kmalloc

I am writing a device driver for a DMA device in Linux. In Linux Device Drivers, Chapter 15 , it says: For devices with this kind of limitation, memory should be allocated from the DMA zone by adding the GFP_DMA flag to the kmalloc or get_free_pages call. When this flag is present, only memory that can be addressed with 24 bits is allocated. Alternatively, you can use the generic DMA layer (which we discuss shortly) to allocate buffers that work around your device’s limitations I am calling kmalloc like this: physical_pointer0 = kmalloc(number_of_bytes, GFP_DMA); and printing the result like

As far as I understand, the use in the GFP_USER flag (in the call to kmalloc ) is used to allocate memory for user space. Does it means that the allocated page are in kernel space, which are accessible to user? Does these pages required to be mmapp 'ed in user space or the address is directly accessible to user. If they need to mmapp then what is difference between GFP_USER and GFP_KERNEL ? Brief explanation can be found in kernel sources : GFP_KERNEL is typical for kernel-internal allocations. The caller requires ZONE_NORMAL or a lower zone for direct access but can direct reclaim. GFP_USER

I've googled around and found most people advocating the use of kmalloc , as you're guaranteed to get contiguous physical blocks of memory. However, it also seems as though kmalloc can fail if a contiguous physical block that you want can't be found. What are the advantages of having a contiguous block of memory? Specifically, why would I need to have a contiguous physical block of memory in a system call ? Is there any reason I couldn't just use vmalloc ? Finally, if I were to allocate memory during the handling of a system call, should I specify GFP_ATOMIC ? Is a system call executed in an