On 8/26/22 11:09, Vlastimil Babka wrote: > Joel reports [1] that increasing the rcu_head size for debugging > purposes used to work before struct slab was split from struct page, but > now runs into the various SLAB_MATCH() sanity checks of the layout. > > This is because the rcu_head in struct page is in union with large > sub-structures and has space to grow without exceeding their size, while > in struct slab (for SLAB and SLUB) it's in union only with a list_head. > > On closer inspection (and after the previous patch) we can put all > fields except slab_cache to a union with rcu_head, as slab_cache is > sufficient for the rcu freeing callbacks to work and the rest can be > overwritten by rcu_head without causing issues. > > This is only somewhat complicated by the need to keep SLUB's > freelist+counters aligned for cmpxchg_double. As a result the fields > need to be reordered so that slab_cache is first (after page flags) and > the union with rcu_head follows. For consistency, do that for SLAB as > well, although not necessary there. > > As a result, the rcu_head field in struct page and struct slab is no > longer at the same offset, but that doesn't matter as there is no > casting that would rely on that in the slab freeing callbacks, so we can > just drop the respective SLAB_MATCH() check. > > Also we need to update the SLAB_MATCH() for compound_head to reflect the > new ordering. > > While at it, also add a static_assert to check the alignment needed for > cmpxchg_double so mistakes are found sooner than a runtime GPF. > > [1] https://lore.kernel.org/all/85afd876-d8bb-0804-b2c5-48ed3055e702@xxxxxxxxxxxxxxxxx/ > > Reported-by: Joel Fernandes <joel@xxxxxxxxxxxxxxxxx> > Signed-off-by: Vlastimil Babka <vbabka@xxxxxxx> Both patches now pushed to slab.git for-6.1/fit_rcu_head and for-next > --- > mm/slab.h | 54 ++++++++++++++++++++++++++++++++---------------------- > 1 file changed, 32 insertions(+), 22 deletions(-) > > diff --git a/mm/slab.h b/mm/slab.h > index 4ec82bec15ec..2c248864ea91 100644 > --- a/mm/slab.h > +++ b/mm/slab.h > @@ -11,37 +11,43 @@ struct slab { > > #if defined(CONFIG_SLAB) > > + struct kmem_cache *slab_cache; > union { > - struct list_head slab_list; > + struct { > + struct list_head slab_list; > + void *freelist; /* array of free object indexes */ > + void *s_mem; /* first object */ > + }; > struct rcu_head rcu_head; > }; > - struct kmem_cache *slab_cache; > - void *freelist; /* array of free object indexes */ > - void *s_mem; /* first object */ > unsigned int active; > > #elif defined(CONFIG_SLUB) > > - union { > - struct list_head slab_list; > - struct rcu_head rcu_head; > -#ifdef CONFIG_SLUB_CPU_PARTIAL > - struct { > - struct slab *next; > - int slabs; /* Nr of slabs left */ > - }; > -#endif > - }; > struct kmem_cache *slab_cache; > - /* Double-word boundary */ > - void *freelist; /* first free object */ > union { > - unsigned long counters; > struct { > - unsigned inuse:16; > - unsigned objects:15; > - unsigned frozen:1; > + union { > + struct list_head slab_list; > +#ifdef CONFIG_SLUB_CPU_PARTIAL > + struct { > + struct slab *next; > + int slabs; /* Nr of slabs left */ > + }; > +#endif > + }; > + /* Double-word boundary */ > + void *freelist; /* first free object */ > + union { > + unsigned long counters; > + struct { > + unsigned inuse:16; > + unsigned objects:15; > + unsigned frozen:1; > + }; > + }; > }; > + struct rcu_head rcu_head; > }; > unsigned int __unused; > > @@ -66,9 +72,10 @@ struct slab { > #define SLAB_MATCH(pg, sl) \ > static_assert(offsetof(struct page, pg) == offsetof(struct slab, sl)) > SLAB_MATCH(flags, __page_flags); > -SLAB_MATCH(compound_head, slab_list); /* Ensure bit 0 is clear */ > #ifndef CONFIG_SLOB > -SLAB_MATCH(rcu_head, rcu_head); > +SLAB_MATCH(compound_head, slab_cache); /* Ensure bit 0 is clear */ > +#else > +SLAB_MATCH(compound_head, slab_list); /* Ensure bit 0 is clear */ > #endif > SLAB_MATCH(_refcount, __page_refcount); > #ifdef CONFIG_MEMCG > @@ -76,6 +83,9 @@ SLAB_MATCH(memcg_data, memcg_data); > #endif > #undef SLAB_MATCH > static_assert(sizeof(struct slab) <= sizeof(struct page)); > +#if defined(CONFIG_HAVE_CMPXCHG_DOUBLE) && defined(CONFIG_SLUB) > +static_assert(IS_ALIGNED(offsetof(struct slab, freelist), 16)); > +#endif > > /** > * folio_slab - Converts from folio to slab.
