[Xen-devel] [PATCH 1/2] fbdev: Make deferred I/O work as advertized

[Markus Armbruster <armbru@xxxxxxxxxx>]

Deferred I/O was utterly broken.  Reading the mmap()ed framebuffer
worked, but writing it made the VM endlessly invoke
vm_ops.page_mkwrite().  That happened because we failed to set
page->mapping and page->index.

The fix is to set them, and clean up properly before the framebuffer
gets released.

Fix extracted from this linux-fbdev-devel message:

    Subject: [PATCH 1/1 2.6.24] fbdev: defio and Metronomefb
    From: Jaya Kumar <jayakumar.lkml@xxxxxxxxx>
    Date: 2008-02-18 13:41:26

Signed-off-by: Jaya Kumar <jayakumar.lkml@xxxxxxxxx>
Signed-off-by: Markus Armbruster <armbru@xxxxxxxxxx>

---

 drivers/video/fb_defio.c |   22 ++++++++++++++++++++--
 1 file changed, 20 insertions(+), 2 deletions(-)

diff --git a/drivers/video/fb_defio.c b/drivers/video/fb_defio.c
index 0f8cfb9..24843fd 100644
--- a/drivers/video/fb_defio.c
+++ b/drivers/video/fb_defio.c
@@ -4,7 +4,7 @@
  *  Copyright (C) 2006 Jaya Kumar
  *
  * This file is subject to the terms and conditions of the GNU General Public
- * License.  See the file COPYING in the main directory of this archive
+ * License. See the file COPYING in the main directory of this archive
  * for more details.
  */
 
@@ -31,7 +31,7 @@ static int fb_deferred_io_fault(struct vm_area_struct *vma,
        unsigned long offset;
        struct page *page;
        struct fb_info *info = vma->vm_private_data;
-       /* info->screen_base is in System RAM */
+       /* info->screen_base is virtual memory */
        void *screen_base = (void __force *) info->screen_base;
 
        offset = vmf->pgoff << PAGE_SHIFT;
@@ -43,6 +43,15 @@ static int fb_deferred_io_fault(struct vm_area_struct *vma,
                return VM_FAULT_SIGBUS;
 
        get_page(page);
+
+       if (vma->vm_file)
+               page->mapping = vma->vm_file->f_mapping;
+       else
+               printk(KERN_ERR "no mapping available\n");
+
+       BUG_ON(!page->mapping);
+       page->index = vmf->pgoff;
+
        vmf->page = page;
        return 0;
 }
@@ -138,11 +147,20 @@ EXPORT_SYMBOL_GPL(fb_deferred_io_init);
 
 void fb_deferred_io_cleanup(struct fb_info *info)
 {
+       void *screen_base = (void __force *) info->screen_base;
        struct fb_deferred_io *fbdefio = info->fbdefio;
+       struct page *page;
+       int i;
 
        BUG_ON(!fbdefio);
        cancel_delayed_work(&info->deferred_work);
        flush_scheduled_work();
+
+       /* clear out the mapping that we setup */
+       for (i = 0 ; i < info->fix.smem_len; i += PAGE_SIZE) {
+               page = vmalloc_to_page(screen_base + i);
+               page->mapping = NULL;
+       }
 }
 EXPORT_SYMBOL_GPL(fb_deferred_io_cleanup);
 

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