Re: (BTF) [PATCH] mm/page_alloc: Work around a pahole limitation with zero-sized struct pagesets

[Andrii Nakryiko <andrii.nakryiko@gmail.com>]

On Wed, May 26, 2021 at 1:33 AM Michal Suchánek <msuchanek@suse.de> wrote:
>
> On Wed, May 26, 2021 at 09:07:41AM +0100, Mel Gorman wrote:
> > Michal Suchanek reported the following problem with linux-next
> >
> >   [    0.000000] Linux version 5.13.0-rc2-next-20210519-1.g3455ff8-vanilla (geeko@buildhost) (gcc (SUSE Linux) 10.3.0, GNU ld (GNU Binutils; openSUSE Tumbleweed) 2.36.1.20210326-3) #1 SMP Wed May 19 10:05:10 UTC 2021 (3455ff8)
> >   [    0.000000] Command line: BOOT_IMAGE=/boot/vmlinuz-5.13.0-rc2-next-20210519-1.g3455ff8-vanilla root=UUID=ec42c33e-a2c2-4c61-afcc-93e9527 8f687 plymouth.enable=0 resume=/dev/disk/by-uuid/f1fe4560-a801-4faf-a638-834c407027c7 mitigations=auto earlyprintk initcall_debug nomodeset earlycon ignore_loglevel console=ttyS0,115200
> > ...
> >   [   26.093364] calling  tracing_set_default_clock+0x0/0x62 @ 1
> >   [   26.098937] initcall tracing_set_default_clock+0x0/0x62 returned 0 after 0 usecs
> >   [   26.106330] calling  acpi_gpio_handle_deferred_request_irqs+0x0/0x7c @ 1
> >   [   26.113033] initcall acpi_gpio_handle_deferred_request_irqs+0x0/0x7c returned 0 after 3 usecs
> >   [   26.121559] calling  clk_disable_unused+0x0/0x102 @ 1
> >   [   26.126620] initcall clk_disable_unused+0x0/0x102 returned 0 after 0 usecs
> >   [   26.133491] calling  regulator_init_complete+0x0/0x25 @ 1
> >   [   26.138890] initcall regulator_init_complete+0x0/0x25 returned 0 after 0 usecs
> >   [   26.147816] Freeing unused decrypted memory: 2036K
> >   [   26.153682] Freeing unused kernel image (initmem) memory: 2308K
> >   [   26.165776] Write protecting the kernel read-only data: 26624k
> >   [   26.173067] Freeing unused kernel image (text/rodata gap) memory: 2036K
> >   [   26.180416] Freeing unused kernel image (rodata/data gap) memory: 1184K
> >   [   26.187031] Run /init as init process
> >   [   26.190693]   with arguments:
> >   [   26.193661]     /init
> >   [   26.195933]   with environment:
> >   [   26.199079]     HOME=/
> >   [   26.201444]     TERM=linux
> >   [   26.204152]     BOOT_IMAGE=/boot/vmlinuz-5.13.0-rc2-next-20210519-1.g3455ff8-vanilla
> >   [   26.254154] BPF:      type_id=35503 offset=178440 size=4
> >   [   26.259125] BPF:
> >   [   26.261054] BPF:Invalid offset
> >   [   26.264119] BPF:
> >   [   26.264119]
> >   [   26.267437] failed to validate module [efivarfs] BTF: -22
> >
> > Andrii Nakryiko bisected the problem to the commit "mm/page_alloc: convert
> > per-cpu list protection to local_lock" currently staged in mmotm. In his
> > own words
> >
> >   The immediate problem is two different definitions of numa_node per-cpu
> >   variable. They both are at the same offset within .data..percpu ELF
> >   section, they both have the same name, but one of them is marked as
> >   static and another as global. And one is int variable, while another
> >   is struct pagesets. I'll look some more tomorrow, but adding Jiri and
> >   Arnaldo for visibility.
> >
> >   [110907] DATASEC '.data..percpu' size=178904 vlen=303
> >   ...
> >         type_id=27753 offset=163976 size=4 (VAR 'numa_node')
> >         type_id=27754 offset=163976 size=4 (VAR 'numa_node')
> >
> >   [27753] VAR 'numa_node' type_id=27556, linkage=static
> >   [27754] VAR 'numa_node' type_id=20, linkage=global
> >
> >   [20] INT 'int' size=4 bits_offset=0 nr_bits=32 encoding=SIGNED
> >
> >   [27556] STRUCT 'pagesets' size=0 vlen=1
> >         'lock' type_id=507 bits_offset=0
> >
> >   [506] STRUCT '(anon)' size=0 vlen=0
> >   [507] TYPEDEF 'local_lock_t' type_id=506
> >
> > The patch in question introduces a zero-sized per-cpu struct and while
> > this is not wrong, versions of pahole prior to 1.22 (unreleased) get
> > confused during BTF generation with two separate variables occupying the
> > same address.
> >
> > This patch checks for older versions of pahole and forces struct pagesets
> > to be non-zero sized as a workaround when CONFIG_DEBUG_INFO_BTF is set. A
> > warning is omitted so that distributions can update pahole when 1.22
> > is released.
> >
> > Reported-by: Michal Suchanek <msuchanek@suse.de>
> > Reported-by: Hritik Vijay <hritikxx8@gmail.com>
> > Debugged-by: Andrii Nakryiko <andrii.nakryiko@gmail.com>
> > Signed-off-by: Mel Gorman <mgorman@techsingularity.net>
> > ---
> >  lib/Kconfig.debug |  3 +++
> >  mm/page_alloc.c   | 11 +++++++++++
> >  2 files changed, 14 insertions(+)
> >
> > diff --git a/lib/Kconfig.debug b/lib/Kconfig.debug
> > index 678c13967580..f88a155b80a9 100644
> > --- a/lib/Kconfig.debug
> > +++ b/lib/Kconfig.debug
> > @@ -313,6 +313,9 @@ config DEBUG_INFO_BTF
> >  config PAHOLE_HAS_SPLIT_BTF
> >       def_bool $(success, test `$(PAHOLE) --version | sed -E 's/v([0-9]+)\.([0-9]+)/\1\2/'` -ge "119")
> >
> > +config PAHOLE_HAS_ZEROSIZE_PERCPU_SUPPORT
> > +     def_bool $(success, test `$(PAHOLE) --version | sed -E 's/v([0-9]+)\.([0-9]+)/\1\2/'` -ge "122")
> > +
>
> This does not seem workable with dummy-tools.
>
> Do we even have dummy pahole?
>

