[PATCH RFC 03/14] xen/grant-table: stop setting PG_private on pages for grant mapping

[Zi Yan]

gnttab_alloc_pages() stores xen_page_foreign in allocated page->private.
On 32-bit, a pointer to xen_page_foreign is stored; on 64-bit,
xen_page_foreign is stored inline. Checking page->private != NULL is enough
to tell whether a xen_page_foreign needs to be freed on 32-bit and
page->private is zeroed unconditionally on 64-bit.

It prepares for a future commit that remove PG_private.

No funtional change intended.

Assisted-by: Claude:claude-opus-4-8
Assisted-by: Codex:gpt-5
Signed-off-by: Zi Yan <ziy@xxxxxxxxxx>
To: Juergen Gross <jgross@xxxxxxxx>
To: Stefano Stabellini <sstabellini@xxxxxxxxxx>
Cc: Oleksandr Tyshchenko <oleksandr_tyshchenko@xxxxxxxx>
Cc: xen-devel@xxxxxxxxxxxxxxxxxxxx
Cc: linux-kernel@xxxxxxxxxxxxxxx
---
 drivers/xen/balloon.c     | 5 +++++
 drivers/xen/grant-table.c | 7 +++----
 2 files changed, 8 insertions(+), 4 deletions(-)

diff --git a/drivers/xen/balloon.c b/drivers/xen/balloon.c
index e7f1d4ca6d753..7f47b0ad05607 100644
--- a/drivers/xen/balloon.c
+++ b/drivers/xen/balloon.c
@@ -182,6 +182,11 @@ static struct page *balloon_retrieve(bool require_lowmem)
 
 	__ClearPageOffline(page);
 	dec_node_page_state(page, NR_BALLOON_PAGES);
+	/*
+	 * clear page->private before giving it out, since it might be used to
+	 * store xen_page_foreign info.
+	 */
+	set_page_private(page, 0);
 
 	return page;
 }
diff --git a/drivers/xen/grant-table.c b/drivers/xen/grant-table.c
index 35f879dc5dfb8..cc348ba2e0786 100644
--- a/drivers/xen/grant-table.c
+++ b/drivers/xen/grant-table.c
@@ -875,7 +875,7 @@ int gnttab_pages_set_private(int nr_pages, struct page **pages)
 
 		set_page_private(pages[i], (unsigned long)foreign);
 #endif
-		SetPagePrivate(pages[i]);
+		/* Data is stored in page->private on 64-bit */
 	}
 
 	return 0;
@@ -1031,12 +1031,11 @@ void gnttab_pages_clear_private(int nr_pages, struct page **pages)
 	int i;
 
 	for (i = 0; i < nr_pages; i++) {
-		if (PagePrivate(pages[i])) {
 #if BITS_PER_LONG < 64
+		if (page_private(pages[i]))
 			kfree((void *)page_private(pages[i]));
 #endif
-			ClearPagePrivate(pages[i]);
-		}
+		set_page_private(pages[i], 0);
 	}
 }
 EXPORT_SYMBOL_GPL(gnttab_pages_clear_private);

-- 
2.53.0