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List-Unsubscribe: On Wed, Oct 15, 2025 at 11:54:18AM +0200, Vlastimil Babka wrote: > On 10/15/25 11:25, Harry Yoo wrote: > > 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. > > Hm great point. We should rather make sure it's cleared always, instead of > adjusting the debugging check, which shouldn't be then necessary, right? Yeah folio_memcg_kmem() isn't supposed to be called on slabs anyway (it's not a slab at the time we free it to buddy), so we don't have to adjust the debug check. > >> > 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. > > I thought so too at first... > > >> ~/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. > > And checked the the other debugging checks too. But then thought it's better > that if these are not expected to see slabs, then they should not be > adjusted. I don't see it as a coincidence but as intention to keep it slab > specific. It will be also more future proof for the upcoming separation of > struct slab from struct page. Then we're intentionally using (folio->memcg_data & MEMCG_DATA_OBJEXTS) check as a way to determine whether the folio is a slab (either slabobj_ext array allocation succeeded or failed). That makes sense to me! > >> > --- > >> > 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