Re: [PATCH V1 0/2] Only allow SLAB_OBJ_EXT_IN_OBJ for unmergeable caches

[Johannes Weiner <hannes@cmpxchg.org>]

On Tue, Jan 27, 2026 at 07:31:49PM +0900, Harry Yoo wrote:
> While SLAB_OBJ_EXT_IN_OBJ allows to reduce memory overhead to account
> slab objects, it prevents slab merging because merging can change
> the metadata layout.
>
> As pointed out Vlastimil Babka, disabling merging solely for this memory
> optimization may not be a net win, because disabling slab merging tends
> to increase overall memory usage.

Is this motivated by a production issue or a more a correctness thing?

It's somewhat tangential, but we've had practical problems with slab
merging and ended up disabling it in the Meta fleet. It makes it
difficult to identify culprits when there is a footprint regression in
merged caches. IIRC there was at least one instance where it merged
wildly different lifetimes, which raises fragmentation concerns. In
the end we turned it off and noted no meaningful usage difference.

So I'm curious if there are cases where it tangibly helps. And I
wonder whether default y makes sense given its observability and
predictability implications.