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Message-ID: <YvTA7M8XXu0GgB8v@dhcp22.suse.cz>
Date: Thu, 11 Aug 2022 10:42:20 +0200
From: Michal Hocko <mhocko@...e.com>
To: Christoph Hellwig <hch@....de>
Cc: Baoquan He <bhe@...hat.com>,
Andrew Morton <akpm@...ux-foundation.org>,
John Donnelly <john.p.donnelly@...cle.com>,
David Hildenbrand <david@...hat.com>, linux-mm@...ck.org,
LKML <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH] dma/pool: do not complain if DMA pool is not allocated
On Thu 11-08-22 09:49:46, Christoph Hellwig wrote:
> 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).
Completely agreed!
> 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.
Yes.
--
Michal Hocko
SUSE Labs
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