Revision 2be23c475af8ae4e25f8bab08d815b17593bd547 authored by Russell King on 08 September 2010, 15:27:56 UTC, committed by Russell King on 08 September 2010, 15:27:56 UTC
Dave Hylands reports: | We've observed a problem with dma_alloc_writecombine when the system | is under heavy load (heavy bus traffic). We've managed to reduce the | problem to the following snippet, which is run from a kthread in a | continuous loop: | | void *virtAddr; | dma_addr_t physAddr; | unsigned int numBytes = 256; | | for (;;) { | virtAddr = dma_alloc_writecombine(NULL, | numBytes, &physAddr, GFP_KERNEL); | if (virtAddr == NULL) { | printk(KERN_ERR "Running out of memory\n"); | break; | } | | /* access DMA memory allocated */ | tmp = virtAddr; | *tmp = 0x77; | | /* free DMA memory */ | dma_free_writecombine(NULL, | numBytes, virtAddr, physAddr); | | ...sleep here... | } | | By itself, the code will run forever with no issues. However, as we | increase our bus traffic (typically using DMA) then the *tmp = 0x77 | line will eventually cause a page fault. If we add a small delay (a | few microseconds) before the *tmp = 0x77, then we don't see a page | fault, even under heavy load. A dsb() is required after modifying the PTE entries to ensure that they will always be visible. Add this dsb(). Reported-by: Dave Hylands <dhylands@gmail.com> Tested-by: Dave Hylands <dhylands@gmail.com> Signed-off-by: Russell King <rmk+kernel@arm.linux.org.uk>
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memory.txt
There are several classic problems related to memory on Linux
systems.
1) There are some motherboards that will not cache above
a certain quantity of memory. If you have one of these
motherboards, your system will be SLOWER, not faster
as you add more memory. Consider exchanging your
motherboard.
All of these problems can be addressed with the "mem=XXXM" boot option
(where XXX is the size of RAM to use in megabytes).
It can also tell Linux to use less memory than is actually installed.
If you use "mem=" on a machine with PCI, consider using "memmap=" to avoid
physical address space collisions.
See the documentation of your boot loader (LILO, grub, loadlin, etc.) about
how to pass options to the kernel.
There are other memory problems which Linux cannot deal with. Random
corruption of memory is usually a sign of serious hardware trouble.
Try:
* Reducing memory settings in the BIOS to the most conservative
timings.
* Adding a cooling fan.
* Not overclocking your CPU.
* Having the memory tested in a memory tester or exchanged
with the vendor. Consider testing it with memtest86 yourself.
* Exchanging your CPU, cache, or motherboard for one that works.
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