Re: [PATCH v4 10/17] mm/slab: kmalloc: pass requests larger than order-1 page to page allocator

Vlastimil Babka
2022-10-15T19:39:16Z

On 10/15/22 01:48, Hyeonggon Yoo wrote:
> On Fri, Oct 14, 2022 at 01:58:18PM -0700, Guenter Roeck wrote:
>> Hi,
>>
>> On Wed, Aug 17, 2022 at 07:18:19PM +0900, Hyeonggon Yoo wrote:
>> > There is not much benefit for serving large objects in kmalloc().
>> > Let's pass large requests to page allocator like SLUB for better
>> > maintenance of common code.
>> >
>> > Signed-off-by: Hyeonggon Yoo <42.hyeyoo@gmail.com>
>> > Reviewed-by: Vlastimil Babka <vbabka@suse.cz>
>> > ---
>>
>> This patch results in a WARNING backtrace in all mips and sparc64
>> emulations.
>>
>> ------------[ cut here ]------------
>> WARNING: CPU: 0 PID: 0 at mm/slab_common.c:729 kmalloc_slab+0xc0/0xdc
>> Modules linked in:
>> CPU: 0 PID: 0 Comm: swapper Not tainted 6.0.0-11990-g9c9155a3509a #1
>> Stack : ffffffff 801b2a18 80dd0000 00000004 00000000 00000000 81023cd4 00000000
>> 81040000 811a9930 81040000 8104a628 81101833 00000001 81023c78 00000000
>> 00000000 00000000 80f5d858 81023b98 00000001 00000023 00000000 ffffffff
>> 00000000 00000064 00000002 81040000 81040000 00000001 80f5d858 000002d9
>> 00000000 00000000 80000000 80002000 00000000 00000000 00000000 00000000
>> ...
>> Call Trace:
>> [<8010a2bc>] show_stack+0x38/0x118
>> [<80cf5f7c>] dump_stack_lvl+0xac/0x104
>> [<80130d7c>] __warn+0xe0/0x224
>> [<80cdba5c>] warn_slowpath_fmt+0x64/0xb8
>> [<8028c058>] kmalloc_slab+0xc0/0xdc
>>
>> irq event stamp: 0
>> hardirqs last enabled at (0): [<00000000>] 0x0
>> hardirqs last disabled at (0): [<00000000>] 0x0
>> softirqs last enabled at (0): [<00000000>] 0x0
>> softirqs last disabled at (0): [<00000000>] 0x0
>> ---[ end trace 0000000000000000 ]---
>>
>> Guenter
>
> Hi.
>
> Thank you so much for this report!
>
> Hmm so SLAB tries to find kmalloc cache for freelist index array using
> kmalloc_slab() directly, and it becomes problematic when size of the
> array is larger than PAGE_SIZE * 2.

Hmm interesting, did you find out how exactly that can happen in practice,
or what's special about mips and sparc64 here? Because normally
calculate_slab_order() will only go up to slab_max_order, which AFAICS can
only go up to SLAB_MAX_ORDER_HI, thus 1, unless there's a boot command line
override.

And if we have two pages for objects, surely even with small objects they
can't be smaller than freelist_idx_t, so if the number of objects fits into
two pages (order 1), then the freelist array should also fit in two pages?

Thanks,
Vlastimil

> Will send a fix soon.
>