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

[Nicolin Chen]

Hi Robin,

Thanks for the comments.

On Thu, Nov 01, 2018 at 02:07:55PM +0000, Robin Murphy wrote:
> 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.

Hmm..I don't quite understand the part of worrying its realisticness.
Would you mind elaborating a bit? As I tested this change on Tegra186
board, and saw some single-page allocations have been directed to the
normal allocation; and the "CmaFree" size reported from /proc/meminfo
is also increased. Does this mean it's realistic?

Thank you
Nicolin

-----

> 
> Robin.
> 
> > Signed-off-by: Nicolin Chen <nicoleotsuka@xxxxxxxxx>
> > ---
> >   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)) {
> >