[PATCH 09/13] mm: handle PMD swap entries in non-present PMD walkers

[Usama Arif <usama.arif@linux.dev>]

Teach the remaining non-present PMD walkers about swap entries,
mirroring the PTE-level equivalents.

smaps_pmd_entry() accounts swap and swap_pss via a new shared
smaps_account_swap() helper used by both PTE and PMD paths.

zap_huge_pmd() frees swap slots via swap_put_entries_direct(),
matching zap_nonpresent_ptes().

change_non_present_huge_pmd() skips write-permission changes for swap
entries and only updates uffd_wp, matching change_softleaf_pte().

move_soft_dirty_pmd(), clear_soft_dirty_pmd(), and make_uffd_wp_pmd(),
pagemap_pmd_range_thp() and change_huge_pmd() handle swap entries
alongside migration entries.

madvise_cold_or_pageout_pmd_range() extends its non-present PMD
VM_BUG_ON to allow swap entries; without this, hitting a PMD swap
entry on a DEBUG_VM kernel would BUG().

queue_folios_pmd() in mempolicy silently skips swap entries, matching
the PTE walker which only counts migration entries as failures.
Without this, mbind(MPOL_MF_STRICT) would spuriously return -EIO on
a swapped-out THP.

madvise_free_huge_pmd() handles PMD swap entries directly: for a
full-range MADV_FREE it clears the PMD, frees the deposited page
table, and releases the swap slots; for a partial range it splits to
PTE swap entries. Without this, MADV_FREE silently becomes a no-op
on swapped-out THPs, leaking swap slots.

hmm_vma_handle_absent_pmd() faults in PMD swap entries via
hmm_vma_fault() instead of returning -EFAULT. The first per-page
handle_mm_fault() call triggers do_huge_pmd_swap_page(), which maps
the entire folio; subsequent calls become harmless
huge_pmd_set_accessed() and the walker retries with a present PMD.

Signed-off-by: Usama Arif <usama.arif@linux.dev>
---
 fs/proc/task_mmu.c | 43 +++++++++++++++++++++-------------
 mm/hmm.c           |  3 ++-
 mm/huge_memory.c   | 58 +++++++++++++++++++++++++++++++++++-----------
 mm/khugepaged.c    |  6 +++++
 mm/madvise.c       |  5 ++--
 mm/mempolicy.c     |  2 ++
 6 files changed, 85 insertions(+), 32 deletions(-)

diff --git a/fs/proc/task_mmu.c b/fs/proc/task_mmu.c
index 6d9f43881e62..a6dd91d4cf24 100644
--- a/fs/proc/task_mmu.c
+++ b/fs/proc/task_mmu.c
@@ -1015,6 +1015,23 @@ static void smaps_pte_hole_lookup(unsigned long addr, struct mm_walk *walk)
 #endif
 }
 
+static void smaps_account_swap(struct mem_size_stats *mss,
+		softleaf_t entry, unsigned long size)
+{
+	int mapcount;
+
+	mss->swap += size;
+	mapcount = swp_swapcount(entry);
+	if (mapcount >= 2) {
+		u64 pss_delta = (u64)size << PSS_SHIFT;
+
+		do_div(pss_delta, mapcount);
+		mss->swap_pss += pss_delta;
+	} else {
+		mss->swap_pss += (u64)size << PSS_SHIFT;
+	}
+}
+
 static void smaps_pte_entry(pte_t *pte, unsigned long addr,
 		struct mm_walk *walk)
 {
@@ -1036,18 +1053,7 @@ static void smaps_pte_entry(pte_t *pte, unsigned long addr,
 		const softleaf_t entry = softleaf_from_pte(ptent);
 
