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

[Jens Axboe <axboe@kernel.dk>]

On 11/19/24 12:02 PM, Geert Uytterhoeven wrote:
> 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?

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;

-- 
Jens Axboe