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,

CC Christian (who added the check)
CC Vlastimil (who suggested the check)

On Tue, Nov 19, 2024 at 11:30 PM Jens Axboe <axboe@kernel.dk> wrote:
> On 11/19/24 2:46 PM, Guenter Roeck wrote:
> > On 11/19/24 11:49, Jens Axboe wrote:
> >> On 11/19/24 12:44 PM, Jens Axboe wrote:
> >>>> 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?
> >>>
> >>> It just needs the space, should not matter otherwise. But may as well
> >>> just add the union and align the freeptr so it stop complaining on m68k.
> >>
> >> Ala the below, perhaps alignment takes care of itself then?
> >
> > No, that doesn't work (I tried), at least not on its own, because the pointer
> > is still unaligned on m68k.
>
> Yeah we'll likely need to force it. The below should work, I pressume?
> Feels pretty odd to have to align it to the size of it, when that should
> naturally occur... Crusty legacy archs.
>
> diff --git a/include/linux/io_uring_types.h b/include/linux/io_uring_types.h
> index 593c10a02144..8ed9c6923668 100644
> --- a/include/linux/io_uring_types.h
> +++ b/include/linux/io_uring_types.h
> @@ -674,7 +674,11 @@ struct io_kiocb {
>         struct io_kiocb                 *link;
>         /* custom credentials, valid IFF REQ_F_CREDS is set */
>         const struct cred               *creds;
> -       struct io_wq_work               work;
> +
> +       union {
> +               struct io_wq_work       work;
> +               freeptr_t               freeptr __aligned(sizeof(freeptr_t));

I'd rather add the __aligned() to the definition of freeptr_t, so it
applies to all (future) users.

But my main question stays: why is the slab code checking
IS_ALIGNED(args->freeptr_offset, sizeof(freeptr_t)?
Perhaps that was just intended to be __alignof__ instead of sizeof()?

> +       };
>
>         struct {
>                 u64                     extra1;
> diff --git a/io_uring/io_uring.c b/io_uring/io_uring.c
> index 73af59863300..86ac7df2a601 100644
> --- a/io_uring/io_uring.c
> +++ b/io_uring/io_uring.c
> @@ -3812,7 +3812,7 @@ static int __init io_uring_init(void)
>         struct kmem_cache_args kmem_args = {
>                 .useroffset = offsetof(struct io_kiocb, cmd.data),
>                 .usersize = sizeof_field(struct io_kiocb, cmd.data),
> -               .freeptr_offset = offsetof(struct io_kiocb, work),
> +               .freeptr_offset = offsetof(struct io_kiocb, freeptr),
>                 .use_freeptr_offset = true,
>         };

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