Re: kernel BUG at lib/radix-tree.c:473!

[Ian Campbell <ijc@hellion.org.uk>]

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On Fri, 2008-08-15 at 01:00 +0100, Hugh Dickins wrote:
> On Fri, 15 Aug 2008, Johannes Weiner wrote:
> > Jeremy Fitzhardinge <jeremy@goop.org> writes:
> > > Hugh Dickins wrote:
> > >
> > >> An additional useful input would be: what happens if you replace
> > >> that /dev/fb0 by a symlink /dev/fb0 pointing to an fb0 device node in
> > >> one of your disk filesystems?  I rather expect that to cause the same
> > >> trouble, which would argue that the driver is wrong and shmem right.
> > >
> > > I don't follow. Do you mean make /dev/fb0 a plain file on a
> > > filesystem?  Or make it a disk device node?  Something else?
> > 
> > Creating a device node on a different filesystem to see if the driver
> > only worked with the safe shmem set_page_dirty and now breaks due to
> > exposure to the generic version.  Or if the driver works with the
> > generic version through other mappings and the shmem code screws it up
> > somewhere else.
> 
> Yes, that's it.  I think it was ext2 I referred to, when I worried
> about this when making the change to tmpfs; and my reading of it
> was that ext2 left a device node's a_ops unset, as I was changing
> tmpfs to do.  (Looking at it again, ext2 doesn't even specify its
> .set_page_dirty, so even if it had assigned an a_ops, it wouldn't
> have avoided the default behaviour.)  But I'd like to hear what
> actually happens in practice, rather than relying on my reading.

Creating /tmp/fb0 on an ext3 filesystem gives the same behaviour.

FWIW the patch below apparently makes it work for me, but I'm not going
to pretend I follow what's going on, why or what else it breaks ;-)

Ian.

diff --git a/drivers/video/fb_defio.c b/drivers/video/fb_defio.c
index 59df132..8414646 100644
--- a/drivers/video/fb_defio.c
+++ b/drivers/video/fb_defio.c
@@ -24,6 +24,10 @@
 #include <linux/rmap.h>
 #include <linux/pagemap.h>
 
+static const struct address_space_operations fb_defio_aops = {
+        .set_page_dirty = __set_page_dirty_no_writeback,
+};
+
 /* this is to find and return the vmalloc-ed fb pages */
 static int fb_deferred_io_fault(struct vm_area_struct *vma,
 				struct vm_fault *vmf)
@@ -119,6 +123,7 @@ static int fb_deferred_io_mmap(struct fb_info *info, struct vm_area_struct *vma)
 	vma->vm_ops = &fb_deferred_io_vm_ops;
 	vma->vm_flags |= ( VM_IO | VM_RESERVED | VM_DONTEXPAND );
 	vma->vm_private_data = info;
+	vma->vm_file->f_mapping->a_ops = &fb_defio_aops;
 	return 0;
 }
 

-- 
Ian Campbell

Wharbat darbid yarbou sarbay?

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