Re: [PATCH 1/4] mm/slub: simplify the last resort slab order calculation

[Vlastimil Babka <vbabka@suse.cz>]

On 9/19/23 09:56, Feng Tang wrote:
> Hi Vlastimil,
> 
> On Fri, Sep 08, 2023 at 10:53:04PM +0800, Vlastimil Babka wrote:
>> If calculate_order() can't fit even a single large object within
>> slub_max_order, it will try using the smallest necessary order that may
>> exceed slub_max_order but not MAX_ORDER.
>> 
>> Currently this is done with a call to calc_slab_order() which is
>> unecessary. We can simply use get_order(size). No functional change.
>> 
>> Signed-off-by: Vlastimil Babka <vbabka@suse.cz>
>> ---
>>  mm/slub.c | 2 +-
>>  1 file changed, 1 insertion(+), 1 deletion(-)
>> 
>> diff --git a/mm/slub.c b/mm/slub.c
>> index f7940048138c..c6e694cb17b9 100644
>> --- a/mm/slub.c
>> +++ b/mm/slub.c
>> @@ -4193,7 +4193,7 @@ static inline int calculate_order(unsigned int size)
>>  	/*
>>  	 * Doh this slab cannot be placed using slub_max_order.
>>  	 */
>> -	order = calc_slab_order(size, 1, MAX_ORDER, 1);
>> +	order = get_order(size);
> 
> 
> This patchset is a nice cleanup, and my previous test all looked fine. 
> And one 'slub_min_order' setup reminded by Christopher [1] doesn't
> work as not taking affect with this 1/4 patch.

Hmm I see. Well the trick should keep working if you pass both
slab_min_order=9 slab_max_order=9 ? Maybe Christopher actually does that,
but didn't type it fully in the mail.

> The root cause seems to be, in current kernel, the 'slub_max_order'
> is not ajusted  accordingly with 'slub_min_order', so there is case
> that 'slub_min_order' is bigger than the default 'slub_max_order' (3).
> 
> And it could be fixed by the below patch 
> 
> diff --git a/mm/slub.c b/mm/slub.c
> index 1c91f72c7239..dbe950783105 100644
> --- a/mm/slub.c
> +++ b/mm/slub.c
> @@ -4702,6 +4702,9 @@ static int __init setup_slub_min_order(char *str)
>  {
>  	get_option(&str, (int *)&slub_min_order);
>  
> +	if (slub_min_order > slub_max_order)
> +		slub_max_order = slub_min_order;
> +
>  	return 1;
>  }

Sounds like a good idea. Would also do analogous thing in setup_slub_max_order.

> Though the formal fix may also need to cover case like this kind of
> crazy setting "slub_min_order=6 slub_max_order=5" 

Doing both should cover even this, and AFAICS how param processing works the
last one passed would "win" so it would set min=max=5 in that case. That's
probably the most sane way we can handle such scenarios.

Want to set a full patch or should I finalize it? I would put it as a new
1/5 before the rest. Thanks!

> [1]. https://lore.kernel.org/lkml/21a0ba8b-bf05-0799-7c78-2a35f8c8d52a@os.amperecomputing.com/
> 
> Thanks,
> Feng
> 
>>  	if (order <= MAX_ORDER)
>>  		return order;
>>  	return -ENOSYS;
>> -- 
>> 2.42.0
>> 
>>