Date:   Fri, 27 Jan 2023 07:35:55 +0100
From:   Christoph Hellwig <hch@lst.de>
To:     Dave Hansen <dave.hansen@intel.com>
Cc:     Georgi Djakov <quic_c_gdjako@quicinc.com>, catalin.marinas@arm.com,
        will@kernel.org, dave.hansen@linux.intel.com, luto@kernel.org,
        peterz@infradead.org, tglx@linutronix.de, mingo@redhat.com,
        bp@alien8.de, hpa@zytor.com, hch@lst.de, m.szyprowski@samsung.com,
        robin.murphy@arm.com, linux-arm-kernel@lists.infradead.org,
        iommu@lists.linux.dev, linux-kernel@vger.kernel.org,
        djakov@kernel.org
Subject: Re: [RFC] mm: Allow ZONE_DMA32 to be disabled via kernel command
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On Thu, Jan 26, 2023 at 10:51:17AM -0800, Dave Hansen wrote:
> 
> Also, what are the practical implications here?  There are obviously an
> ever decreasing number of 32-bit DMA devices out there.  Somebody that
> has one and uses this option might be sad because now they're stuck
> using ZONE_DMA which is quite tiny.
> 
> What other ZONE_DMA32 users are left?  Will anyone else care?  There is
> some DMA32 slab and vmalloc() functionality remaining.  Is it impacted?

DMA32 never supported lab.  But < 64-bit DMA device are unfortunately
still not uncommon, and configuring out ZONE_DMA32 breaks them pretty
badly as we guarantee that a DMA mask of 32-bit always works.

So I'm not only very much against this patch, but also the currently
existing way to configure out ZONE_DMA32 on arm64, which needs to
go away.

If people want ZONE_DMA32 to go away we need something to replace
it first, like a large enough CMA region in the 32-bit addressable
range.