Hi Doug,

On Mon, Feb 05, 2024 at 09:02:08AM -0800, Doug Anderson wrote:

Hi,

On Sat, Feb 3, 2024 at 4:18 AM Mark Brown [off-list ref] wrote:

quoted

Doug Anderson observed that ChromeOS crashes are being reported which
include failing allocations of order 7 during core dumps due to ptrace
allocating storage for regsets:

  chrome: page allocation failure: order:7,
          mode:0x40dc0(GFP_KERNEL|__GFP_COMP|__GFP_ZERO),
          nodemask=(null),cpuset=urgent,mems_allowed=0
   ...
  regset_get_alloc+0x1c/0x28
  elf_core_dump+0x3d8/0xd8c
  do_coredump+0xeb8/0x1378

with further investigation showing that this is:

   [   66.957385] DOUG: Allocating 279584 bytes

which is the maximum size of the SVE regset. As Doug observes it is not
entirely surprising that such a large allocation of contiguous memory might
fail on a long running system.

The SVE regset is currently sized to hold SVE registers with a VQ of
SVE_VQ_MAX which is 512, substantially more than the architectural maximum
of 16 which we might see even in a system emulating the limits of the
architecture. Since we don't expose the size we tell the regset core
externally let's define ARCH_SVE_VQ_MAX with the actual architectural
maximum and use that for the regset, we'll still overallocate most of the
time but much less so which will be helpful even if the core is fixed to
not require contiguous allocations.

We could also teach the ptrace core about runtime discoverable regset sizes
but that would be a more invasive change and this is being observed in
practical systems.

Reported-by: Doug Anderson <dianders@chromium.org>
Signed-off-by: Mark Brown <broonie@kernel.org>

Confirmed that when I send a "quit" signal to Chrome now that the
allocation I see for "core_note_type" NT_ARM_SVE goes down from
279,584 bytes (n=17474) to just 8,768 bytes (n=548). I'm not
intimately familiar with this code so I'll skip the Reviewed-by unless
someone thinks it would be valuable for me to analyze more. I think
there are already plenty of people who know this well, so for now,
just:

Tested-by: Douglas Anderson <dianders@chromium.org>

I can pick this up as a short-term hack if it solves the problem for you,
but I also saw that you posted:

https://lore.kernel.org/r/20240205092626.v2.1.Id9ad163b60d21c9e56c2d686b0cc9083a8ba7924@changeid (local)

to fallback onto vmalloc() for large allocations.

What's your preference for a fix?

Cheers,

Will

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