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

[Harry Yoo]

On Thu, Nov 05, 2026 at 03:36:25PM +0100, Vlastimil Babka (SUSE) wrote:
> On 11/5/26 15:30, Harry Yoo wrote:
> > 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@xxxxxxxxxx?part=2
> >> 
> >> Signed-off-by: Vlastimil Babka (SUSE) <vbabka@xxxxxxxxxx>
> >> ---
> > 
> > With the sashiko report to this patch fixed, this looks good to me.
> > Reviewed-by: Harry Yoo (Meta) <harry@xxxxxxxxxx>
> > 
> > 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.
> 
> Maybe not at this point, but unless I made some mistake, later slab_tiny=1
> stops depending on SLUB_DEBUG.

Ah, I see. Right.

> slab_debug=N itself does depend on it but I
> think that's acceptable

Yeah.

> > Sounds like it's time to make SLUB_DEBUG unconditional. It's enabled on
> > all distros listed in kconfigs [1].
> 
> Yes it's my plan to do that next, I did even discuss it with Christoph at LPC :)

Sounds good!

> > 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