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List-Unsubscribe: On Tue, Oct 14, 2025 at 09:12:43AM -0700, Suren Baghdasaryan wrote: > On Tue, Oct 14, 2025 at 8:28 AM Hao Ge wrote: > > > > From: Hao Ge > > > > Since OBJEXTS_ALLOC_FAIL and MEMCG_DATA_OBJEXTS currently share > > the same bit position, we cannot determine whether memcg_data still > > points to the slabobj_ext vector simply by checking > > folio->memcg_data & MEMCG_DATA_OBJEXTS. > > > > If obj_exts allocation failed, slab->obj_exts is set to OBJEXTS_ALLOC_FAIL, > > and during the release of the associated folio, the BUG check is triggered > > because it was mistakenly assumed that a valid folio->memcg_data > > was not cleared before freeing the folio. nit: yesterday I was confused that this is sanity checks in buddy complaining folio->memcg_data not being cleared, but it's actually folio_memcg_kmem() complaining that MEMCG_OBJEXTS_DATA flag is set on non-slab folios (in free_pages_prepare(), if PageMemcgKmem(page) -> __memcg_kmem_uncharge_page())) So the paragraph above should be updated? And as a side question, we clear slab->obj_exts when freeing obj_ext array, but don't clear OBJEXTS_ALLOC_FAIL when freeing a slab? That's not good. > > So let's check for memcg_data != OBJEXTS_ALLOC_FAIL in folio_memcg_kmem. > > > > Fixes: 7612833192d5 ("slab: Reuse first bit for OBJEXTS_ALLOC_FAIL") > > Suggested-by: Harry Yoo > > Signed-off-by: Hao Ge > > Reviewed-by: Suren Baghdasaryan > > nit: I think it would be helpful if the changelog explained why we > need the additional check. We can have the same bit set in two > different situations: > 1. object extension vector allocation failure; > 2. memcg_data pointing to a valid mem_cgroup. > To distinguish between them, we need to check not only the bit itself > but also the rest of this field. If the rest is NULL, we have case 1, > otherwise case 2. Agreed. In general LGTM, Reviewed-by: Harry Yoo By the way, maybe it'd be nice to introduce a new helper function that properly checks MEMCG_DATA_OBJEXTS flag. > ~/slab (slab/for-next-fixes)> git grep -n MEMCG_DATA_OBJEXTS > include/linux/memcontrol.h:337: MEMCG_DATA_OBJEXTS = (1UL << 0), > include/linux/memcontrol.h:344:#define __OBJEXTS_ALLOC_FAIL MEMCG_DATA_OBJEXTS > include/linux/memcontrol.h:358: * MEMCG_DATA_OBJEXTS. > include/linux/memcontrol.h:400: VM_BUG_ON_FOLIO(memcg_data & MEMCG_DATA_OBJEXTS, folio); > include/linux/memcontrol.h:421: VM_BUG_ON_FOLIO(memcg_data & MEMCG_DATA_OBJEXTS, folio); these two, > include/linux/memcontrol.h:492: if (memcg_data & MEMCG_DATA_OBJEXTS) this, > include/linux/memcontrol.h:538: (folio->memcg_data & MEMCG_DATA_OBJEXTS), > include/linux/memcontrol.h:1491: * if MEMCG_DATA_OBJEXTS is set. > mm/kfence/core.c:624: MEMCG_DATA_OBJEXTS; > mm/page_owner.c:513: if (memcg_data & MEMCG_DATA_OBJEXTS) this, > mm/slab.h:541: * MEMCG_DATA_OBJEXTS bit set or be equal to OBJEXTS_ALLOC_FAIL. > mm/slab.h:543: VM_BUG_ON_PAGE(obj_exts && !(obj_exts & MEMCG_DATA_OBJEXTS) && > mm/slub.c:2137: new_exts |= MEMCG_DATA_OBJEXTS; > tools/mm/show_page_info.py:55: MEMCG_DATA_OBJEXTS = prog.constant("MEMCG_DATA_OBJEXTS").value_() > tools/mm/show_page_info.py:59: if memcg_data & MEMCG_DATA_OBJEXTS: and this do not look good. I mean technically they are fine since OBJEXTS_ALLOC_FAIL is set on slabs only, but that's just a coincidence. > > --- > > v3: Simplify the solution, per Harry's suggestion in the v1 comments > > Add Suggested-by: Harry Yoo > > --- > > include/linux/memcontrol.h | 4 +++- > > 1 file changed, 3 insertions(+), 1 deletion(-) > > > > diff --git a/include/linux/memcontrol.h b/include/linux/memcontrol.h > > index 873e510d6f8d..7ed15f858dc4 100644 > > --- a/include/linux/memcontrol.h > > +++ b/include/linux/memcontrol.h > > @@ -534,7 +534,9 @@ static inline struct mem_cgroup *get_mem_cgroup_from_objcg(struct obj_cgroup *ob > > static inline bool folio_memcg_kmem(struct folio *folio) > > { > > VM_BUG_ON_PGFLAGS(PageTail(&folio->page), &folio->page); > > - VM_BUG_ON_FOLIO(folio->memcg_data & MEMCG_DATA_OBJEXTS, folio); > > + VM_BUG_ON_FOLIO((folio->memcg_data != OBJEXTS_ALLOC_FAIL) && > > + (folio->memcg_data & MEMCG_DATA_OBJEXTS), > > + folio); > > return folio->memcg_data & MEMCG_DATA_KMEM; > > } > > > > -- > > 2.25.1 -- Cheers, Harry / Hyeonggon