This problem has been already fixed. See
http://www.x86-64.org/lists/discuss/msg08774.html
That patch is already in 2.6.17-rc2.
Thanks,
Jon
On Mon, Apr 24, 2006 at 06:53:46PM +0100, Neil Turton wrote:
> I've recently tracked down a problem where the IOMMU fills up. The
> machine is a 2 cpu x86_64 (Opteron) with 6G of RAM and is doing lots of
> network activity on a 32-bit bus mastering PCI card at the time. The
> following message gets written to the console and the system ends up in
> a partly-functional state.
>
> PCI-DMA: Out of IOMMU space for 53248 bytes at device 0000:03:05.0
> end_request: I/O error, dev sda, sector 84087213
>
> Since sda holds the root filesystem and swap, that explains the loss of
> functionality.
>
> The problem is caused by a race condition in free_iommu in pci-gart.c.
> It looks like it has been optimized for the case of single page
> allocations to avoid taking iommu_bitmap_lock, however this isn't valid
> on an SMP machine since the atomic instruction in clear_bit can be
> inserted into the middle of one of the non-atomic (read, modify, write)
> instructions in __clear_bit_string or set_bit_string. The effect of
> this is that the value written by the non-atomic instruction overwrites
> the value written by the atomic instruction and so that IOMMU entry
> doesn't get freed. After running for some time with heavy I/O, the
> IOMMU gets full and the system grinds to a halt.
>
> I've attached a patch against 2.6.16.9 to remove the unsafe optimization.
>
> Cheers, Neil.
>
> The IOMMU boot messages were:
> Checking aperture...
> CPU 0: aperture @ f2000000 size 32 MB
> Aperture from northbridge cpu 0 too small (32 MB)
> No AGP bridge found
> Your BIOS doesn't leave a aperture memory hole
> Please enable the IOMMU option in the BIOS setup
> This costs you 64 MB of RAM
> Mapping aperture over 65536 KB of RAM @ 4000000
> ...
> PCI-DMA: Disabling AGP.
> PCI-DMA: aperture base @ 4000000 size 65536 KB
> PCI-DMA: Reserving 64MB of IOMMU area in the AGP aperture
>
>
> diff -ur linux-2.6.16.9/arch/x86_64/kernel/pci-gart.c linux-2.6.16.9-ndt/arch/x86_64/kernel/pci-gart.c
> --- linux-2.6.16.9/arch/x86_64/kernel/pci-gart.c 2006-04-19 07:10:14.000000000 +0100
> +++ linux-2.6.16.9-ndt/arch/x86_64/kernel/pci-gart.c 2006-04-24 15:53:33.000000000 +0100
> @@ -114,10 +114,6 @@
> static void free_iommu(unsigned long offset, int size)
> {
> unsigned long flags;
> - if (size == 1) {
> - clear_bit(offset, iommu_gart_bitmap);
> - return;
> - }
> spin_lock_irqsave(&iommu_bitmap_lock, flags);
> __clear_bit_string(iommu_gart_bitmap, offset, size);
> spin_unlock_irqrestore(&iommu_bitmap_lock, flags);
>
-
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