smaps_pmd_entry() derives a page from a non-present PMD and gives up if
it cannot. A PMD swap entry carries no PFN, so a swapped-out THP is
missing from both Swap and SwapPss.

The slots of a PMD swap entry can have different swap reference counts,
so SwapPss has to be computed one slot at a time rather than divided
once. Factor that loop out of smaps_pte_entry() and call it from both
levels, then return early: a swapped-out THP is not resident and must
not be added to AnonHugePages or Rss.

While touching PMD softleaf accounting, match the PTE path and treat
device-private PMD entries as fake-present. This lets smaps use
mapcount-based PSS and private/shared accounting instead of treating
them like migration entries.

Signed-off-by: Usama Arif <usama.arif@linux.dev>
---
 fs/proc/task_mmu.c | 41 ++++++++++++++++++++++++++++-------------
 1 file changed, 28 insertions(+), 13 deletions(-)

diff --git a/fs/proc/task_mmu.c b/fs/proc/task_mmu.c
index 24425e2308951..97c3fd7acf15d 100644
--- a/fs/proc/task_mmu.c
+++ b/fs/proc/task_mmu.c
@@ -957,6 +957,25 @@ 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 nr_pages)
+{
+	mss->swap += nr_pages << PAGE_SHIFT;
+	do {
+		int swapcount = swp_swapcount(entry);
+
+		if (swapcount >= 2) {
+			u64 pss_delta = (u64)PAGE_SIZE << PSS_SHIFT;
+
+			do_div(pss_delta, swapcount);
+			mss->swap_pss += pss_delta;
+		} else {
+			mss->swap_pss += (u64)PAGE_SIZE << PSS_SHIFT;
+		}
+		entry.val++;
+	} while (--nr_pages);
+}
+
 static void smaps_pte_entry(pte_t *pte, unsigned long addr,
 		struct mm_walk *walk)
 {
@@ -978,18 +997,7 @@ 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, 1);
 		} else if (softleaf_has_pfn(entry)) {
 			if (softleaf_is_device_private(entry))
 				present = true;
@@ -1022,8 +1030,15 @@ static void smaps_pmd_entry(pmd_t *pmd, unsigned long addr,
 	} else if (unlikely(thp_migration_supported())) {
 		const softleaf_t entry = softleaf_from_pmd(*pmd);
 
-		if (softleaf_has_pfn(entry))
+		if (softleaf_is_swap(entry)) {
+			smaps_account_swap(mss, entry, HPAGE_PMD_NR);
+			return;
+		}
+		if (softleaf_has_pfn(entry)) {
+			if (softleaf_is_device_private(entry))
+				present = true;
 			page = softleaf_to_page(entry);
+		}
 	}
 	if (IS_ERR_OR_NULL(page))
 		return;
-- 
2.53.0-Meta