Hi Vlastimil, On Wed, Nov 20, 2024 at 9:47 AM Vlastimil Babka <vbabka@xxxxxxx> wrote: > On 11/20/24 09:19, Geert Uytterhoeven wrote: > > On Tue, Nov 19, 2024 at 11:30 PM Jens Axboe <axboe@xxxxxxxxx> 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 Tue, Nov 19, 2024 at 8:30?PM Jens Axboe <axboe@xxxxxxxxx> wrote: > >> >>>>> On 11/19/24 12:25 PM, Geert Uytterhoeven wrote: > >> >>>>>> On Tue, Nov 19, 2024 at 8:10?PM Jens Axboe <axboe@xxxxxxxxx> wrote: > >> >>>>>>> On 11/19/24 12:02 PM, Geert Uytterhoeven wrote: > >> >>>>>>>> On Tue, Nov 19, 2024 at 8:00?PM Jens Axboe <axboe@xxxxxxxxx> wrote: > >> >>>>>>>>> On 11/19/24 10:49 AM, Geert Uytterhoeven wrote: > >> >>>>>>>>>> On Tue, Nov 19, 2024 at 5:21?PM Guenter Roeck <linux@xxxxxxxxxxxx> 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@xxxxxxxxx> > >> >>>>>>>>>>>>> > >> >>>>>>>>>>>>> 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)); > > > > I'd rather add the __aligned() to the definition of freeptr_t, so it > > applies to all (future) users. > > > > But my main question stays: why is the slab code checking > > IS_ALIGNED(args->freeptr_offset, sizeof(freeptr_t)? > > I believe it's to match how SLUB normally calculates the offset if no > explicit one is given, in calculate_sizes(): > > s->offset = ALIGN_DOWN(s->object_size / 2, sizeof(void *)); > > Yes there's a sizeof(void *) because freepointer used to be just that and we > forgot to update this place when freepointer_t was introduced (by Jann in > 44f6a42d49350) for handling CONFIG_SLAB_FREELIST_HARDENED. In > get_freepointer() you can see how there's a cast to a pointer eventually. > > Does m68k have different alignment for pointer and unsigned long or both are > 2 bytes? Or any other arch, i.e. should get_freepointer be a union with > unsigned long and void * instead? (or it doesn't matter?) The default alignment for int, long, and pointer is 2 on m68k. On CRIS (no longer supported by Linux), it was 1, IIRC. So the union won't make a difference. > > Perhaps that was just intended to be __alignof__ instead of sizeof()? > > Would it do the right thing everywhere, given the explanation above? It depends. Does anything rely on the offset being a multiple of (at least) 4? E.g. does anything counts in multiples of longs (hi BCPL! ;-), or are the 2 LSB used for a special purpose? (cfr. maple_tree, which uses bit 0 (https://elixir.bootlin.com/linux/v6.12/source/include/linux/maple_tree.h#L46)? Gr{oetje,eeting}s, Geert -- Geert Uytterhoeven -- There's lots of Linux beyond ia32 -- geert@xxxxxxxxxxxxxx In personal conversations with technical people, I call myself a hacker. But when I'm talking to journalists I just say "programmer" or something like that. -- Linus Torvalds
