Re: [PATCH v3 04/10] mm, slab: introduce slab_debug=N

[Harry Yoo <harry@kernel.org>]

On Thu, Nov 05, 2026 at 09:43:53AM +0100, Vlastimil Babka (SUSE) wrote:
> Enabling slab_debug has the side-effect of disabling all percpu caching
> of objects (now via sheaves), which can be sometimes useful for saving
> memory for e.g. kdump kernels. To make this possible without the
> overhead (CPU or memory) of actual debugging options such as poisoning,
> introduce a No-op debug option that only forces the debugging slow paths
> and zero sheaf capacity. The new corresponding slab flag is
> SLAB_DEBUG_NOOP.
> 
> This will also allow reimplementing CONFIG_SLUB_TINY as a boot-time
> option.
> 
> Two details have to be considered, as suggested by Harry Yoo [1].
> 
> - While SLAB_DEBUG_NOOP is part of SLAB_DEBUG_FLAGS, it should not
>   prevent cache merging as it doesn't affect object metadata, and lack
>   of merging would go against the memory footprint reduction. Thus
>   exclude the flag from SLAB_NEVER_MERGE.
> 
>   - However add the flag to SLAB_MERGE_SAME, per sashiko.
> 
> - SLAB_DEBUG_NOOP alone should not request disabling printk pointer
>   hashing. Thus rework the call to hash_pointers_finalize() to not rely
>   on the slub_debug static key state. Instead determine whether the
>   global or per-cache slab_debug options include any flags that are not
>   SLAB_DEBUG_NOOP. This does not change the fact that caches created
>   with explicit debug flags (i.e. slub_kunit tests) later in boot have
>   no effect on pointer hashing, which is determined in
>   kmem_cache_init().
> 
> While documenting the new N option, remove obsolete SLAB references from
> the F option description.
> 
> [1] https://lore.kernel.org/all/arUx56ee_nkxYP_f@thinkstation/
> [2] https://sashiko.dev/#/patchset/20261024-slub_tiny_rework-v2-0-bf1419ab5ffd@kernel.org?part=2
> 
> Signed-off-by: Vlastimil Babka (SUSE) <vbabka@kernel.org>
> ---

With the sashiko report to this patch fixed, this looks good to me.
Reviewed-by: Harry Yoo (Meta) <harry@kernel.org>

One nit comment:

specifying SLAB_DEBUG_FLAGS depends on CONFIG_SLUB_DEBUG, and with this
patchset there is no way to minimize slab memory usage w/o SLUB_DEBUG.
Sounds like it's time to make SLUB_DEBUG unconditional. It's enabled on
all distros listed in kconfigs [1].

I built kernels w/o SLUB_DEBUG and the difference in the kernel
binary was about 40 KiB.

[1] https://oracle.github.io/kconfigs/?config=UTS_RELEASE&config=SLUB_DEBUG

-- 
Cheers,
Harry / Hyeonggon