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.
