Messages in this thread Patch in this message | | | Date | Mon, 24 Apr 2006 18:53:46 +0100 | From | Neil Turton <> | Subject | PROBLEM/PATCH: free_iommu leaks entries (x86_64) |
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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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