linux-kernel - Re: [PATCH RESEND v2 0/5] Avoid requesting page from DMA zone when no managed pages

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Message-ID: <20211213074919.GC20758@lst.de>
Date:   Mon, 13 Dec 2021 08:49:19 +0100
From:   Christoph Hellwig <hch@....de>
To:     Baoquan He <bhe@...hat.com>
Cc:     Christoph Lameter <cl@...two.org>, linux-kernel@...r.kernel.org,
        linux-mm@...ck.org, akpm@...ux-foundation.org, hch@....de,
        robin.murphy@....com, penberg@...nel.org, rientjes@...gle.com,
        iamjoonsoo.kim@....com, vbabka@...e.cz, m.szyprowski@...sung.com,
        John.p.donnelly@...cle.com, kexec@...ts.infradead.org
Subject: Re: [PATCH RESEND v2 0/5] Avoid requesting page from DMA zone when
 no managed pages

On Mon, Dec 13, 2021 at 03:39:25PM +0800, Baoquan He wrote:
> > > As said at above, ia64 and riscv don't have ZONE_DMA at all, they just
> > > cover low 4G with ZONE_DMA32 alone.
> > 
> > If you do not have devices that are crap and cannot address the full
> > memory then you dont need these special zones.
> 
> I am not a DMA expert, with my understanding, on x86_64 and arm64, we
> have PCIe devices which dma mask is 32bit

Yes, way to many, and they keep getting newly introduce as well.  Also
weirdo masks like 40, 44 or 48 bits.

> , means they can only address
> ZONE_DMA32.

Yes and no.  Offset between cpu physical and device address make this
complicated, even ignoring iommus.