linux-kernel - Re: [PATCH v2 RFC/RFT] dma-contiguous: Get normal pages for single-page allocations

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Date:   Fri, 22 Mar 2019 10:57:13 +0000
From:   Catalin Marinas <catalin.marinas@....com>
To:     Nicolin Chen <nicoleotsuka@...il.com>
Cc:     hch@....de, robin.murphy@....com, vdumpa@...dia.com,
        will.deacon@....com, chris@...kel.net, jcmvbkbc@...il.com,
        joro@...tes.org, dwmw2@...radead.org, m.szyprowski@...sung.com,
        linux-arm-kernel@...ts.infradead.org, linux-kernel@...r.kernel.org,
        linux-xtensa@...ux-xtensa.org, iommu@...ts.linux-foundation.org,
        jonathanh@...dia.com
Subject: Re: [PATCH v2 RFC/RFT] dma-contiguous: Get normal pages for
 single-page allocations

Hi Nicolin,

On Thu, Mar 21, 2019 at 04:32:49PM -0700, Nicolin Chen wrote:
> On Tue, Mar 19, 2019 at 02:43:01PM +0000, Catalin Marinas wrote:
> > On Tue, Mar 05, 2019 at 10:32:02AM -0800, Nicolin Chen wrote:
> > > The addresses within a single page are always contiguous, so it's
> > > not so necessary to always allocate one single page from CMA area.
> > > Since the CMA area has a limited predefined size of space, it may
> > > run out of space in heavy use cases, where there might be quite a
> > > lot CMA pages being allocated for single pages.
> > > 
> > > However, there is also a concern that a device might care where a
> > > page comes from -- it might expect the page from CMA area and act
> > > differently if the page doesn't.
> > > 
> > > This patch tries to get normal pages for single-page allocations
> > > unless the device has its own CMA area. This would save resources
> > > from the CMA area for more CMA allocations. And it'd also reduce
> > > CMA fragmentations resulted from trivial allocations.
> > 
> > This is not sufficient. Some architectures/platforms declare limits on
> > the CMA range so that DMA is possible with all expected devices. For
> > example, on arm64 we keep the CMA in the lower 4GB of the address range,
> > though with this patch you only covered the iommu ops allocation.
> 
> I will follow the way of v1 by adding alloc_page()/free_page()
> function to those callers who don't have fallback allocations.
> In this way, archs may use different callbacks to alloc pages.
> 
> > Do you have any numbers to back this up? You don't seem to address
> > dma_direct_alloc() either but, as I said above, it's not trivial since
> > some platforms expect certain physical range for DMA allocations.
> 
> What's the dma_direct_alloc() here about? Mind elaborating?

I just did a grep for dma_alloc_from_contiguous() in the 5.1-rc1 kernel
and came up with __dma_direct_alloc_pages(). Should your patch cover
this as well?

-- 
Catalin