Re: [PATCH v3] mm/page_alloc: Require pahole v1.22 to cope with zero-sized struct pagesets

From: Andrii Nakryiko
Date: Thu May 27 2021 - 18:18:04 EST

On Thu, May 27, 2021 at 10:19 AM Mel Gorman <mgorman@xxxxxxxxxxxxxxxxxxx> wrote:
>
> This patch replaces
> mm-page_alloc-convert-per-cpu-list-protection-to-local_lock-fix.patch in
> Andrew's tree.
>
> 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 get confused during
> BTF generation with two separate variables occupying the same address.
>
> This patch adds a requirement for pahole 1.22 before setting
> DEBUG_INFO_BTF. While pahole 1.22 does not exist yet, a fix is in the
> pahole git tree as ("btf_encoder: fix and complete filtering out zero-sized
> per-CPU variables").
>
> Reported-by: Michal Suchanek <msuchanek@xxxxxxx>
> Reported-by: Hritik Vijay <hritikxx8@xxxxxxxxx>
> Debugged-by: Andrii Nakryiko <andrii.nakryiko@xxxxxxxxx>
> Signed-off-by: Mel Gorman <mgorman@xxxxxxxxxxxxxxxxxxx>
> ---

I still think that v1 ([0]) is a more appropriate temporary solution
until pahole 1.22 is released and widely packaged. Suddenly raising
the minimum version to 1.22, which is not even released even, is a
pretty big compatibility concern for all the users that rely on
CONFIG_DEBUG_INFO_BTF. Just a few days ago pahole 1.16 worked fine and
here we suddenly (and silently due to how Kconfig functions) raise
that to a version that doesn't exist. That's going to break workflows
for a lot of people.

I'm asking to have that ugly work-around to ensure sizeof(struct
pagesets) > 0 as a temporary solution only. If we have to raise the
minimum pahole version to 1.22 we should also use that as an
opportunity to clean up and simplify pahole integration in Kbuild.
Kernel-side work-around will give us enough time to test and validate
everything instead of rushing bug fix release. Once 1.22 is widely
available we'll get rid of work-around and make
scripts/link-vmlinux.sh simpler (see discussion in [1]).

[0] https://lore.kernel.org/bpf/20210526080741.GW30378@xxxxxxxxxxxxxxxxxxx/
[1] https://lore.kernel.org/bpf/CAEf4BzaTP_jULKMN_hx6ZOqwESOmsR6_HxWW-LnrA5xwRNtSWg@xxxxxxxxxxxxxx/

> lib/Kconfig.debug | 4 ++++
> 1 file changed, 4 insertions(+)
>
> diff --git a/lib/Kconfig.debug b/lib/Kconfig.debug
> index 678c13967580..825be101767e 100644
> --- a/lib/Kconfig.debug
> +++ b/lib/Kconfig.debug
> @@ -301,10 +301,14 @@ config DEBUG_INFO_DWARF5
>
> endchoice # "DWARF version"
>
> +config PAHOLE_HAS_ZEROSIZE_PERCPU_SUPPORT
> + def_bool $(success, test `$(PAHOLE) --version | sed -E 's/v([0-9]+)\.([0-9]+)/\1\2/'` -ge "122")
> +
> config DEBUG_INFO_BTF
> bool "Generate BTF typeinfo"
> depends on !DEBUG_INFO_SPLIT && !DEBUG_INFO_REDUCED
> depends on !GCC_PLUGIN_RANDSTRUCT || COMPILE_TEST
> + depends on PAHOLE_HAS_ZEROSIZE_PERCPU_SUPPORT
> help
> Generate deduplicated BTF type information from DWARF debug info.
> Turning this on expects presence of pahole tool, which will convert