Messages in this thread | | | Date | Thu, 14 Aug 2008 22:03:43 +0100 (BST) | From | Hugh Dickins <> | Subject | Re: kernel BUG at lib/radix-tree.c:473! |
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On Thu, 14 Aug 2008, Jeremy Fitzhardinge wrote: > Hugh Dickins wrote: > > In both cases it's handling a page fault: I'm curious as to what kind > > of vma this fault is occurring on. Could you devise a way of getting > > us /proc/<pid>/maps output, together with the faulting address, when > > it hits one of these BUGs? Or should I try to put together a patch > > for that? > > It's a /dev/fb0 mapping: > > open("/dev/fb0", O_RDWR) = 8 > ... > mmap(NULL, 2097152, PROT_READ|PROT_WRITE, MAP_SHARED, 8, 0) = 0x7fed69a08000 > > The fault is 1 page into this mapping: > > WARNING: at /home/jeremy/hg/xen/paravirt/linux/fs/buffer.c:711 > __set_page_dirty+0x7e/0x113() > kernel BUG at /home/jeremy/hg/xen/paravirt/linux/lib/radix-tree.c:473! > radix_tree_tag_set+0x17/0x9b > CR2: 00007fed69a09000 CR3: 000000000cbc4000 CR4: 0000000000002620 > ^^^^^^^^^^^^^^^^ > Process X (pid: 1357, threadinfo ffff88000b048000, task ffff88000cb2ecc0)
Brilliant, thanks a lot, Jeremy. That fits, I'd been inching towards forming the thought that it was likely to involve a block or char device (rather than a directory, which is what had prompted the patch).
I'd thought about them when making the patch, but quickly decided that a device node may live in a tmpfs (and usually does with udev), but redirects off to somewhere else entirely.
If I open /dev/sda and mmap it, then I don't expect to see pages of shmem, I expect to see pages from my disk. Though if I open /dev/zero and mmap it, that character device does happen to be the one which comes back and delivers pages of shmem.
Now if I open /dev/fb0 here and mmap it as you did, and try to write to it through those pages, I see nothing bad happening: I don't know for sure what pages it's making available to me, but I hope they're pages belonging to that driver.
tmpfs doesn't associate its shmem_file_operations with a device node, so there wouldn't be a way to mmap it, unless the device driver gives the struct file its own file_operations, including an .mmap method.
It looks like your fb driver is providing a backing_dev_info which tells vma_wants_writenotify that it wants mapping_cap_account_dirty: hmm, I suppose the default one would do that, though shmem provided one which says not. But not providing any address_space_operations with a .set_page_dirty which would keep it out of trouble.
Before my patch, the device node happened to stay pointing to shmem_aops, whose set_page_dirty was safe; now it's getting default behaviour, and hitting these problems.
As you can see, I'm still groping towards the right answer. The driver probably needs to provide its own backing_dev_info (or point to a suitable default), and its own address_space_ops, and perhaps more (there should be examples elsewhere). But whether it is actually wrong, or whether I was wrong to mess it up, I've not yet decided.
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.
Hugh
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