On Sat, 2020-10-10 at 12:36 +0200, Ard Biesheuvel wrote:

On Fri, 9 Oct 2020 at 19:10, Catalin Marinas [off-list ref] wrote:

quoted

On Fri, Oct 09, 2020 at 06:23:06PM +0200, Ard Biesheuvel wrote:

quoted

On Fri, 9 Oct 2020 at 17:24, Lorenzo Pieralisi
[off-list ref] wrote:

quoted

We can move this check to IORT code and call it from arm64 if it
can be made to work.

Finding the smallest value in the IORT, and assigning it to
zone_dma_bits if it is < 32 should be easy. But as I understand it,
having these separate DMA and DMA32 zones is what breaks kdump, no? So
how is this going to fix the underlying issue?

If zone_dma_bits is 32, ZONE_DMA32 disappears into ZONE_DMA (GFP_DMA32
allocations fall back to ZONE_DMA).

kdump wants DMA-able memory and, without a 30-bit ZONE_DMA, that would
be the bottom 32-bit. With the introduction of ZONE_DMA, this suddenly
became 1GB. We could change kdump to allocate ZONE_DMA32 but this one
may also be small as it lost 1GB to ZONE_DMA. However, the kdump kernel
would need to be rebuilt without ZONE_DMA since it won't have any. IIRC
(it's been a while since I looked), the kdump allocation couldn't span
multiple zones.

In a separate thread, we try to fix kdump to use allocations above 4G as
a fallback but this only fixes platforms with enough RAM (and maybe it's
only those platforms that care about kdump).

One thing that strikes me as odd is that we are applying the same
shifting logic to ZONE_DMA as we are applying to ZONE_DMA32, i.e., if
DRAM starts outside of the zone, it is shifted upwards.

On a typical ARM box, this gives me

[    0.000000] Zone ranges:
[    0.000000]   DMA      [mem 0x0000000080000000-0x00000000bfffffff]
[    0.000000]   DMA32    [mem 0x00000000c0000000-0x00000000ffffffff]
[    0.000000]   Normal   [mem 0x0000000100000000-0x0000000fffffffff]

i.e., the 30-bit addressable range has bit 31 set, which is weird.

Yes I agree it's weird, and IMO plain useless. I sent a series this summer to
address this[1], which ultimately triggered the discussion we're having right
now.

Although with with your latest patch and the DT counterpart, we should be OK.
It would be weird for a HW description to define DMA constraints that are
impossible to reach on that system.

I wonder if it wouldn't be better (and less problematic in the general
case) to drop this logic for ZONE_DMA, and simply let it remain empty
unless there is really some memory there.

From my experience, you can't have empty ZONE_DMA when enabled. Empty
ZONE_DMA32 is OK though. Although I'm sure it's something that can be changed.

Regards,
Nicolas

[1] https://lkml.org/lkml/2020/8/19/1022