Re: [RFC PATCH 11/30] mm: introduce slabobj_ext to support slab object extensions

Roman Gushchin
2022-09-01T23:36:32Z

On Tue, Aug 30, 2022 at 02:49:00PM -0700, Suren Baghdasaryan wrote:
> Currently slab pages can store only vectors of obj_cgroup pointers in
> page->memcg_data. Introduce slabobj_ext structure to allow more data
> to be stored for each slab object. Wraps obj_cgroup into slabobj_ext
> to support current functionality while allowing to extend slabobj_ext
> in the future.
>
> Note: ideally the config dependency should be turned the other way around:
> MEMCG should depend on SLAB_OBJ_EXT and {page|slab|folio}.memcg_data would
> be renamed to something like {page|slab|folio}.objext_data. However doing
> this in RFC would introduce considerable churn unrelated to the overall
> idea, so avoiding this until v1.

Hi Suren!

I'd say CONFIG_MEMCG_KMEM and CONFIG_YOUR_NEW_STUFF should both depend on
SLAB_OBJ_EXT.
CONFIG_MEMCG_KMEM depend on CONFIG_MEMCG anyway.

>
> Signed-off-by: Suren Baghdasaryan <surenb@google.com>
> ---
> include/linux/memcontrol.h | 18 ++++--
> init/Kconfig | 5 ++
> mm/kfence/core.c | 2 +-
> mm/memcontrol.c | 60 ++++++++++---------
> mm/page_owner.c | 2 +-
> mm/slab.h | 119 +++++++++++++++++++++++++------------
> 6 files changed, 131 insertions(+), 75 deletions(-)
>
> diff --git a/include/linux/memcontrol.h b/include/linux/memcontrol.h
> index 6257867fbf95..315399f77173 100644
> --- a/include/linux/memcontrol.h
> +++ b/include/linux/memcontrol.h
> @@ -227,6 +227,14 @@ struct obj_cgroup {
> };
> };
>
> +/*
> + * Extended information for slab objects stored as an array in page->memcg_data
> + * if MEMCG_DATA_OBJEXTS is set.
> + */
> +struct slabobj_ext {
> + struct obj_cgroup *objcg;
> +} __aligned(8);

Why do we need this aligment requirement?

