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

[Geert Uytterhoeven]

Hi Ard,

On Thu, Dec 23, 2021 at 3:30 PM Ard Biesheuvel <ardb at kernel.org> wrote:
> On Tue, 21 Dec 2021 at 22:56, Marek Szyprowski <m.szyprowski at samsung.com> wrote:
> > On 21.12.2021 17:20, Ard Biesheuvel wrote:
> > > On Tue, 21 Dec 2021 at 14:51, Marek Szyprowski <m.szyprowski at samsung.com> wrote:
> > >> On 21.12.2021 14:34, Ard Biesheuvel wrote:
> > >>> On Tue, 21 Dec 2021 at 12:15, Marek Szyprowski <m.szyprowski at samsung.com> wrote:
> > >>>> On 21.12.2021 11:44, Ard Biesheuvel wrote:
> > >>>>> On Tue, 21 Dec 2021 at 11:39, Marek Szyprowski <m.szyprowski at 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 at kernel.org>
> > >>>>>>> Tested-by: Keith Packard <keithpac at 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.

Wish I had seen this thread before...

I've just bisected a resume after s2ram failure on R-Car Gen2 to the same
commit a1c510d0adc604bb ("ARM: implement support for vmap'ed stacks")
in arm/for-next.

Expected output:

    PM: suspend entry (deep)
    Filesystems sync: 0.000 seconds
    Freezing user space processes ... (elapsed 0.010 seconds) done.
    OOM killer disabled.
    Freezing remaining freezable tasks ... (elapsed 0.009 seconds) done.
    Disabling non-boot CPUs ...

[system suspended, this is also where it hangs on failure]

    Enabling non-boot CPUs ...
    CPU1 is up
    sh-eth ee700000.ethernet eth0: Link is Down
    Micrel KSZ8041RNLI ee700000.ethernet-ffffffff:01: attached PHY
driver (mii_bus:phy_addr=ee700000.ethernet-ffffffff:01, irq=193)
    OOM killer enabled.
    Restarting tasks ... done.
    PM: suspend exit

Both wake-on-LAN and wake-up by gpio-keys fail.
Nothing interesting in the kernel log, cfr. above.

Disabling CONFIG_VMAP_STACK fixes the issue for me.

Just like arch/arm/mach-exynos/ (and others), arch/arm/mach-shmobile/
has several *.S files related to secondary CPU bringup.

Gr{oetje,eeting}s,

                        Geert

--
Geert Uytterhoeven -- There's lots of Linux beyond ia32 -- geert at linux-m68k.org

In personal conversations with technical people, I call myself a hacker. But
when I'm talking to journalists I just say "programmer" or something like that.
                                -- Linus Torvalds