linux-kernel - Re: [LSF/MM TOPIC NOTES] x86 ZONE

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

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.