Re: [PATCH RFC v2] Randomized slab caches for kmalloc()

[GONG, Ruiqi]

On 2023/05/22 16:03, Hyeonggon Yoo wrote:
> On Mon, May 22, 2023 at 4:35 PM Gong Ruiqi <gongruiqi1@xxxxxxxxxx> wrote:
>> On 2023/05/17 6:35, Hyeonggon Yoo wrote:
> [...]
>>>>>> +#ifdef CONFIG_RANDOM_KMALLOC_CACHES
>>>>>> +# define SLAB_RANDOMSLAB       ((slab_flags_t __force)0x01000000U)
>>>>>> +#else
>>>>>> +# define SLAB_RANDOMSLAB       0
>>>>>> +#endif
>>>
>>> There is already the SLAB_KMALLOC flag that indicates if a cache is a
>>> kmalloc cache. I think that would be enough for preventing merging
>>> kmalloc caches?
>>
>> After digging into the code of slab merging (e.g. slab_unmergeable(),
>> find_mergeable(), SLAB_NEVER_MERGE, SLAB_MERGE_SAME etc), I haven't
>> found an existing mechanism that prevents normal kmalloc caches with
>> SLAB_KMALLOC from being merged with other slab caches. Maybe I missed
>> something?
>>
>> While SLAB_RANDOMSLAB, unlike SLAB_KMALLOC, is added into
>> SLAB_NEVER_MERGE, which explicitly indicates the no-merge policy.
> 
> I mean, why not make slab_unmergable()/find_mergeable() not to merge kmalloc
> caches when CONFIG_RANDOM_KMALLOC_CACHES is enabled, instead of a new flag?
> 
> Something like this:
> 
> diff --git a/mm/slab_common.c b/mm/slab_common.c
> index 607249785c07..13ac08e3e6a0 100644
> --- a/mm/slab_common.c
> +++ b/mm/slab_common.c
> @@ -140,6 +140,9 @@ int slab_unmergeable(struct kmem_cache *s)
>   if (slab_nomerge || (s->flags & SLAB_NEVER_MERGE))
>   return 1;
> 
> + if (IS_ENALBED(CONFIG_RANDOM_KMALLOC_CACHES) && (flags & SLAB_KMALLOC))
> + return 1;
> +
>   if (s->ctor)
>   return 1;
> 
> @@ -176,6 +179,9 @@ struct kmem_cache *find_mergeable(unsigned int
> size, unsigned int align,
>   if (flags & SLAB_NEVER_MERGE)
>   return NULL;
> 
> + if (IS_ENALBED(CONFIG_RANDOM_KMALLOC_CACHES) && (flags & SLAB_KMALLOC))
> + return NULL;
> +
>   list_for_each_entry_reverse(s, &slab_caches, list) {
>   if (slab_unmergeable(s))
>   continue;

Ah I see. My concern is that it would affect not only normal kmalloc
caches, but kmalloc_{dma,cgroup,rcl} as well: since they were all marked
with SLAB_KMALLOC when being created, this code could potentially change
their mergeablity. I think it's better not to influence those irrelevant
caches.