> +
> /*
> * The memory controller data structure. The memory controller controls both
> * page cache and RSS per cgroup. We would eventually like to provide
> @@ -363,7 +371,7 @@ extern struct mem_cgroup *root_mem_cgroup;
>
> enum page_memcg_data_flags {
> /* page->memcg_data is a pointer to an objcgs vector */
> - MEMCG_DATA_OBJCGS = (1UL << 0),
> + MEMCG_DATA_OBJEXTS = (1UL << 0),
> /* page has been accounted as a non-slab kernel page */
> MEMCG_DATA_KMEM = (1UL << 1),
> /* the next bit after the last actual flag */
> @@ -401,7 +409,7 @@ static inline struct mem_cgroup *__folio_memcg(struct folio *folio)
> unsigned long memcg_data = folio->memcg_data;
>
> VM_BUG_ON_FOLIO(folio_test_slab(folio), folio);
> - VM_BUG_ON_FOLIO(memcg_data & MEMCG_DATA_OBJCGS, folio);
> + VM_BUG_ON_FOLIO(memcg_data & MEMCG_DATA_OBJEXTS, folio);
> VM_BUG_ON_FOLIO(memcg_data & MEMCG_DATA_KMEM, folio);
>
> return (struct mem_cgroup *)(memcg_data & ~MEMCG_DATA_FLAGS_MASK);
> @@ -422,7 +430,7 @@ static inline struct obj_cgroup *__folio_objcg(struct folio *folio)
> unsigned long memcg_data = folio->memcg_data;
>
> VM_BUG_ON_FOLIO(folio_test_slab(folio), folio);
> - VM_BUG_ON_FOLIO(memcg_data & MEMCG_DATA_OBJCGS, folio);
> + VM_BUG_ON_FOLIO(memcg_data & MEMCG_DATA_OBJEXTS, folio);
> VM_BUG_ON_FOLIO(!(memcg_data & MEMCG_DATA_KMEM), folio);
>
> return (struct obj_cgroup *)(memcg_data & ~MEMCG_DATA_FLAGS_MASK);
> @@ -517,7 +525,7 @@ static inline struct mem_cgroup *page_memcg_check(struct page *page)
> */
> unsigned long memcg_data = READ_ONCE(page->memcg_data);
>
> - if (memcg_data & MEMCG_DATA_OBJCGS)
> + if (memcg_data & MEMCG_DATA_OBJEXTS)
> return NULL;
>
> if (memcg_data & MEMCG_DATA_KMEM) {
> @@ -556,7 +564,7 @@ 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_OBJCGS, folio);
> + VM_BUG_ON_FOLIO(folio->memcg_data & MEMCG_DATA_OBJEXTS, folio);
> return folio->memcg_data & MEMCG_DATA_KMEM;
> }
>
> diff --git a/init/Kconfig b/init/Kconfig
> index 532362fcfe31..82396d7a2717 100644
> --- a/init/Kconfig
> +++ b/init/Kconfig
> @@ -958,6 +958,10 @@ config MEMCG
> help
> Provides control over the memory footprint of tasks in a cgroup.
>
> +config SLAB_OBJ_EXT
> + bool
> + depends on MEMCG
> +
> config MEMCG_SWAP
> bool
> depends on MEMCG && SWAP
> @@ -966,6 +970,7 @@ config MEMCG_SWAP
> config MEMCG_KMEM
> bool
> depends on MEMCG && !SLOB
> + select SLAB_OBJ_EXT
> default y
>
> config BLK_CGROUP
> diff --git a/mm/kfence/core.c b/mm/kfence/core.c
> index c252081b11df..c0958e4a32e2 100644
> --- a/mm/kfence/core.c
> +++ b/mm/kfence/core.c
> @@ -569,7 +569,7 @@ static unsigned long kfence_init_pool(void)
> __folio_set_slab(slab_folio(slab));
> #ifdef CONFIG_MEMCG
> slab->memcg_data = (unsigned long)&kfence_metadata[i / 2 - 1].objcg |
> - MEMCG_DATA_OBJCGS;
> + MEMCG_DATA_OBJEXTS;
> #endif
> }
>
> diff --git a/mm/memcontrol.c b/mm/memcontrol.c
> index b69979c9ced5..3f407ef2f3f1 100644
> --- a/mm/memcontrol.c
> +++ b/mm/memcontrol.c
> @@ -2793,7 +2793,7 @@ static void commit_charge(struct folio *folio, struct mem_cgroup *memcg)
> folio->memcg_data = (unsigned long)memcg;
> }
>
> -#ifdef CONFIG_MEMCG_KMEM
> +#ifdef CONFIG_SLAB_OBJ_EXT
> /*
> * The allocated objcg pointers array is not accounted directly.
> * Moreover, it should not come from DMA buffer and is not readily
> @@ -2801,38 +2801,20 @@ static void commit_charge(struct folio *folio, struct mem_cgroup *memcg)
> */
> #define OBJCGS_CLEAR_MASK (__GFP_DMA | __GFP_RECLAIMABLE | __GFP_ACCOUNT)
>
> -/*
> - * mod_objcg_mlstate() may be called with irq enabled, so
> - * mod_memcg_lruvec_state() should be used.
> - */
> -static inline void mod_objcg_mlstate(struct obj_cgroup *objcg,
> - struct pglist_data *pgdat,
> - enum node_stat_item idx, int nr)
> -{
> - struct mem_cgroup *memcg;
> - struct lruvec *lruvec;
> -
> - rcu_read_lock();
> - memcg = obj_cgroup_memcg(objcg);
> - lruvec = mem_cgroup_lruvec(memcg, pgdat);
> - mod_memcg_lruvec_state(lruvec, idx, nr);
> - rcu_read_unlock();
> -}
> -
> -int memcg_alloc_slab_cgroups(struct slab *slab, struct kmem_cache *s,
> - gfp_t gfp, bool new_slab)
> +int alloc_slab_obj_exts(struct slab *slab, struct kmem_cache *s,
> + gfp_t gfp, bool new_slab)
> {
> unsigned int objects = objs_per_slab(s, slab);
> unsigned long memcg_data;
