Re: [PATCH] arm64/sve: Lower the maximum allocation for the SVE ptrace regset

[Doug Anderson]

Hi,

On Sat, Feb 3, 2024 at 4:18 AM Mark Brown <broonie@xxxxxxxxxx> wrote:
>
> 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@xxxxxxxxxxxx>
> Signed-off-by: Mark Brown <broonie@xxxxxxxxxx>

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@xxxxxxxxxxxx>