Re: [PATCH v2] dma-direct: do not allocate a single page from CMA area

Nicolin Chen
2019-02-05T23:05:38Z

Hi Christoph

On Mon, Feb 04, 2019 at 09:23:07AM +0100, Christoph Hellwig wrote:
> On Tue, Jan 15, 2019 at 01:51:40PM -0800, Nicolin Chen wrote:
> > The addresses within a single page are always contiguous, so it's
> > not so necessary to allocate one single page from CMA area. Since
> > the CMA area has a limited predefined size of space, it might run
> > out of space in some heavy use case, where there might be quite a
> > lot CMA pages being allocated for single pages.
> >
> > This patch tries to skip CMA allocations of single pages and lets
> > them go through normal page allocations unless the allocation has
> > a DMA_ATTR_FORCE_CONTIGUOUS attribute. This'd save some resources
> > in the CMA area for further more CMA allocations, and it can also
> > reduce CMA fragmentations resulted from trivial allocations.
>
> That DMA_ATTR_FORCE_CONTIGUOUS flag does not make sense. A single
> page allocation is per defintion always contigous.
>
> > again:
> > - /* CMA can be used only in the context which permits sleeping */
> > - if (gfpflags_allow_blocking(gfp)) {
> > + /*
> > + * CMA can be used only in the context which permits sleeping.
> > + * Since addresses within one PAGE are always contiguous, skip
> > + * CMA allocation for a single page to save CMA reserved space
> > + * unless DMA_ATTR_FORCE_CONTIGUOUS is flagged.
> > + */
> > + if (gfpflags_allow_blocking(gfp) &&
> > + (count > 1 || attrs & DMA_ATTR_FORCE_CONTIGUOUS)) {
>
> And my other concern is that this skips allocating from the per-device
> pool, which drivers might rely on.

Actually Robin had the same concern at v1 and suggested that we could
always use DMA_ATTR_FORCE_CONTIGUOUS to enforce into per-device pool.

> To be honest I'm not sure there is
> much of a point in the per-device CMA pool vs the traditional per-device
> coherent pool, but I'd rather change that behavior in a clearly documented
> commit with intentions rather as a side effect from a random optimization.

Hmm..sorry, I don't really follow this suggestion. Is it possible for
you to make it clear that what should I do for the change?

Thanks
Nicolin