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

[Guenter Roeck <linux@roeck-us.net>]

On 11/19/24 11:49, Jens Axboe wrote:
> On 11/19/24 12:44 PM, Jens Axboe wrote:
>> On 11/19/24 12:41 PM, Geert Uytterhoeven wrote:
>>> 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?
>>
>> 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.

Guenter

> 
> diff --git a/include/linux/io_uring_types.h b/include/linux/io_uring_types.h
> index 593c10a02144..a83ec7f7849d 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;
> +	};
>   
>   	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,
>   	};
>   
>