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.