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 12:25 PM, Geert Uytterhoeven wrote:
> Hi Jens,
> 
> 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.

-- 
Jens Axboe