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

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

Hi Vlastimil,

On Wed, Nov 20, 2024 at 9:47 AM Vlastimil Babka <vbabka@suse.cz> wrote:
> On 11/20/24 09:19, Geert Uytterhoeven wrote:
> > 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)?
>
> I believe it's to match how SLUB normally calculates the offset if no
> explicit one is given, in calculate_sizes():
>
> s->offset = ALIGN_DOWN(s->object_size / 2, sizeof(void *));
>
> Yes there's a sizeof(void *) because freepointer used to be just that and we
> forgot to update this place when freepointer_t was introduced (by Jann in
> 44f6a42d49350) for handling CONFIG_SLAB_FREELIST_HARDENED. In
> get_freepointer() you can see how there's a cast to a pointer eventually.
>
> Does m68k have different alignment for pointer and unsigned long or both are
> 2 bytes? Or any other arch, i.e. should get_freepointer be a union with
> unsigned long and void * instead? (or it doesn't matter?)

The default alignment for int, long, and pointer is 2 on m68k.
On CRIS (no longer supported by Linux), it was 1, IIRC.
So the union won't make a difference.

> > Perhaps that was just intended to be __alignof__ instead of sizeof()?
>
> Would it do the right thing everywhere, given the explanation above?

It depends. Does anything rely on the offset being a multiple of (at
least) 4?
E.g. does anything counts in multiples of longs (hi BCPL! ;-), or are
the 2 LSB used for a special purpose? (cfr. maple_tree, which uses
bit 0 (https://elixir.bootlin.com/linux/v6.12/source/include/linux/maple_tree.h#L46)?

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