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([2600:1700:e321:62f0:329c:23ff:fee3:9d7c]) by smtp.gmail.com with ESMTPSA id d9443c01a7336-211d0f35143sm78578575ad.148.2024.11.19.13.46.47 (version=TLS1_3 cipher=TLS_AES_128_GCM_SHA256 bits=128/128); Tue, 19 Nov 2024 13:46:48 -0800 (PST) Sender: Guenter Roeck Message-ID: <47a16a83-52c7-4779-9ed3-f16ea547b9f0@roeck-us.net> Date: Tue, 19 Nov 2024 13:46:46 -0800 Precedence: bulk X-Mailing-List: io-uring@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 User-Agent: Mozilla Thunderbird Subject: Re: [PATCH 03/14] io_uring: specify freeptr usage for SLAB_TYPESAFE_BY_RCU io_kiocb cache To: Jens Axboe , Geert Uytterhoeven Cc: io-uring@vger.kernel.org, linux-m68k References: <20241029152249.667290-1-axboe@kernel.dk> <20241029152249.667290-4-axboe@kernel.dk> <37c588d4-2c32-4aad-a19e-642961f200d7@roeck-us.net> <5a7528c4-4391-4bd9-bbdb-a0247f3c76a9@kernel.dk> <5851cd28-b369-4c09-876c-62c4a47c5982@kernel.dk> <358710e8-a826-46df-9846-5a9e0f7c6851@kernel.dk> <82b97543-ad01-4e42-b79c-12d97c1df194@kernel.dk> <4623f30c-a12e-4ba6-ad99-835764611c67@kernel.dk> Content-Language: en-US From: Guenter Roeck Autocrypt: addr=linux@roeck-us.net; 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charset=UTF-8; format=flowed Content-Transfer-Encoding: 7bit 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 wrote: >>>> On 11/19/24 12:25 PM, Geert Uytterhoeven wrote: >>>>> On Tue, Nov 19, 2024 at 8:10?PM Jens Axboe wrote: >>>>>> On 11/19/24 12:02 PM, Geert Uytterhoeven wrote: >>>>>>> On Tue, Nov 19, 2024 at 8:00?PM Jens Axboe wrote: >>>>>>>> On 11/19/24 10:49 AM, Geert Uytterhoeven wrote: >>>>>>>>> On Tue, Nov 19, 2024 at 5:21?PM Guenter Roeck 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 >>>>>>>>>>>> >>>>>>>>>>>> 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, > }; > >