[RESEND v7 15/29] fs/proc: account PMD swap entries in smaps

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

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 accounted
nowhere: its bytes show up in neither Swap nor SwapPss, and VmSwap silently
loses them.

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.

pagemap needs no equivalent change - it already emits PM_SWAP and
type | offset+idx for a PFN-less softleaf entry - but its
thp_migration_supported() gate answers a different question than the decode
requires, so use pmd_is_valid_softleaf() there instead.

Signed-off-by: Usama Arif <usama.arif@linux.dev>
---
 fs/proc/task_mmu.c | 42 ++++++++++++++++++++++++++++--------------
 1 file changed, 28 insertions(+), 14 deletions(-)

diff --git a/fs/proc/task_mmu.c b/fs/proc/task_mmu.c
index 24425e2308951..aa3f4d54a8580 100644
--- a/fs/proc/task_mmu.c
+++ b/fs/proc/task_mmu.c
@@ -957,6 +957,27 @@ 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)
+{
+	unsigned long nr_pages = size >> PAGE_SHIFT;
+
+	mss->swap += size;
+	do {
+		int 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;
+		}
+		entry.val++;
+	} while (--nr_pages);
+}
+
 static void smaps_pte_entry(pte_t *pte, unsigned long addr,
 		struct mm_walk *walk)
 {
@@ -978,18 +999,7 @@ 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;
@@ -1019,9 +1029,13 @@ 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);
 	}
@@ -2000,7 +2014,7 @@ 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_valid_softleaf(pmd)) {
 		const softleaf_t entry = softleaf_from_pmd(pmd);
 		unsigned long offset;
 
-- 
2.53.0-Meta