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Date: Fri, 27 Apr 2018 09:18:13 -0700
From: Matthew Wilcox <willy@infradead.org>
Subject: Re: [LSF/MM TOPIC NOTES] x86 ZONE_DMA love
Message-ID: <20180427161813.GD8161@bombadil.infradead.org>
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To: Christopher Lameter <cl@linux.com>
Cc: Michal Hocko <mhocko@kernel.org>, Christoph Hellwig <hch@infradead.org>, "Luis R. Rodriguez" <mcgrof@kernel.org>, linux-mm@kvack.org, Jan Kara <jack@suse.cz>, matthew@wil.cx, x86@kernel.org, luto@amacapital.net, martin.petersen@oracle.com, jthumshirn@suse.de, broonie@kernel.org, linux-spi@vger.kernel.org, linux-scsi@vger.kernel.org, linux-kernel@vger.kernel.org, "lsf-pc@lists.linux-foundation.org" <lsf-pc@lists.linux-foundation.org>

On Fri, Apr 27, 2018 at 11:07:07AM -0500, Christopher Lameter wrote:
> Well it looks like what we are using it for is to force allocation from
> low physical memory if we fail to obtain proper memory through a normal
> channel.  The use of ZONE_DMA is only there for emergency purposes.
> I think we could subsitute ZONE_DMA32 on x87 without a problem.
> 
> Which means that ZONE_DMA has no purpose anymore.
> 
> Can we make ZONE_DMA on x86 refer to the low 32 bit physical addresses
> instead and remove ZONE_DMA32?
> 
> That would actually improve the fallback because you have more memory for
> the old devices.

Some devices have incredibly bogus hardware like 28 bit addressing
or 39 bit addressing.  We don't have a good way to allocate memory by
physical address other than than saying "GFP_DMA for anything less than
32, GFP_DMA32 (or GFP_KERNEL on 32-bit) for anything less than 64 bit".

Even CMA doesn't have a "cma_alloc_phys()".  Maybe that's the right place
to put such an allocation API.