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

Christoph Hellwig
2019-02-04T08:23:10Z

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. 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.