linux-kernel - [RFC PATCH 5/6] mm/util: Add flag to track device private PFNs in page snapshots

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Message-Id: <20251128044146.80050-6-jniethe@nvidia.com>
Date: Fri, 28 Nov 2025 15:41:45 +1100
From: Jordan Niethe <jniethe@...dia.com>
To: linux-mm@...ck.org
Cc: balbirs@...dia.com,
	matthew.brost@...el.com,
	akpm@...ux-foundation.org,
	linux-kernel@...r.kernel.org,
	dri-devel@...ts.freedesktop.org,
	david@...hat.com,
	ziy@...dia.com,
	apopple@...dia.com,
	lorenzo.stoakes@...cle.com,
	lyude@...hat.com,
	dakr@...nel.org,
	airlied@...il.com,
	simona@...ll.ch,
	rcampbell@...dia.com,
	mpenttil@...hat.com,
	jgg@...dia.com,
	willy@...radead.org
Subject: [RFC PATCH 5/6] mm/util: Add flag to track device private PFNs in page snapshots

A future change will remove device private pages from the physical
address space. This will mean that device private pages no longer have
normal PFN and must be handled separately.

Add a new flag PAGE_SNAPSHOT_DEVICE_PRIVATE to track when the pfn of a
page snapshot is a device private page.

Signed-off-by: Jordan Niethe <jniethe@...dia.com>
Signed-off-by: Alistair Popple <apopple@...dia.com>
---
 fs/proc/page.c     | 6 ++++--
 include/linux/mm.h | 7 ++++---
 mm/util.c          | 3 +++
 3 files changed, 11 insertions(+), 5 deletions(-)

diff --git a/fs/proc/page.c b/fs/proc/page.c
index fc64f23e05e5..c3e88a199c19 100644
--- a/fs/proc/page.c
+++ b/fs/proc/page.c
@@ -192,10 +192,12 @@ u64 stable_page_flags(const struct page *page)
 	         folio_test_large_rmappable(folio)) {
 		/* Note: we indicate any THPs here, not just PMD-sized ones */
 		u |= 1 << KPF_THP;
-	} else if (is_huge_zero_pfn(ps.pfn)) {
+	} else if (!(ps.flags & PAGE_SNAPSHOT_DEVICE_PRIVATE) &&
+		   is_huge_zero_pfn(ps.pfn)) {
 		u |= 1 << KPF_ZERO_PAGE;
 		u |= 1 << KPF_THP;
-	} else if (is_zero_pfn(ps.pfn)) {
+	} else if (!(ps.flags & PAGE_SNAPSHOT_DEVICE_PRIVATE)
+		   && is_zero_pfn(ps.pfn)) {
 		u |= 1 << KPF_ZERO_PAGE;
 	}
 
diff --git a/include/linux/mm.h b/include/linux/mm.h
index 7c79b3369b82..6b8c299a6687 100644
--- a/include/linux/mm.h
+++ b/include/linux/mm.h
@@ -4317,9 +4317,10 @@ static inline bool page_pool_page_is_pp(const struct page *page)
 }
 #endif
 
-#define PAGE_SNAPSHOT_FAITHFUL (1 << 0)
-#define PAGE_SNAPSHOT_PG_BUDDY (1 << 1)
-#define PAGE_SNAPSHOT_PG_IDLE  (1 << 2)
+#define PAGE_SNAPSHOT_FAITHFUL		(1 << 0)
+#define PAGE_SNAPSHOT_PG_BUDDY		(1 << 1)
+#define PAGE_SNAPSHOT_PG_IDLE		(1 << 2)
+#define PAGE_SNAPSHOT_DEVICE_PRIVATE	(1 << 3)
 
 struct page_snapshot {
 	struct folio folio_snapshot;
diff --git a/mm/util.c b/mm/util.c
index 8989d5767528..2472b7381b11 100644
--- a/mm/util.c
+++ b/mm/util.c
@@ -1215,6 +1215,9 @@ static void set_ps_flags(struct page_snapshot *ps, const struct folio *folio,
 
 	if (folio_test_idle(folio))
 		ps->flags |= PAGE_SNAPSHOT_PG_IDLE;
+
+	if (is_device_private_page(page))
+		ps->flags |= PAGE_SNAPSHOT_DEVICE_PRIVATE;
 }
 
 /**
-- 
2.34.1