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

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Message-ID: <20190321233248.GA6941@Asurada-Nvidia>
Date:   Thu, 21 Mar 2019 16:32:49 -0700
From:   Nicolin Chen <nicoleotsuka@...il.com>
To:     Catalin Marinas <catalin.marinas@....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 Catalin,

Thank you for the review. And I realized that the free() path
is missing too.

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?