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Message-ID: <e0b2a00c8cf86cb1a91804942d35c9d4b98e9f9f.camel@suse.de>
Subject: Re: [PATCH v2 2/2] dma-pool: Only allocate from CMA when in same
 memory zone
From:   Nicolas Saenz Julienne <nsaenzjulienne@suse.de>
To:     Christoph Hellwig <hch@lst.de>
Cc:     amit.pundir@linaro.org, linux-kernel@vger.kernel.org,
        Marek Szyprowski <m.szyprowski@samsung.com>,
        Robin Murphy <robin.murphy@arm.com>, rientjes@google.com,
        jeremy.linton@arm.com, linux-rpi-kernel@lists.infradead.org,
        iommu@lists.linux-foundation.org
Date:   Tue, 04 Aug 2020 11:43:15 +0200
In-Reply-To: <20200804060633.GA7368@lst.de>
References: <20200803160956.19235-1-nsaenzjulienne@suse.de>
         <20200803160956.19235-3-nsaenzjulienne@suse.de>
         <20200804060633.GA7368@lst.de>
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On Tue, 2020-08-04 at 08:06 +0200, Christoph Hellwig wrote:
> On Mon, Aug 03, 2020 at 06:09:56PM +0200, Nicolas Saenz Julienne wrote:
> > +	if (IS_ENABLED(CONFIG_ZONE_DMA) && (gfp & GFP_DMA))
> > +		return end <=3D DMA_BIT_MASK(zone_dma_bits);
> > +	if (IS_ENABLED(CONFIG_ZONE_DMA32) && (gfp & GFP_DMA32))
> > +		return end <=3D DMA_BIT_MASK(32);
> > +	if (gfp & GFP_KERNEL)
> > +		return end > DMA_BIT_MASK(32);
>=20
> So the GFP_KERNEL one here looks weird.  For one I don't think the if
> line is needed at all, and it just confuses things.

Yes, sorry, shoud've seen that.

> Second I don't see the need (and actually some harm) in preventing GFP_KE=
RNEL
> allocations from dipping into lower CMA areas - something that we did sup=
port
> 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 no=
w
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 pla=
in
abusing CMA? If you need low memory allocations, you should be defining DMA
zones.

Regards,
Nicolas


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