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Date:   Thu, 11 Aug 2022 09:49:46 +0200
From:   Christoph Hellwig <hch@lst.de>
To:     Baoquan He <bhe@redhat.com>
Cc:     Michal Hocko <mhocko@suse.com>, Christoph Hellwig <hch@lst.de>,
        Andrew Morton <akpm@linux-foundation.org>,
        John Donnelly <john.p.donnelly@oracle.com>,
        David Hildenbrand <david@redhat.com>, linux-mm@kvack.org,
        LKML <linux-kernel@vger.kernel.org>
Subject: Re: [PATCH] dma/pool: do not complain if DMA pool is not allocated
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On Thu, Aug 04, 2022 at 07:01:28PM +0800, Baoquan He wrote:
> After attempts, I realize it's time to let one zone DMA or DMA32 cover
> the whole low 4G memory on x86_64. That's the real fix. The tiny 16M DMA
> on 64bit system is root cause.

We can't for two reasons:

 - people still use ISA cards on x86, including the industrial PC104
   version, and we still have drivers that rely on it
 - we still have PCI and PCIe devices with small than 26, 28, 30 and 31
   bit addressing limitations

We could try to get the 24-bit DMA entirely out of the zone allocator
and only fill a genpool at bootmem time.  But that requires fixing up
all the direct users of page and slab allocations on it first (of
which 90+% look bogus, with the s390 drivers being the obvious
exception).

Or we could make 'low' memory a special ZONE_MOVABLE and have an
allocator that can search by physical address an replace ZONE_DMA
and ZONE_DMA32 with that.  Which sounds like a nice idea to me, but
is pretty invasive.