 		if (softleaf_is_swap(entry)) {
-			int mapcount;
-
-			mss->swap += PAGE_SIZE;
-			mapcount = swp_swapcount(entry);
-			if (mapcount >= 2) {
-				u64 pss_delta = (u64)PAGE_SIZE << PSS_SHIFT;
-
-				do_div(pss_delta, mapcount);
-				mss->swap_pss += pss_delta;
-			} else {
-				mss->swap_pss += (u64)PAGE_SIZE << PSS_SHIFT;
-			}
+			smaps_account_swap(mss, entry, PAGE_SIZE);
 		} else if (softleaf_has_pfn(entry)) {
 			if (softleaf_is_device_private(entry))
 				present = true;
@@ -1077,9 +1083,13 @@ static void smaps_pmd_entry(pmd_t *pmd, unsigned long addr,
 	if (pmd_present(*pmd)) {
 		page = vm_normal_page_pmd(vma, addr, *pmd);
 		present = true;
-	} else if (unlikely(thp_migration_supported())) {
+	} else {
 		const softleaf_t entry = softleaf_from_pmd(*pmd);
 
+		if (softleaf_is_swap(entry)) {
+			smaps_account_swap(mss, entry, HPAGE_PMD_SIZE);
+			return;
+		}
 		if (softleaf_has_pfn(entry))
 			page = softleaf_to_page(entry);
 	}
@@ -1665,7 +1675,7 @@ static inline void clear_soft_dirty_pmd(struct vm_area_struct *vma,
 		pmd = pmd_clear_soft_dirty(pmd);
 
 		set_pmd_at(vma->vm_mm, addr, pmdp, pmd);
-	} else if (pmd_is_migration_entry(pmd)) {
+	} else if (pmd_is_migration_entry(pmd) || pmd_is_swap_entry(pmd)) {
 		pmd = pmd_swp_clear_soft_dirty(pmd);
 		set_pmd_at(vma->vm_mm, addr, pmdp, pmd);
 	}
@@ -2025,7 +2035,8 @@ static int pagemap_pmd_range_thp(pmd_t *pmdp, unsigned long addr,
 			flags |= PM_UFFD_WP;
 		if (pm->show_pfn)
 			frame = pmd_pfn(pmd) + idx;
-	} else if (thp_migration_supported()) {
+	} else if (pmd_is_swap_entry(pmd) ||
+		   (thp_migration_supported() && pmd_is_migration_entry(pmd))) {
 		const softleaf_t entry = softleaf_from_pmd(pmd);
 		unsigned long offset;
 
@@ -2463,7 +2474,7 @@ static void make_uffd_wp_pmd(struct vm_area_struct *vma,
 		old = pmdp_invalidate_ad(vma, addr, pmdp);
 		pmd = pmd_mkuffd_wp(old);
 		set_pmd_at(vma->vm_mm, addr, pmdp, pmd);
-	} else if (pmd_is_migration_entry(pmd)) {
+	} else if (pmd_is_migration_entry(pmd) || pmd_is_swap_entry(pmd)) {
 		pmd = pmd_swp_mkuffd_wp(pmd);
 		set_pmd_at(vma->vm_mm, addr, pmdp, pmd);
 	}
diff --git a/mm/hmm.c b/mm/hmm.c
index 5955f2f0c83d..2bd3ebd1b8d6 100644
--- a/mm/hmm.c
+++ b/mm/hmm.c
@@ -370,7 +370,8 @@ static int hmm_vma_handle_absent_pmd(struct mm_walk *walk, unsigned long start,
 	required_fault = hmm_range_need_fault(hmm_vma_walk, hmm_pfns,
 					      npages, 0);
 	if (required_fault) {
-		if (softleaf_is_device_private(entry))
+		if (softleaf_is_device_private(entry) ||
+		    softleaf_is_swap(entry))
 			return hmm_vma_fault(addr, end, required_fault, walk);
 		else
 			return -EFAULT;
diff --git a/mm/huge_memory.c b/mm/huge_memory.c
index 42887cf518cd..109e4dc4a167 100644
--- a/mm/huge_memory.c
+++ b/mm/huge_memory.c
@@ -2375,6 +2375,14 @@ vm_fault_t do_huge_pmd_numa_page(struct vm_fault *vmf)
 	return 0;
 }
 
