Re: [PATCH] mm/slub: serve slabobj_ext array from a strictly larger kmalloc cache

[Shakeel Butt <shakeel.butt@linux.dev>]

On Sat, Jun 27, 2026 at 07:58:12PM -0700, Shakeel Butt wrote:
> On Fri, Jun 26, 2026 at 07:11:33PM +0200, Vlastimil Babka (SUSE) wrote:
> [...]
> > >> > Fix it structurally by removing cycles of every shape: serve the array
> > >> > from a cache strictly larger than the one it describes whenever it would
> > >> > otherwise come from the same or a smaller cache.  Every reference edge
> > >> > then points from a smaller to a larger cache (here kmalloc-1k's array
> > >> > moves to kmalloc-2k), so the relation is a DAG and cannot contain a cycle.
> > >> 
> > >> This will fix the problem.
> > >> 
> > >> But this will waste memory as we need smaller obj_exts array
> > >> as the size gets larger.
> > >> 
> > >> We should probably create a new kmalloc type to avoid cycles instead?
> > >> (needed only when memory profiling is enabled, though)
> > >> 
> > >> That would also prevent recursion even further.
> > > 
> > > Yes but I assume that would add kmem caches even for users not using memory
> > > profiling. Anyways, I think that is a separate discussion. Am I understanding
> > > correctly that you don't have any concerns with this approach?
> > 
> > Umm, the memory waste is a concern?
> > 
> > Minimally I'd now want to only do that size bumping when allocation
> > profiling is enabled. Ideally that means both configured in and not booted
> > with "never".
> > 
> > We probably should have done that already in 280ea9c3154b2. Because AFAIU
> > memcg-only obj_exts array don't have this issue (or maybe they do have the
> > [1] issue? Harry?). But if memcg-only should keep avoiding the same size
> > bucket, it can keep what it was doing and only memalloc profiling would do
> > the strictly larger thing.
> 
> memcg should not have this issue as normal kmalloc caches do not serve memcg
> charged objects. 

I am wrong here as I went back and see d8df600b67d7.

> 
> So here we can do dedicated caches as Harry suggested or make this size bumping
> very specialized as Vlastimil suggested. What do we want long term? Orthogonally
> we do want this fix to be backported easily to older stable kernels. I will see
> how does this narrowed down size bumping looks like.
> 

BTW I think we need something like the following, right?

	if (mem_alloc_profiling_enabled()) {
		if (obj_exts_cache->object_size <= s->object_size)
			return s->object_size + 1;
	} else {
		if (obj_exts_cache->object_size == s->object_size)
			return s->object_size + 1;
	}

> > 
> > Suren's input would be also nice to have.
> > 
> > Thanks!
> > 
> > [1] https://lore.kernel.org/oe-lkp/202601231457.f7b31e09-lkp@intel.com
> >