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Is there a way to allocate contiguous physical memory from userspace in linux? At least few guaranteed contiguous memory pages. One huge page isn't the answer.
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Memory that the malloc() function allocates is contiguous in the virtual memory address space, but any underlying physical pages are unlikely to be contiguous physically. As such, the host must be able to allocate physically-contiguous memory regions.
Virtual memory is a function provided by many operating systems where the operating system creates a virtual memory space that applications can access as if it were a single piece of contiguous memory. This virtual memory space can be a combination of actual physical memory as well as disk-based resources in concert.
Whilst a user-space program is not allowed to access kernel memory, it is possible for the kernel to access user memory. However, the kernel must never execute user-space memory and it must also never access user-space memory without explicit expectation to do so.
DMA stands for Designated Market Area. A DMA is a geographic region used to analyze and quantify consumer activity. In the Luminate products DMAs are used to geographically categorize music sales and online streaming of music in the United States. CMA stands for Census Metropolitan Area.
No. There is not. You do need to do this from Kernel space.
If you say "we need to do this from User Space" - without anything going on in kernel-space it makes little sense - because a user space program has no way of controlling or even knowing if the underlying memory is contiguous or not.
The only reason where you would need to do this - is if you were working in-conjunction with a piece of hardware, or some other low-level (i.e. Kernel) service that needed this requirement. So again, you would have to deal with it at that level.
So the answer isn't just "you can't" - but "you should never need to".
I have written such memory managers that do allow me to do this - but it was always because of some underlying issue at the kernel level, which had to be addressed at the kernel level. Generally because some other agent on the bus (PCI card, BIOS or even another computer over RDMA interface) had the physical contiguous memory requirement. Again, all of this had to be addressed in kernel space.
When you talk about "cache lines" - you don't need to worry. You can be assured that each page of your user-space memory is contiguous, and each page is much larger than a cache-line (no matter what architecture you're talking about).
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