+static inline void zap_deposited_table(struct mm_struct *mm, pmd_t *pmd)
+{
+	pgtable_t pgtable;
+
+	pgtable = pgtable_trans_huge_withdraw(mm, pmd);
+	pte_free(mm, pgtable);
+	mm_dec_nr_ptes(mm);
+}
 /*
  * Return true if we do MADV_FREE successfully on entire pmd page.
  * Otherwise, return false.
@@ -2399,8 +2407,23 @@ bool madvise_free_huge_pmd(struct mmu_gather *tlb, struct vm_area_struct *vma,
 		goto out;
 
 	if (unlikely(!pmd_present(orig_pmd))) {
+		if (pmd_is_swap_entry(orig_pmd)) {
+			if (next - addr != HPAGE_PMD_SIZE) {
+				spin_unlock(ptl);
+				__split_huge_pmd(vma, pmd, addr, false);
+				goto out_unlocked;
+			}
+			softleaf_t sl = softleaf_from_pmd(orig_pmd);
+
+			pmdp_huge_get_and_clear(mm, addr, pmd);
+			zap_deposited_table(mm, pmd);
+			spin_unlock(ptl);
+			swap_put_entries_direct(sl, HPAGE_PMD_NR);
+			add_mm_counter(mm, MM_SWAPENTS, -HPAGE_PMD_NR);
+			return true;
+		}
 		VM_BUG_ON(thp_migration_supported() &&
-				  !pmd_is_migration_entry(orig_pmd));
+			  !pmd_is_migration_entry(orig_pmd));
 		goto out;
 	}
 
@@ -2449,15 +2472,6 @@ bool madvise_free_huge_pmd(struct mmu_gather *tlb, struct vm_area_struct *vma,
 	return ret;
 }
 
-static inline void zap_deposited_table(struct mm_struct *mm, pmd_t *pmd)
-{
-	pgtable_t pgtable;
-
-	pgtable = pgtable_trans_huge_withdraw(mm, pmd);
-	pte_free(mm, pgtable);
-	mm_dec_nr_ptes(mm);
-}
-
 static void zap_huge_pmd_folio(struct mm_struct *mm, struct vm_area_struct *vma,
 		pmd_t pmdval, struct folio *folio, bool is_present)
 {
@@ -2550,6 +2564,16 @@ bool zap_huge_pmd(struct mmu_gather *tlb, struct vm_area_struct *vma,
 	arch_check_zapped_pmd(vma, orig_pmd);
 	tlb_remove_pmd_tlb_entry(tlb, pmd, addr);
 
+	if (pmd_is_swap_entry(orig_pmd)) {
+		softleaf_t sl = softleaf_from_pmd(orig_pmd);
+
+		zap_deposited_table(mm, pmd);
+		spin_unlock(ptl);
+		swap_put_entries_direct(sl, HPAGE_PMD_NR);
+		add_mm_counter(mm, MM_SWAPENTS, -HPAGE_PMD_NR);
+		return true;
+	}
+
 	is_present = pmd_present(orig_pmd);
 	folio = normal_or_softleaf_folio_pmd(vma, addr, orig_pmd, is_present);
 	has_deposit = has_deposited_pgtable(vma, orig_pmd, folio);
@@ -2582,7 +2606,8 @@ static inline int pmd_move_must_withdraw(spinlock_t *new_pmd_ptl,
 static pmd_t move_soft_dirty_pmd(pmd_t pmd)
 {
 	if (pgtable_supports_soft_dirty()) {
-		if (unlikely(pmd_is_migration_entry(pmd)))
+		if (unlikely(pmd_is_migration_entry(pmd) ||
+			     pmd_is_swap_entry(pmd)))
 			pmd = pmd_swp_mksoft_dirty(pmd);
 		else if (pmd_present(pmd))
 			pmd = pmd_mksoft_dirty(pmd);
@@ -2662,7 +2687,14 @@ static void change_non_present_huge_pmd(struct mm_struct *mm,
 	pmd_t newpmd;
 