I don't know what dummy-tools is, so probably no. But if you don't
have pahole on the build host, you can't have DEBUG_INFO_BTF=y
anyways. As in, your build will fail because it will be impossible to
generate BTF information. So you'll have to disable DEBUG_INFO_BTF if
you can't get pahole onto your build host for some reason.

> Thanks
>
> Michal
>
> >  config DEBUG_INFO_BTF_MODULES
> >       def_bool y
> >       depends on DEBUG_INFO_BTF && MODULES && PAHOLE_HAS_SPLIT_BTF
> > diff --git a/mm/page_alloc.c b/mm/page_alloc.c
> > index ff8f706839ea..1d56d3de8e08 100644
> > --- a/mm/page_alloc.c
> > +++ b/mm/page_alloc.c
> > @@ -124,6 +124,17 @@ static DEFINE_MUTEX(pcp_batch_high_lock);
> >
> >  struct pagesets {
> >       local_lock_t lock;
> > +#if defined(CONFIG_DEBUG_INFO_BTF) &&                        \
> > +    !defined(CONFIG_DEBUG_LOCK_ALLOC) &&             \
> > +    !defined(CONFIG_PAHOLE_HAS_ZEROSIZE_PERCPU_SUPPORT)
> > +     /*
> > +      * pahole 1.21 and earlier gets confused by zero-sized per-CPU
> > +      * variables and produces invalid BTF. Ensure that
> > +      * sizeof(struct pagesets) != 0 for older versions of pahole.
> > +      */
> > +     char __pahole_hack;
> > +     #warning "pahole too old to support zero-sized struct pagesets"
> > +#endif
> >  };
> >  static DEFINE_PER_CPU(struct pagesets, pagesets) = {
> >       .lock = INIT_LOCAL_LOCK(lock),