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

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Message-ID: <3a530804ad5aa96d2502da8ee3e8650b0b477c0f.camel@suse.de>
Date:   Thu, 06 Aug 2020 13:50:29 +0200
From:   Nicolas Saenz Julienne <nsaenzjulienne@...e.de>
To:     Christoph Hellwig <hch@....de>
Cc:     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 Thu, 2020-08-06 at 07:18 +0200, Christoph Hellwig wrote:
> 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.

Fair enough, should I send a v3 with everything cleaned-up/rebased, or you'd
rather pick it up from your version?


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