 	VM_WARN_ON(!pmd_is_valid_softleaf(*pmd));
-	if (softleaf_is_migration_write(entry)) {
+
+	/*
+	 * PMD swap entries don't encode write permission in the entry type,
+	 * so only uffd_wp flag changes apply. No folio lookup needed.
+	 */
+	if (softleaf_is_swap(entry)) {
+		newpmd = *pmd;
+	} else if (softleaf_is_migration_write(entry)) {
 		const struct folio *folio = softleaf_to_folio(entry);
 
 		/*
@@ -2719,7 +2751,7 @@ int change_huge_pmd(struct mmu_gather *tlb, struct vm_area_struct *vma,
 	if (!ptl)
 		return 0;
 
-	if (thp_migration_supported() && pmd_is_valid_softleaf(*pmd)) {
+	if (pmd_is_valid_softleaf(*pmd)) {
 		change_non_present_huge_pmd(mm, addr, pmd, uffd_wp,
 					    uffd_wp_resolve);
 		goto unlock;
diff --git a/mm/khugepaged.c b/mm/khugepaged.c
index b8452dbdb043..a7cc65c3d06a 100644
--- a/mm/khugepaged.c
+++ b/mm/khugepaged.c
@@ -950,6 +950,12 @@ static inline enum scan_result check_pmd_state(pmd_t *pmd)
 	 */
 	if (pmd_is_migration_entry(pmde))
 		return SCAN_PMD_MAPPED;
+	/*
+	 * A PMD-mapped THP that has been swapped out is still a THP from
+	 * khugepaged's perspective; treat it like a present huge PMD.
+	 */
+	if (pmd_is_swap_entry(pmde))
+		return SCAN_PMD_MAPPED;
 	if (!pmd_present(pmde))
 		return SCAN_NO_PTE_TABLE;
 	if (pmd_trans_huge(pmde))
diff --git a/mm/madvise.c b/mm/madvise.c
index 69708e953cf5..2702eb0b1134 100644
--- a/mm/madvise.c
+++ b/mm/madvise.c
@@ -390,7 +390,8 @@ static int madvise_cold_or_pageout_pte_range(pmd_t *pmd,
 
 		if (unlikely(!pmd_present(orig_pmd))) {
 			VM_BUG_ON(thp_migration_supported() &&
-					!pmd_is_migration_entry(orig_pmd));
+					!pmd_is_migration_entry(orig_pmd) &&
+					!pmd_is_swap_entry(orig_pmd));
 			goto huge_unlock;
 		}
 
@@ -666,7 +667,7 @@ static int madvise_free_pte_range(pmd_t *pmd, unsigned long addr,
 	int nr, max_nr;
 
 	next = pmd_addr_end(addr, end);
-	if (pmd_trans_huge(*pmd))
+	if (pmd_trans_huge(*pmd) || pmd_is_swap_entry(*pmd))
 		if (madvise_free_huge_pmd(tlb, vma, pmd, addr, next))
 			return 0;
 
diff --git a/mm/mempolicy.c b/mm/mempolicy.c
index 4e4421b22b59..55b38fe13a63 100644
--- a/mm/mempolicy.c
+++ b/mm/mempolicy.c
@@ -658,6 +658,8 @@ static void queue_folios_pmd(pmd_t *pmd, struct mm_walk *walk)
 		qp->nr_failed++;
 		return;
 	}
+	if (unlikely(pmd_is_swap_entry(*pmd)))
+		return;
 	folio = pmd_folio(*pmd);
 	if (is_huge_zero_folio(folio)) {
 		walk->action = ACTION_CONTINUE;
-- 
2.52.0