Messages in this thread | | | Subject | Re: [PATCH RFC] dma-direct: do not allocate a single page from CMA area | From | Robin Murphy <> | Date | Thu, 1 Nov 2018 14:07:55 +0000 |
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On 31/10/2018 20:03, 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. This would save resource > in the CMA area for further more CMA allocations.
In general, this seems to make sense to me. It does represent a theoretical change in behaviour for devices which have their own CMA area somewhere other than kernel memory, and only ever make non-atomic allocations, but I'm not sure whether that's a realistic or common enough case to really worry about.
Robin.
> Signed-off-by: Nicolin Chen <nicoleotsuka@gmail.com> > --- > kernel/dma/direct.c | 8 ++++++-- > 1 file changed, 6 insertions(+), 2 deletions(-) > > diff --git a/kernel/dma/direct.c b/kernel/dma/direct.c > index 22a12ab5a5e9..14c5d49eded2 100644 > --- a/kernel/dma/direct.c > +++ b/kernel/dma/direct.c > @@ -120,8 +120,12 @@ void *dma_direct_alloc_pages(struct device *dev, size_t size, > gfp |= __dma_direct_optimal_gfp_mask(dev, dev->coherent_dma_mask, > &phys_mask); > 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 > + */ > + if (gfpflags_allow_blocking(gfp) && count > 1) { > page = dma_alloc_from_contiguous(dev, count, page_order, > gfp & __GFP_NOWARN); > if (page && !dma_coherent_ok(dev, page_to_phys(page), size)) { >
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