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

[Hyeonggon Yoo <42.hyeyoo@gmail.com>]

On Mon, May 22, 2023 at 04:58:25PM +0800, GONG, Ruiqi wrote:
> 
> 
> On 2023/05/22 16:03, Hyeonggon Yoo wrote:
> > On Mon, May 22, 2023 at 4:35 PM Gong Ruiqi <gongruiqi1@huawei.com> 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.

I see. no problem at all as we're not running out of cache flags.

By the way, is there any reason to only randomize normal caches
and not dma/cgroup/rcl caches?

Thanks,

-- 
Hyeonggon Yoo

Doing kernel stuff as a hobby
Undergraduate | Chungnam National University
Dept. Computer Science & Engineering