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 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.

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));
+	};
 
 	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,
 	};
 
-- 
Jens Axboe