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

[Christoph Lameter]

Hmmm...Doesnt this belong into memcg_create_kmem_cache() or into
kmem_cache_create() in mm/slab_common.h? Definitely not in an allocator
specific function since this is an issue for all allocators.

memcg_create_kmem_cache() simply assumes that it can pass flags from the
kmem_cache structure to kmem_cache_create(). However, those flags may
contain slab specific options.

kmem_cache_create() could filter out flags that cannot be specified.

Maybe create SLAB_FLAGS_PERMITTED in linux/mm/slab.h and mask other bits
out in kmem_cache_create()?

Slub also has internal flags and those also should not be passed to
kmem_cache_create(). If we define the valid ones we can mask them out.

The cleanest approach would be if kmem_cache_create() would reject invalid
flags and fail and if memcg_create_kmem_cache() would mask out the invalid
flags using SLAB_FLAGS_PERMITTED or so.



On Wed, 26 Oct 2016, 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
>  	/*
>