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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