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

Hyeonggon Yoo
2021-12-13T14:21:58Z

On Thu, Dec 09, 2021 at 01:59:58PM +0100, Christoph Lameter wrote:
> On Thu, 9 Dec 2021, Baoquan He wrote:
>
> > > The slab allocators guarantee that all kmalloc allocations are DMA able
> > > indepent of specifying ZONE_DMA/ZONE_DMA32
> >
> > Here you mean we guarantee dma-kmalloc will be DMA able independent of
> > specifying ZONE_DMA/DMA32, or the whole sla/ub allocator?
>
> All memory obtained via kmalloc --independent of "dma-alloc", ZONE_DMA
> etc-- must be dmaable.
>
> > With my understanding, isn't the reasonable sequence zone DMA firstly if
> > GFP_DMA, then zone DMA32, finaly zone NORMAL. At least, on x86_64, I
> > believe device driver developer prefer to see this because most of time,
> > zone DMA and zone DMA32 are both used for dma buffer allocation, if
> > IOMMU is not enabled. However, memory got from zone NORMAL when required
> > with GFP_DMA, and it succeeds, does it mean that the developer doesn't
> > take the GFP_DMA flag seriously, just try to get buffer for allocation?
>
> ZONE_NORMAL is also used for DMA allocations. ZONE_DMA and ZONE_DMA32 are
> only used if the physical range of memory supported by a device does not
> include all of normal memory.
>
> > > 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.
> >
> > As said at above, ia64 and riscv don't have ZONE_DMA at all, they just
> > cover low 4G with ZONE_DMA32 alone.
>
> If you do not have devices that are crap and cannot address the full
> memory then you dont need these special zones.
>
> Sorry this subject has caused confusion multiple times over the years and
> there are still arches that are not implementing this in a consistent way.

Hello Baoquan and Christoph.

I'm the confused one here too. :)

So the point is that ZONE_NORMAL is also dma-able if the device can access
normal memory. (which is false for ISA devices, ancient PCI devices,
...etc.)

Then if I understand right, I think the patch 5/5 (mm/slub: Avoid ...) should be removing
GFP_DMA flag from the function sr_probe() -> get_capabilities, rather than
copying copying normal kmalloc caches to dma kmalloc caches. (If the device does
not have limitation in its address space.)

Please let me know If I got it wrong :)

Thanks,
Hyeonggon.