Re: [PATCH 03/14] io_uring: specify freeptr usage for SLAB_TYPESAFE_BY_RCU io_kiocb cache

[Geert Uytterhoeven <geert@linux-m68k.org>]

Hi Jens.

On Tue, Nov 19, 2024 at 8:00 PM Jens Axboe <axboe@kernel.dk> wrote:
> On 11/19/24 10:49 AM, Geert Uytterhoeven wrote:
> > On Tue, Nov 19, 2024 at 5:21?PM Guenter Roeck <linux@roeck-us.net> wrote:
> >> On 11/19/24 08:02, Jens Axboe wrote:
> >>> On 11/19/24 8:36 AM, Guenter Roeck wrote:
> >>>> On Tue, Oct 29, 2024 at 09:16:32AM -0600, Jens Axboe wrote:
> >>>>> Doesn't matter right now as there's still some bytes left for it, but
> >>>>> let's prepare for the io_kiocb potentially growing and add a specific
> >>>>> freeptr offset for it.
> >>>>>
> >>>>> Signed-off-by: Jens Axboe <axboe@kernel.dk>
> >>>>
> >>>> This patch triggers:
> >>>>
> >>>> Kernel panic - not syncing: __kmem_cache_create_args: Failed to create slab 'io_kiocb'. Error -22
> >>>> CPU: 0 UID: 0 PID: 1 Comm: swapper Not tainted 6.12.0-mac-00971-g158f238aa69d #1
> >>>> Stack from 00c63e5c:
> >>>>          00c63e5c 00612c1c 00612c1c 00000300 00000001 005f3ce6 004b9044 00612c1c
> >>>>          004ae21e 00000310 000000b6 005f3ce6 005f3ce6 ffffffea ffffffea 00797244
> >>>>          00c63f20 000c6974 005ee588 004c9051 005f3ce6 ffffffea 000000a5 00c614a0
> >>>>          004a72c2 0002cb62 000c675e 004adb58 0076f28a 005f3ce6 000000b6 00c63ef4
> >>>>          00000310 00c63ef4 00000000 00000016 0076f23e 00c63f4c 00000010 00000004
> >>>>          00000038 0000009a 01000000 00000000 00000000 00000000 000020e0 0076f23e
> >>>> Call Trace: [<004b9044>] dump_stack+0xc/0x10
> >>>>   [<004ae21e>] panic+0xc4/0x252
> >>>>   [<000c6974>] __kmem_cache_create_args+0x216/0x26c
> >>>>   [<004a72c2>] strcpy+0x0/0x1c
> >>>>   [<0002cb62>] parse_args+0x0/0x1f2
> >>>>   [<000c675e>] __kmem_cache_create_args+0x0/0x26c
> >>>>   [<004adb58>] memset+0x0/0x8c
> >>>>   [<0076f28a>] io_uring_init+0x4c/0xca
> >>>>   [<0076f23e>] io_uring_init+0x0/0xca
> >>>>   [<000020e0>] do_one_initcall+0x32/0x192
> >>>>   [<0076f23e>] io_uring_init+0x0/0xca
> >>>>   [<0000211c>] do_one_initcall+0x6e/0x192
> >>>>   [<004a72c2>] strcpy+0x0/0x1c
> >>>>   [<0002cb62>] parse_args+0x0/0x1f2
> >>>>   [<000020ae>] do_one_initcall+0x0/0x192
> >>>>   [<0075c4e2>] kernel_init_freeable+0x1a0/0x1a4
> >>>>   [<0076f23e>] io_uring_init+0x0/0xca
> >>>>   [<004b911a>] kernel_init+0x0/0xec
> >>>>   [<004b912e>] kernel_init+0x14/0xec
> >>>>   [<004b911a>] kernel_init+0x0/0xec
> >>>>   [<0000252c>] ret_from_kernel_thread+0xc/0x14
> >>>>
> >>>> when trying to boot the m68k:q800 machine in qemu.
> >>>>
> >>>> An added debug message in create_cache() shows the reason:
> >>>>
> >>>> #### freeptr_offset=154 object_size=182 flags=0x310 aligned=0 sizeof(freeptr_t)=4
> >>>>
> >>>> freeptr_offset would need to be 4-byte aligned but that is not the
> >>>> case on m68k.
> >>>
> >>> Why is ->work 2-byte aligned to begin with on m68k?!
> >>
> >> My understanding is that m68k does not align pointers.
> >
> > The minimum alignment for multi-byte integral values on m68k is
> > 2 bytes.
> >
> > See also the comment at
> > https://elixir.bootlin.com/linux/v6.12/source/include/linux/maple_tree.h#L46
>
> Maybe it's time we put m68k to bed? :-)
>
> We can add a forced alignment ->work to be 4 bytes, won't change
> anything on anything remotely current. But does feel pretty hacky to
> need to align based on some ancient thing.

Why does freeptr_offset need to be 4-byte aligned?

Gr{oetje,eeting}s,

                        Geert

-- 
Geert Uytterhoeven -- There's lots of Linux beyond ia32 -- geert@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