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RECEIVED_SPAMHAUS_BLOCKED_OPENRESOLVER(0.00)[2a07:de40:b281:106:10:150:64:167:received]; RCVD_TLS_ALL(0.00)[]; DKIM_TRACE(0.00)[suse.cz:+]; DBL_BLOCKED_OPENRESOLVER(0.00)[imap1.dmz-prg2.suse.org:rdns,imap1.dmz-prg2.suse.org:helo] X-Spam-Score: -4.51 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? >> > 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. >> > --- >> > 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 >