Re: [PATCH RESEND v2 0/5] Avoid requesting page from DMA zone when no managed pages

[Christoph Hellwig]

On Tue, Dec 07, 2021 at 09:05:26AM +0100, Christoph Lameter wrote:
> On Tue, 7 Dec 2021, Baoquan He wrote:
> 
> > into ZONE_DMA32 by default. The zone DMA covering low 16M is used to
> > take care of antique ISA devices. In fact, on 64bit system, it rarely
> > need ZONE_DMA (which is low 16M) to support almost extinct ISA devices.
> > However, some components treat DMA as a generic concept, e.g
> > kmalloc-dma, slab allocator initializes it for later any DMA related
> > buffer allocation, but not limited to ISA DMA.
> 
> The idea of the slab allocator DMA support is to have memory available
> for devices that can only support a limited range of physical addresses.
> These are only to be enabled for platforms that have such requirements.
> 
> The slab allocators guarantee that all kmalloc allocations are DMA able
> indepent of specifying ZONE_DMA/ZONE_DMA32

Yes.  And we never supported slab for ZONE_DMA32 and should work on
getting rid of it for ZONE_DMA as well.  The only thing that guarantees
device addressability is the DMA API.  The DMA API needs ZONE_DMA/DMA32
to back its page allocations, but supporting this in slab is a bad idea
only explained by historic reasons from before when we had a DMA API.

> > On arm64, even though both CONFIG_ZONE_DMA and CONFIG_ZONE_DMA32
> > are enabled, it makes ZONE_DMA covers the low 4G area, and ZONE_DMA32
> > empty. Unless on specific platforms (e.g. 30-bit on Raspberry Pi 4),
> > then zone DMA covers the 1st 1G area, zone DMA32 covers the rest of
> > the 32-bit addressable memory.
> 
> ZONE_NORMAL should cover all memory. ARM does not need ZONE_DMA32.

arm32 not, arm64 does.  And the Pi 4 is an arm64 device.

> > I am wondering if we can also change the size of DMA and DMA32 ZONE as
> > dynamically adjusted, just as arm64 is doing? On x86_64, we can make
> > zone DMA covers the 32-bit addressable memory, and empty zone DMA32 by
> > default. Once ISA_DMA_API is enabled, we go back to make zone DMA covers
> > low 16M area, zone DMA32 covers the rest of 32-bit addressable memory.
> > (I am not familiar with ISA_DMA_API, will it require 24-bit addressable
> > memory when enabled?)
> 
> The size of ZONE_DMA is traditionally depending on the platform. On some
> it is 16MB, on some 1G and on some 4GB. ZONE32 is always 4GB and should
> only be used if ZONE_DMA has already been used.

ZONE32 should be (and generally is) used whenever there is zone covering
the 32-bit CPU physical address limit.

> 
> ZONE_DMA is dynamic in the sense of being different on different
> platforms.

Agreed.