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 8:30 PM Jens Axboe <axboe@kernel.dk> wrote:
> On 11/19/24 12:25 PM, Geert Uytterhoeven wrote:
> > On Tue, Nov 19, 2024 at 8:10?PM Jens Axboe <axboe@kernel.dk> wrote:
> >> On 11/19/24 12:02 PM, Geert Uytterhoeven wrote:
> >>> 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?
> >>
> >> Didn't check, but it's slab/slub complaining using a 2-byte aligned
> >> address for the free pointer offset. It's explicitly checking:
> >>
> >>         /* If a custom freelist pointer is requested make sure it's sane. */
> >>         err = -EINVAL;
> >>         if (args->use_freeptr_offset &&
> >>             (args->freeptr_offset >= object_size ||
> >>              !(flags & SLAB_TYPESAFE_BY_RCU) ||
> >>              !IS_ALIGNED(args->freeptr_offset, sizeof(freeptr_t))))
> >>                 goto out;
> >
> > It is not guaranteed that alignof(freeptr_t) >= sizeof(freeptr_t)
> > (free_ptr is sort of a long). If freeptr_offset must be a multiple of
> > 4 or 8 bytes,
> > the code that assigns it must make sure that is true.
>
> Right, this is what the email is about...
>
> > I guess this is the code in fs/file_table.c:
> >
> >     .freeptr_offset = offsetof(struct file, f_freeptr),
> >
> > which references:
> >
> >     include/linux/fs.h:           freeptr_t               f_freeptr;
> >
> > I guess the simplest solution is to add an __aligned(sizeof(freeptr_t))
> > (or __aligned(sizeof(long)) to the definition of freeptr_t:
> >
> >     include/linux/slab.h:typedef struct { unsigned long v; } freeptr_t;
>
> It's not, it's struct io_kiocb->work, as per the stack trace in this
> email.

Sorry, I was falling out of thin air into this thread...

linux-next/master:io_uring/io_uring.c:          .freeptr_offset =
offsetof(struct io_kiocb, work),
linux-next/master:io_uring/io_uring.c:          .use_freeptr_offset = true,

Apparently io_kiocb.work is of type struct io_wq_work, not freeptr_t?
Isn't that a bit error-prone, as the slab core code expects a freeptr_t?

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