linux-kernel - Re: [PATCH] dma-pool: use single atomic pool for both DMA zones

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Message-ID: <20200708161010.GA30184@lst.de>
Date:   Wed, 8 Jul 2020 18:10:10 +0200
From:   Christoph Hellwig <hch@....de>
To:     Nicolas Saenz Julienne <nsaenzjulienne@...e.de>
Cc:     Christoph Hellwig <hch@....de>,
        Marek Szyprowski <m.szyprowski@...sung.com>,
        Robin Murphy <robin.murphy@....com>,
        David Rientjes <rientjes@...gle.com>,
        linux-rpi-kernel@...ts.infradead.org, jeremy.linton@....com,
        iommu@...ts.linux-foundation.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH] dma-pool: use single atomic pool for both DMA zones

On Wed, Jul 08, 2020 at 06:00:35PM +0200, Nicolas Saenz Julienne wrote:
> On Wed, 2020-07-08 at 17:35 +0200, Christoph Hellwig wrote:
> > On Tue, Jul 07, 2020 at 02:28:04PM +0200, Nicolas Saenz Julienne wrote:
> > > When allocating atomic DMA memory for a device, the dma-pool core
> > > queries __dma_direct_optimal_gfp_mask() to check which atomic pool to
> > > use. It turns out the GFP flag returned is only an optimistic guess.
> > > The pool selected might sometimes live in a zone higher than the
> > > device's view of memory.
> > > 
> > > As there isn't a way to grantee a mapping between a device's DMA
> > > constraints and correct GFP flags this unifies both DMA atomic pools.
> > > The resulting pool is allocated in the lower DMA zone available, if any,
> > > so as for devices to always get accessible memory while having the
> > > flexibility of using dma_pool_kernel for the non constrained ones.
> > > 
> > > Fixes: c84dc6e68a1d ("dma-pool: add additional coherent pools to map to gfp
> > > mask")
> > > Reported-by: Jeremy Linton <jeremy.linton@....com>
> > > Suggested-by: Robin Murphy <robin.murphy@....com>
> > > Signed-off-by: Nicolas Saenz Julienne <nsaenzjulienne@...e.de>
> > 
> > Hmm, this is not what I expected from the previous thread.  I thought
> > we'd just use one dma pool based on runtime available of the zones..
> 
> I may be misunderstanding you, but isn't that going back to how things used to
> be before pulling in David Rientjes' work? The benefit of having a GFP_KERNEL
> pool is that non-address-constrained devices can get their atomic memory there,
> instead of consuming somewhat scarcer low memory.

Yes, I think we are misunderstanding each other.  I don't want to remove
any pool, just make better runtime decisions when to use them.