Re: [PATCH] dma/pool: do not complain if DMA pool is not allocated

Michal Hocko
2022-08-11T08:42:27Z

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