On 14/06/2019 15:50, 'Christoph Hellwig' wrote:

On Fri, Jun 14, 2019 at 02:15:44PM +0000, David Laight wrote:

quoted

Does this still guarantee that requests for 16k will not cross a 16k boundary?
It looks like you are losing the alignment parameter.

The DMA API never gave you alignment guarantees to start with,
and you can get not naturally aligned memory from many of our
current implementations.

Well, apart from the bit in DMA-API-HOWTO which has said this since 
forever (well, before Git history, at least):

"The CPU virtual address and the DMA address are both
guaranteed to be aligned to the smallest PAGE_SIZE order which
is greater than or equal to the requested size.  This invariant
exists (for example) to guarantee that if you allocate a chunk
which is smaller than or equal to 64 kilobytes, the extent of the
buffer you receive will not cross a 64K boundary."

That said, I don't believe this particular patch should make any 
appreciable difference - alloc_pages_exact() is still going to give back 
the same base address as the rounded up over-allocation would, and 
PAGE_ALIGN()ing the size passed to get_order() already seemed to be 
pointless.

Robin.