Re: [PATCH v4 7/7] ARM: implement support for vmap'ed stacks

[Ard Biesheuvel <ardb@kernel.org>]

On Tue, 21 Dec 2021 at 22:56, Marek Szyprowski <m.szyprowski@samsung.com> wrote:
>
> Hi,
>
> On 21.12.2021 17:20, Ard Biesheuvel wrote:
> > On Tue, 21 Dec 2021 at 14:51, Marek Szyprowski <m.szyprowski@samsung.com> wrote:
> >> On 21.12.2021 14:34, Ard Biesheuvel wrote:
> >>> On Tue, 21 Dec 2021 at 12:15, Marek Szyprowski <m.szyprowski@samsung.com> wrote:
> >>>> On 21.12.2021 11:44, Ard Biesheuvel wrote:
> >>>>> On Tue, 21 Dec 2021 at 11:39, Marek Szyprowski <m.szyprowski@samsung.com> wrote:
> >>>>>> On 22.11.2021 10:28, Ard Biesheuvel wrote:
> >>>>>>> Wire up the generic support for managing task stack allocations via vmalloc,
> >>>>>>> and implement the entry code that detects whether we faulted because of a
> >>>>>>> stack overrun (or future stack overrun caused by pushing the pt_regs array)
> >>>>>>>
> >>>>>>> While this adds a fair amount of tricky entry asm code, it should be
> >>>>>>> noted that it only adds a TST + branch to the svc_entry path. The code
> >>>>>>> implementing the non-trivial handling of the overflow stack is emitted
> >>>>>>> out-of-line into the .text section.
> >>>>>>>
> >>>>>>> Since on ARM, we rely on do_translation_fault() to keep PMD level page
> >>>>>>> table entries that cover the vmalloc region up to date, we need to
> >>>>>>> ensure that we don't hit such a stale PMD entry when accessing the
> >>>>>>> stack. So we do a dummy read from the new stack while still running from
> >>>>>>> the old one on the context switch path, and bump the vmalloc_seq counter
> >>>>>>> when PMD level entries in the vmalloc range are modified, so that the MM
> >>>>>>> switch fetches the latest version of the entries.
> >>>>>>>
> >>>>>>> Note that we need to increase the per-mode stack by 1 word, to gain some
> >>>>>>> space to stash a GPR until we know it is safe to touch the stack.
> >>>>>>> However, due to the cacheline alignment of the struct, this does not
> >>>>>>> actually increase the memory footprint of the struct stack array at all.
> >>>>>>>
> >>>>>>> Signed-off-by: Ard Biesheuvel <ardb@kernel.org>
> >>>>>>> Tested-by: Keith Packard <keithpac@amazon.com>
> >>>>>> This patch landed recently in linux-next 20211220 as commit a1c510d0adc6
> >>>>>> ("ARM: implement support for vmap'ed stacks"). Sadly it breaks
> >>>>>> suspend/resume operation on all ARM 32bit Exynos SoCs. Probably the
> >>>>>> suspend/resume related code must be updated somehow (it partially works
> >>>>>> on physical addresses and disabled MMU), but I didn't analyze it yet. If
> >>>>>> you have any hints, let me know.
> >>>>>>
> >>>>> Are there any such systems in KernelCI? We caught a suspend/resume
> >>>>> related issue in development, which is why the hunk below was added.
> >>>> I think that some Exynos-based Odroids (U3 and XU3) were some time ago
> >>>> available in KernelCI, but I don't know if they are still there.
> >>>>
> >>>>
> >>>>> In general, any virt-to-phys translation involving and address on the
> >>>>> stack will become problematic.
> >>>>>
> >>>>> Could you please confirm whether the issue persists with the patch
> >>>>> applied but with CONFIG_VMAP_STACK turned off? Just so we know we are
> >>>>> looking in the right place?
> >>>> I've just checked. After disabling CONFIG_VMAP_STACK suspend/resume
> >>>> works fine both on commit a1c510d0adc6 and linux-next 20211220.
> >>>>
> >>> Thanks. Any other context you can provide beyond 'does not work' ?
> >> Well, the board properly suspends, but it doesn't wake then (tested
> >> remotely with rtcwake command). So far I cannot provide anything more.
> >>
> > Thanks. Does the below help? Or otherwise, could you try doubling the
> > size of the overflow stack at arch/arm/include/asm/thread_info.h:34?
>
> I've tried both (but not at the same time) on the current linux-next and
> none helped. This must be something else... :/
>

Thanks.

As i don't have access to this hardware, I am going to have to rely on
someone who does to debug this further. The only alternative is
marking CONFIG_VMAP_STACK broken on MACH_EXYNOS but that would be
unfortunate.

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