linux-kernel - Re: [PATCH v2 2/2] dma-pool: Only allocate from CMA when in same memory zone

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Date:   Thu, 6 Aug 2020 07:18:14 +0200
From:   Christoph Hellwig <hch@....de>
To:     Nicolas Saenz Julienne <nsaenzjulienne@...e.de>
Cc:     Christoph Hellwig <hch@....de>, amit.pundir@...aro.org,
        linux-kernel@...r.kernel.org,
        Marek Szyprowski <m.szyprowski@...sung.com>,
        Robin Murphy <robin.murphy@....com>, rientjes@...gle.com,
        jeremy.linton@....com, linux-rpi-kernel@...ts.infradead.org,
        iommu@...ts.linux-foundation.org
Subject: Re: [PATCH v2 2/2] dma-pool: Only allocate from CMA when in same
 memory zone

On Tue, Aug 04, 2020 at 11:43:15AM +0200, Nicolas Saenz Julienne wrote:
> > Second I don't see the need (and actually some harm) in preventing GFP_KERNEL
> > allocations from dipping into lower CMA areas - something that we did support
> > before 5.8 with the single pool.
> 
> My thinking is the least we pressure CMA the better, it's generally scarse, and
> it'll not grow as the atomic pools grow. As far as harm is concerned, we now
> check addresses for correctness, so we shouldn't run into problems.
> 
> There is a potential case for architectures defining a default CMA but not
> defining DMA zones where this could be problematic. But isn't that just plain
> abusing CMA? If you need low memory allocations, you should be defining DMA
> zones.

The latter is pretty much what I expect, as we only support the default and
per-device DMA CMAs.