fbdev/defio: Early-out if page is already enlisted

Return early if a page is already in the list of dirty pages for
deferred I/O. This can be detected if the page's list head is not
empty. Keep the list head initialized while the page is not enlisted
to make this work reliably.

v2:
	* update comment and fix spelling (Sam)
Signed-off-by: Thomas Zimmermann <tzimmermann@suse.de>
Acked-by: Sam Ravnborg <sam@ravnborg.org>
Link: https://patchwork.freedesktop.org/patch/msgid/20220211094640.21632-2-tzimmermann@suse.de

drivers/video/fbdev/core/fb_defio.c

@@ -59,6 +59,7 @@ static vm_fault_t fb_deferred_io_fault(struct vm_fault *vmf)
 		printk(KERN_ERR "no mapping available\n");
 
 	BUG_ON(!page->mapping);
+	INIT_LIST_HEAD(&page->lru);
 	page->index = vmf->pgoff;
 
 	vmf->page = page;
@@ -122,17 +123,24 @@ static vm_fault_t fb_deferred_io_mkwrite(struct vm_fault *vmf)
 	 */
 	lock_page(page);
 
+	/*
+	 * This check is to catch the case where a new process could start
+	 * writing to the same page through a new PTE. This new access
+	 * can cause a call to .page_mkwrite even if the original process'
+	 * PTE is marked writable.
+	 *
+	 * TODO: The lru field is owned by the page cache; hence the name.
+	 *       We dequeue in fb_deferred_io_work() after flushing the
+	 *       page's content into video memory. Instead of lru, fbdefio
+	 *       should have it's own field.
+	 */
+	if (!list_empty(&page->lru))
+		goto page_already_added;
+
 	/* we loop through the pagelist before adding in order
 	to keep the pagelist sorted */
 	list_for_each_entry(cur, &fbdefio->pagelist, lru) {
-		/* this check is to catch the case where a new
-		process could start writing to the same page
-		through a new pte. this new access can cause the
-		mkwrite even when the original ps's pte is marked
-		writable */
-		if (unlikely(cur == page))
-			goto page_already_added;
-		else if (cur->index > page->index)
+		if (cur->index > page->index)
 			break;
 	}
 
@@ -194,7 +202,7 @@ static void fb_deferred_io_work(struct work_struct *work)
 
 	/* clear the list */
 	list_for_each_safe(node, next, &fbdefio->pagelist) {
-		list_del(node);
+		list_del_init(node);
 	}
 	mutex_unlock(&fbdefio->lock);
 }