Re: [PATCH] mm: slub: Fix slub_debug disablement for list of slabs

[Vijayanand Jitta]

On 7/19/2021 11:51 AM, David Rientjes wrote:
> On Tue, 13 Jul 2021, vjitta@xxxxxxxxxxxxxx wrote:
> 
>> From: Vijayanand Jitta <vjitta@xxxxxxxxxxxxxx>
>>
>> Consider the scenario where CONFIG_SLUB_DEBUG_ON is set
>> and we would want to disable slub_debug for few slabs.
>> Using boot parameter with slub_debug=-,slab_name syntax
>> doesn't work as expected i.e; only disabling debugging for
>> the specified list of slabs, instead it disables debugging
>> for all slabs. Fix this.
>>
>> Signed-off-by: Vijayanand Jitta <vjitta@xxxxxxxxxxxxxx>
>> ---
>>  mm/slub.c | 6 ++++--
>>  1 file changed, 4 insertions(+), 2 deletions(-)
>>
>> diff --git a/mm/slub.c b/mm/slub.c
>> index dc863c1..5a88418 100644
>> --- a/mm/slub.c
>> +++ b/mm/slub.c
>> @@ -1429,6 +1429,7 @@ static int __init setup_slub_debug(char *str)
>>  	char *slab_list;
>>  	bool global_slub_debug_changed = false;
>>  	bool slab_list_specified = false;
>> +	bool slab_list_debug_disable = true;
>>  
>>  	slub_debug = DEBUG_DEFAULT_FLAGS;
>>  	if (*str++ != '=' || !*str)
>> @@ -1436,7 +1437,6 @@ static int __init setup_slub_debug(char *str)
>>  		 * No options specified. Switch on full debugging.
>>  		 */
>>  		goto out;
>> -
>>  	saved_str = str;
>>  	while (str) {
>>  		str = parse_slub_debug_flags(str, &flags, &slab_list, true);
>> @@ -1445,6 +1445,8 @@ static int __init setup_slub_debug(char *str)
>>  			slub_debug = flags;
>>  			global_slub_debug_changed = true;
>>  		} else {
>> +			if (flags || !IS_ENABLED(CONFIG_SLUB_DEBUG_ON))
> 
> Could you explain the check for CONFIG_SLUB_DEBUG_ON here?
>
Thanks for the review.

The check for CONFIG_SLUB_DEBUG_ON is for below scenario:

Consider CONFIG_SLUB_DEBUG_ON is not set and the below boot parameter is
set incorrectly "slub_debug=-,slab_name", Now without the additional
CONFIG_SLUB_DEBUG_ON check slab_list_debug_disable will still be true
and slub_debug will be set to DEBUG_DEFAULT_FLAGS instead of 0 as the
below check fails.

if (!global_slub_debug_changed && !slab_list_debug_disable)
	slub_debug = 0;

> I don't believe that using `-' for slub_debug is *only* useful when 
> CONFIG_SLUB_DEBUG_ON is enabled.
> 

'-' is still useful in CONFIG_SLUB_DEBUG_ON disabled case even with the
above check, all the possible cases using '-' work as expected.

i.e;
slub_debug=-.slab_name
slub_debug=flag,slab_name;-,slab_name

Thanks,
Vijay

>> +				slab_list_debug_disable = false;
>>  			slab_list_specified = true;
>>  		}
>>  	}
>> @@ -1456,7 +1458,7 @@ static int __init setup_slub_debug(char *str)
>>  	 * long as there is no option specifying flags without a slab list.
>>  	 */
>>  	if (slab_list_specified) {
>> -		if (!global_slub_debug_changed)
>> +		if (!global_slub_debug_changed && !slab_list_debug_disable)
>>  			slub_debug = 0;
>>  		slub_debug_string = saved_str;
>>  	}

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