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

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Date:   Fri, 27 Apr 2018 11:07:07 -0500 (CDT)
From:   Christopher Lameter <cl@...ux.com>
To:     Michal Hocko <mhocko@...nel.org>
cc:     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, 27 Apr 2018, Michal Hocko wrote:

> On Thu 26-04-18 22:35:56, Christoph Hellwig wrote:
> > On Thu, Apr 26, 2018 at 09:54:06PM +0000, Luis R. Rodriguez wrote:
> > > In practice if you don't have a floppy device on x86, you don't need ZONE_DMA,
> >
> > I call BS on that, and you actually explain later why it it BS due
> > to some drivers using it more explicitly.  But even more importantly
> > we have plenty driver using it through dma_alloc_* and a small DMA
> > mask, and they are in use - we actually had a 4.16 regression due to
> > them.
>
> Well, but do we need a zone for that purpose? The idea was to actually
> replace the zone by a CMA pool (at least on x86). With the current
> implementation of the CMA we would move the range [0-16M] pfn range into
> zone_movable so it can be used and we would get rid of all of the
> overhead each zone brings (a bit in page flags, kmalloc caches and who
> knows what else)

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.