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

[Will Deacon]

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 <broonie at kernel.org> 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 at chromium.org>
> > Signed-off-by: Mark Brown <broonie at 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 at 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

to fallback onto vmalloc() for large allocations.

What's your preference for a fix?

Cheers,

Will