Trying to alloc many continous pages

["EXTERNAL Werner Philipp (Praktikant; CR/AEM5)" <external.Philipp.Werner@xxxxxxxxxxxx>]

Hi,

I'm writing a driver using DMA to transfer data to a performance critical device. I allocate a temporary DMA buffer which will be freed immediately after the DMA is completed. If there is enough continous memory available for all data I would like to transfer, I use it. Otherwise I split up the transfer in multiple transfers.

What's the right way to allocate the continous buffer? I didn't find how to "try allocating memory" without having an effect, if the memory in not available.

If I use the following code and there is not enough free memory, my linux hangs for several minutes after the DMA has finished.


unsigned int order = get_order(data_size);
while (1) {
     	kbuf = (void *) __get_free_pages(__GFP_NORETRY | __GFP_NOWARN, order);
	if (kbuf)
		break;		// success: we got a buffer
	if (order == 0)
		return -ENOMEM;
	order--;			// retry with a smaller order
}
buffer_size = PAGE_SIZE << order;



I think the memory management tries to reduce fragmentation in memory, but I would like to avoid this behaviour. It's not really a problem if I can't get a huge buffer, because in this case I will use a smaller one. But if there is a huge chunk of free memory I would like to use it, because it yields performance benefits.

I just want to look if I can alloc a number of continous pages, and if there aren't enough I want to handle it myself... How can I do that?


Best regards 
Philipp Werner 
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