Re: [PATCH v1] memcg: Prevent caches to be both OFF_SLAB & OBJFREELIST_SLAB

[Thomas Garnier]

On Thu, Oct 27, 2016 at 12:25 AM, Michal Hocko <mhocko@xxxxxxxxxx> wrote:
> The patch is marked for memcg but I do not see any direct relation.
> I am not familiar with this code enough probably but if this really is
> memcg kmem related, please do not forget to CC Vladimir
>

Yes, the next iteration should be closer to memcg. I will CC Vladimir.

Thanks for the heads-up.

> On Wed 26-10-16 10:41:28, Thomas Garnier wrote:
>> While testing OBJFREELIST_SLAB integration with pagealloc, we found a
>> bug where kmem_cache(sys) would be created with both CFLGS_OFF_SLAB &
>> CFLGS_OBJFREELIST_SLAB.
>>
>> The original kmem_cache is created early making OFF_SLAB not possible.
>> When kmem_cache(sys) is created, OFF_SLAB is possible and if pagealloc
>> is enabled it will try to enable it first under certain conditions.
>> Given kmem_cache(sys) reuses the original flag, you can have both flags
>> at the same time resulting in allocation failures and odd behaviors.
>>
>> The proposed fix removes these flags by default at the entrance of
>> __kmem_cache_create. This way the function will define which way the
>> freelist should be handled at this stage for the new cache.
>>
>> Fixes: b03a017bebc4 ("mm/slab: introduce new slab management type, OBJFREELIST_SLAB")
>> Signed-off-by: Thomas Garnier <thgarnie@xxxxxxxxxx>
>> Signed-off-by: Greg Thelen <gthelen@xxxxxxxxxx>
>> ---
>> Based on next-20161025
>> ---
>>  mm/slab.c | 8 ++++++++
>>  1 file changed, 8 insertions(+)
>>
>> diff --git a/mm/slab.c b/mm/slab.c
>> index 3c83c29..efe280a 100644
>> --- a/mm/slab.c
>> +++ b/mm/slab.c
>> @@ -2027,6 +2027,14 @@ __kmem_cache_create (struct kmem_cache *cachep, unsigned long flags)
>>       int err;
>>       size_t size = cachep->size;
>>
>> +     /*
>> +      * memcg re-creates caches with the flags of the originals. Remove
>> +      * the freelist related flags to ensure they are re-defined at this
>> +      * stage. Prevent having both flags on edge cases like with pagealloc
>> +      * if the original cache was created too early to be OFF_SLAB.
>> +      */
>> +     flags &= ~(CFLGS_OBJFREELIST_SLAB|CFLGS_OFF_SLAB);
>> +
>>  #if DEBUG
>>  #if FORCED_DEBUG
>>       /*
>> --
>> 2.8.0.rc3.226.g39d4020
>>
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>
> --
> Michal Hocko
> SUSE Labs



-- 
Thomas