fbdev/deferred-io: Clean up pageref on lastclose

Clean up the pageref state as part of the lastclose helper. This
only requires to clear the page's mapping field. The pageref and
page can stay in place for the next opened instance of the frame-
buffer file.

With the change in the clean-up logic, there's no further need
to look up pages during the lastclose cleanup. The code instead
uses the existing pagerefs in its look-up table. It also avoids
using smem_len, which some driver might not set correctly.

v2:
- fix typos in commit message (Javier)
Signed-off-by: Thomas Zimmermann <tzimmermann@suse.de>
Reviewed-by: Javier Martinez Canillas <javierm@redhat.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20240419083331.7761-4-tzimmermann@suse.de

drivers/video/fbdev/core/fb_defio.c

@@ -62,6 +62,14 @@ static struct fb_deferred_io_pageref *fb_deferred_io_pageref_lookup(struct fb_in
 	return pageref;
 }
 
+static void fb_deferred_io_pageref_clear(struct fb_deferred_io_pageref *pageref)
+{
+	struct page *page = pageref->page;
+
+	if (page)
+		page->mapping = NULL;
+}
+
 static struct fb_deferred_io_pageref *fb_deferred_io_pageref_get(struct fb_info *info,
 								 unsigned long offset,
 								 struct page *page)
@@ -330,16 +338,13 @@ EXPORT_SYMBOL_GPL(fb_deferred_io_open);
 
 static void fb_deferred_io_lastclose(struct fb_info *info)
 {
-	struct page *page;
-	int i;
+	unsigned long i;
 
 	flush_delayed_work(&info->deferred_work);
 
 	/* clear out the mapping that we setup */
-	for (i = 0 ; i < info->fix.smem_len; i += PAGE_SIZE) {
-		page = fb_deferred_io_page(info, i);
-		page->mapping = NULL;
-	}
+	for (i = 0; i < info->npagerefs; ++i)
+		fb_deferred_io_pageref_clear(&info->pagerefs[i]);
 }
 
 void fb_deferred_io_release(struct fb_info *info)