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

[Hyeonggon Yoo <42.hyeyoo@xxxxxxxxx>]

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;