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 5:08 PM, Guenter Roeck wrote:
> On 11/19/24 14:30, Jens Axboe 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 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.
>>
>> 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.
>>
> 
> Yes, that works. Feel free to add
> 
> Tested-by: Guenter Roeck <linux@roeck-us.net>
> 
> to an official patch.

Thanks for testing, will add that and send it out (and queue it up for
later this merge window).

-- 
Jens Axboe