swp_entry(pte)));
+	if (pte_present(pte)) {
+		WARN_ON_ONCE(pte_uffd_wp(pte) && pte_write(pte));
+	} else if (pte_swp_uffd_wp(pte)) {
+		const swp_entry_t entry = pte_to_swp_entry(pte);
+
+		WARN_ON_ONCE(swap_cached_writable(entry));
+	}
 }
 
 void __page_table_check_ptes_set(struct mm_struct *mm, pte_t *ptep, pte_t pte,
diff --git a/mm/page_vma_mapped.c b/mm/page_vma_mapped.c
index 75a8fbb788b7..2e5ac6572630 100644
--- a/mm/page_vma_mapped.c
+++ b/mm/page_vma_mapped.c
@@ -16,6 +16,7 @@ static inline bool not_found(struct page_vma_mapped_walk *pvmw)
 static bool map_pte(struct page_vma_mapped_walk *pvmw, pmd_t *pmdvalp,
 		    spinlock_t **ptlp)
 {
+	bool is_migration;
 	pte_t ptent;
 
 	if (pvmw->flags & PVMW_SYNC) {
@@ -26,6 +27,7 @@ static bool map_pte(struct page_vma_mapped_walk *pvmw, pmd_t *pmdvalp,
 		return !!pvmw->pte;
 	}
 
+	is_migration = pvmw->flags & PVMW_MIGRATION;
 again:
 	/*
 	 * It is important to return the ptl corresponding to pte,
@@ -41,11 +43,14 @@ static bool map_pte(struct page_vma_mapped_walk *pvmw, pmd_t *pmdvalp,
 
 	ptent = ptep_get(pvmw->pte);
 
-	if (pvmw->flags & PVMW_MIGRATION) {
-		if (!is_swap_pte(ptent))
+	if (pte_none(ptent)) {
+		return false;
+	} else if (pte_present(ptent)) {
+		if (is_migration)
 			return false;
-	} else if (is_swap_pte(ptent)) {
+	} else if (!is_migration) {
 		swp_entry_t entry;
+
 		/*
 		 * Handle un-addressable ZONE_DEVICE memory.
 		 *
@@ -66,8 +71,6 @@ static bool map_pte(struct page_vma_mapped_walk *pvmw, pmd_t *pmdvalp,
 		if (!is_device_private_entry(entry) &&
 		    !is_device_exclusive_entry(entry))
 			return false;
-	} else if (!pte_present(ptent)) {
-		return false;
 	}
 	spin_lock(*ptlp);
 	if (unlikely(!pmd_same(*pmdvalp, pmdp_get_lockless(pvmw->pmd)))) {
@@ -107,27 +110,23 @@ static bool check_pte(struct page_vma_mapped_walk *pvmw, unsigned long pte_nr)
 	pte_t ptent = ptep_get(pvmw->pte);
 
 	if (pvmw->flags & PVMW_MIGRATION) {
-		swp_entry_t entry = pte_to_swp_entry_or_zero(ptent);
+		const swp_entry_t entry = pte_to_swp_entry_or_zero(ptent);
 
 		if (!is_migration_entry(entry))
 			return false;
 
 		pfn = swp_offset_pfn(entry);
-	} else if (is_swap_pte(ptent)) {
-		swp_entry_t entry;
+	} else if (pte_present(ptent)) {
+		pfn = pte_pfn(ptent);
+	} else {
+		const swp_entry_t entry = pte_to_swp_entry(ptent);
 
 		/* Handle un-addressable ZONE_DEVICE memory */
-		entry = pte_to_swp_entry(ptent);
 		if (!is_device_private_entry(entry) &&
 		    !is_device_exclusive_entry(entry))
 			return false;
 
 		pfn = swp_offset_pfn(entry);
-	} else {
-		if (!pte_present(ptent))
-			return false;
-
-		pfn = pte_pfn(ptent);
 	}
 
 	if ((pfn + pte_nr - 1) < pvmw->pfn)
-- 
2.51.0

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