RE: [PATCH v3 1/2] dma-direct: provide the ability to reserve per-numa CMA

[Song Bao Hua (Barry Song)]

> -----Original Message-----
> From: Christoph Hellwig [mailto:hch@xxxxxx]
> Sent: Thursday, July 23, 2020 2:30 AM
> To: Song Bao Hua (Barry Song) <song.bao.hua@xxxxxxxxxxxxx>
> Cc: hch@xxxxxx; m.szyprowski@xxxxxxxxxxx; robin.murphy@xxxxxxx;
> will@xxxxxxxxxx; ganapatrao.kulkarni@xxxxxxxxxx;
> catalin.marinas@xxxxxxx; iommu@xxxxxxxxxxxxxxxxxxxxxxxxxx; Linuxarm
> <linuxarm@xxxxxxxxxx>; linux-arm-kernel@xxxxxxxxxxxxxxxxxxx;
> linux-kernel@xxxxxxxxxxxxxxx; Jonathan Cameron
> <jonathan.cameron@xxxxxxxxxx>; Nicolas Saenz Julienne
> <nsaenzjulienne@xxxxxxx>; Steve Capper <steve.capper@xxxxxxx>; Andrew
> Morton <akpm@xxxxxxxxxxxxxxxxxxxx>; Mike Rapoport <rppt@xxxxxxxxxxxxx>
> Subject: Re: [PATCH v3 1/2] dma-direct: provide the ability to reserve
> per-numa CMA
> 
+cc Prime and Daode who are interested in this patchset.

> On Sun, Jun 28, 2020 at 11:12:50PM +1200, Barry Song wrote:
> >  struct page *dma_alloc_contiguous(struct device *dev, size_t size, gfp_t
> gfp)
> >  {
> >  	size_t count = size >> PAGE_SHIFT;
> >  	struct page *page = NULL;
> >  	struct cma *cma = NULL;
> > +	int nid = dev ? dev_to_node(dev) : NUMA_NO_NODE;
> > +	bool alloc_from_pernuma = false;
> > +
> > +	if ((count <= 1) && !(dev && dev->cma_area))
> > +		return NULL;
> >
> >  	if (dev && dev->cma_area)
> >  		cma = dev->cma_area;
> > -	else if (count > 1)
> > +	else if ((nid != NUMA_NO_NODE) &&
> dma_contiguous_pernuma_area[nid]
> > +		&& !(gfp & (GFP_DMA | GFP_DMA32))) {
> > +		cma = dma_contiguous_pernuma_area[nid];
> > +		alloc_from_pernuma = true;
> > +	} else {
> >  		cma = dma_contiguous_default_area;
> > +	}
> 
> I find the function rather confusing now.  What about something
> like (this relies on the fact that dev should never be NULL in the
> DMA API)
> 
> struct page *dma_alloc_contiguous(struct device *dev, size_t size, gfp_t gfp)
> {
> 	size_t cma_align = min_t(size_t, get_order(size),
> CONFIG_CMA_ALIGNMENT);
>  	size_t count = size >> PAGE_SHIFT;
> 
> 	if (gfpflags_allow_blocking(gfp))
> 		return NULL;
> 	gfp &= __GFP_NOWARN;
> 
> 	if (dev->cma_area)

I got a kernel robot warning which said dev should be checked before being accessed
when I did a similar change in v1. Probably it was an invalid warning if dev should
never be null.

> 		return cma_alloc(dev->cma_area, count, cma_align, gfp);
> 	if (count <= 1)
> 		return NULL;
> 
> 	if (IS_ENABLED(CONFIG_PERNODE_CMA) && !(gfp & (GFP_DMA |
> GFP_DMA32)) {
> 		int nid = dev_to_node(dev);
>  		struct cma *cma = dma_contiguous_pernuma_area[nid];
> 		struct page *page;
> 
> 		if (cma) {
> 			page = cma_alloc(cma, count, cma_align, gfp);
> 			if (page)
> 				return page;
> 		}
> 	}
> 
> 	return cma_alloc(dma_contiguous_default_area, count, cma_align, gfp);
> }

Yes, it looks much better.

> 
> > +		/*
> > +		 * otherwise, page is from either per-numa cma or default cma
> > +		 */
> > +		int nid = page_to_nid(page);
> > +
> > +		if (nid != NUMA_NO_NODE) {
> > +			if (cma_release(dma_contiguous_pernuma_area[nid], page,
> > +						PAGE_ALIGN(size) >> PAGE_SHIFT))
> > +				return;
> > +		}
> > +
> > +		if (cma_release(dma_contiguous_default_area, page,
> 
> How can page_to_nid ever return NUMA_NO_NODE?

I thought page_to_nid would return NUMA_NO_NODE if CONFIG_NUMA is
not enabled. Probably I was wrong. Will get it fixed in v4.

Thanks
Barry