fbdev/deferred-io: Always call get_page() for framebuffer pages

Unconditionally call get_page() after looking up a page from the
framebuffer memory. Guarantees that we always hold a reference.

This change also refactors the code such that it can support a
driver-supplied get_page helper. This will be useful for DRM's
fbdev emulation.
Signed-off-by: Thomas Zimmermann <tzimmermann@suse.de>
Reviewed-by: Javier Martinez Canillas <javierm@redhat.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20240419083331.7761-7-tzimmermann@suse.de

drivers/video/fbdev/core/fb_defio.c

@@ -23,7 +23,7 @@
 #include <linux/rmap.h>
 #include <linux/pagemap.h>
 
-static struct page *fb_deferred_io_page(struct fb_info *info, unsigned long offs)
+static struct page *fb_deferred_io_get_page(struct fb_info *info, unsigned long offs)
 {
 	const void *screen_buffer = info->screen_buffer;
 	struct page *page = NULL;
@@ -33,6 +33,9 @@ static struct page *fb_deferred_io_page(struct fb_info *info, unsigned long offs
 	else if (info->fix.smem_start)
 		page = pfn_to_page((info->fix.smem_start + offs) >> PAGE_SHIFT);
 
+	if (page)
+		get_page(page);
+
 	return page;
 }
 
@@ -129,12 +132,10 @@ static vm_fault_t fb_deferred_io_fault(struct vm_fault *vmf)
 	if (offset >= info->fix.smem_len)
 		return VM_FAULT_SIGBUS;
 
-	page = fb_deferred_io_page(info, offset);
+	page = fb_deferred_io_get_page(info, offset);
 	if (!page)
 		return VM_FAULT_SIGBUS;
 
-	get_page(page);
-
 	if (vmf->vma->vm_file)
 		page->mapping = vmf->vma->vm_file->f_mapping;
 	else