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owner-majordomo@kvack.org List-ID: List-Subscribe: List-Unsubscribe: On Mon, Jun 09, 2025 at 10:27:23PM +0900, Harry Yoo wrote: > When running tools/mm/page-types (with flags, page-counts, MB omitted), > it reports that some slabs have KPF_ANON and KPF_KSM set: > > $ sudo ./page-types | grep slab > _______S___________________________________ slab > _______S____a________x_____________________ slab,anonymous,ksm > > This is unexpected, as slab memory should never be treated as anonymous > memory. This is because slab->slabs shares the same offset as > page->mapping and setting the lower two bits of ->slabs field means > folio_test_anon() and folio_test_ksm() will return true on the slab: > > [ field ] [ offset ] [ size ] > page->mapping 24 8 > page->lru 8 16 > > slab->next 16 8 > slab->slabs 24 4 > > Reorder ->slabs and ->next, so that the layout will be: > > slab->slabs 16 4 > slab->next 24 8 > > After reordering, slab->slabs shares its offset with page->lru.prev, > which is not a problem. slab->next now shares the offset with > page->mapping, but that's fine, as a slab is double-word aligned. > > With the change, the nonsense slab pages disappear: > > $ sudo ./page-types | grep slab > _______S___________________________________ slab > > Fixes: 130d4df57390 ("mm/sl[au]b: rearrange struct slab fields to allow larger rcu_head") > Signed-off-by: Harry Yoo > --- Oops, I should have mentioned that this may not _completely_ fix the false positive. Three years ago, there was a report [1] saying that CC_OPTIMIZE_FOR_SIZE causes the function rcu_free_slab()'s alignment to be less than 4 bytes, which can also lead to false positives :( I don't know how the minimum function alignement has changed since then, need to check. [1] https://lore.kernel.org/all/ea96c78c-e1dc-1364-e91-51909f82388b@google.com/ > No Cc: stable because we don't usually check folio_test_anon() on slabs. > > mm/slab.h | 2 +- > 1 file changed, 1 insertion(+), 1 deletion(-) > > diff --git a/mm/slab.h b/mm/slab.h > index 05a21dc796e0..2e7064f7709e 100644 > --- a/mm/slab.h > +++ b/mm/slab.h > @@ -59,8 +59,8 @@ struct slab { > struct list_head slab_list; > #ifdef CONFIG_SLUB_CPU_PARTIAL > struct { > - struct slab *next; > int slabs; /* Nr of slabs left */ > + struct slab *next; > }; > #endif > }; > -- > 2.43.0 > -- Cheers, Harry / Hyeonggon