Trying to alloc many continous pages

Hi,

I'm writing a driver using DMA to transfer data to a performance critic=
al device. I allocate a temporary DMA buffer which will be freed immedi=
ately after the DMA is completed. If there is enough continous memory a=
vailable for all data I would like to transfer, I use it. Otherwise I s=
plit up the transfer in multiple transfers.

What's the right way to allocate the continous buffer? I didn't find ho=
w 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 lin=
ux hangs for several minutes after the DMA has finished.


unsigned int order =3D get_order(data_size);
while (1) {
kbuf =3D (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 =3D 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 on=
e. 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=20
Philipp Werner=20
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