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

diff --git a/drivers/video/fbdev/core/fb_defio.c b/drivers/video/fbdev/core/fb_defio.c
index a591d29..169a81c 100644 (file)
--- a/drivers/video/fbdev/core/fb_defio.c
+++ b/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);
 }