> void *vec;
>
> gfp &= ~OBJCGS_CLEAR_MASK;
> - vec = kcalloc_node(objects, sizeof(struct obj_cgroup *), gfp,
> + vec = kcalloc_node(objects, sizeof(struct slabobj_ext), gfp,
> slab_nid(slab));
> if (!vec)
> return -ENOMEM;
>
> - memcg_data = (unsigned long) vec | MEMCG_DATA_OBJCGS;
> + memcg_data = (unsigned long) vec | MEMCG_DATA_OBJEXTS;
> if (new_slab) {
> /*
> * If the slab is brand new and nobody can yet access its
> @@ -2843,7 +2825,7 @@ int memcg_alloc_slab_cgroups(struct slab *slab, struct kmem_cache *s,
> } else if (cmpxchg(&slab->memcg_data, 0, memcg_data)) {
> /*
> * If the slab is already in use, somebody can allocate and
> - * assign obj_cgroups in parallel. In this case the existing
> + * assign slabobj_exts in parallel. In this case the existing
> * objcg vector should be reused.
> */
> kfree(vec);
> @@ -2853,6 +2835,26 @@ int memcg_alloc_slab_cgroups(struct slab *slab, struct kmem_cache *s,
> kmemleak_not_leak(vec);
> return 0;
> }
> +#endif /* CONFIG_SLAB_OBJ_EXT */
> +
> +#ifdef CONFIG_MEMCG_KMEM
> +/*
> + * mod_objcg_mlstate() may be called with irq enabled, so
> + * mod_memcg_lruvec_state() should be used.
> + */
> +static inline void mod_objcg_mlstate(struct obj_cgroup *objcg,
> + struct pglist_data *pgdat,
> + enum node_stat_item idx, int nr)
> +{
> + struct mem_cgroup *memcg;
> + struct lruvec *lruvec;
> +
> + rcu_read_lock();
> + memcg = obj_cgroup_memcg(objcg);
> + lruvec = mem_cgroup_lruvec(memcg, pgdat);
> + mod_memcg_lruvec_state(lruvec, idx, nr);
> + rcu_read_unlock();
> +}
>
> static __always_inline
> struct mem_cgroup *mem_cgroup_from_obj_folio(struct folio *folio, void *p)
> @@ -2863,18 +2865,18 @@ struct mem_cgroup *mem_cgroup_from_obj_folio(struct folio *folio, void *p)
> * slab->memcg_data.
> */
> if (folio_test_slab(folio)) {
> - struct obj_cgroup **objcgs;
> + struct slabobj_ext *obj_exts;
> struct slab *slab;
> unsigned int off;
>
> slab = folio_slab(folio);
> - objcgs = slab_objcgs(slab);
> - if (!objcgs)
> + obj_exts = slab_obj_exts(slab);
> + if (!obj_exts)
> return NULL;
>
> off = obj_to_index(slab->slab_cache, slab, p);
> - if (objcgs[off])
> - return obj_cgroup_memcg(objcgs[off]);
> + if (obj_exts[off].objcg)
> + return obj_cgroup_memcg(obj_exts[off].objcg);
>
> return NULL;
> }
> diff --git a/mm/page_owner.c b/mm/page_owner.c
> index e4c6f3f1695b..fd4af1ad34b8 100644
> --- a/mm/page_owner.c
> +++ b/mm/page_owner.c
> @@ -353,7 +353,7 @@ static inline int print_page_owner_memcg(char *kbuf, size_t count, int ret,
> if (!memcg_data)
> goto out_unlock;
>
> - if (memcg_data & MEMCG_DATA_OBJCGS)
> + if (memcg_data & MEMCG_DATA_OBJEXTS)
> ret += scnprintf(kbuf + ret, count - ret,
> "Slab cache page\n");
>
> diff --git a/mm/slab.h b/mm/slab.h
> index 4ec82bec15ec..c767ce3f0fe2 100644
> --- a/mm/slab.h
> +++ b/mm/slab.h
> @@ -422,36 +422,94 @@ static inline bool kmem_cache_debug_flags(struct kmem_cache *s, slab_flags_t fla
> return false;
> }
>
> +#ifdef CONFIG_SLAB_OBJ_EXT
> +
> +static inline bool is_kmem_only_obj_ext(void)
> +{
> #ifdef CONFIG_MEMCG_KMEM
> + return sizeof(struct slabobj_ext) == sizeof(struct obj_cgroup *);
> +#else
> + return false;
> +#endif
> +}
> +
> /*
> - * slab_objcgs - get the object cgroups vector associated with a slab
> + * slab_obj_exts - get the pointer to the slab object extension vector
> + * associated with a slab.
> * @slab: a pointer to the slab struct
> *
> - * Returns a pointer to the object cgroups vector associated with the slab,
> + * Returns a pointer to the object extension vector associated with the slab,
> * or NULL if no such vector has been associated yet.
> */
> -static inline struct obj_cgroup **slab_objcgs(struct slab *slab)
> +static inline struct slabobj_ext *slab_obj_exts(struct slab *slab)
> {
> unsigned long memcg_data = READ_ONCE(slab->memcg_data);
>
> - VM_BUG_ON_PAGE(memcg_data && !(memcg_data & MEMCG_DATA_OBJCGS),
> + VM_BUG_ON_PAGE(memcg_data && !(memcg_data & MEMCG_DATA_OBJEXTS),
> slab_page(slab));
> VM_BUG_ON_PAGE(memcg_data & MEMCG_DATA_KMEM, slab_page(slab));
>
> - return (struct obj_cgroup **)(memcg_data & ~MEMCG_DATA_FLAGS_MASK);
> + return (struct slabobj_ext *)(memcg_data & ~MEMCG_DATA_FLAGS_MASK);
> }
>
> -int memcg_alloc_slab_cgroups(struct slab *slab, struct kmem_cache *s,
> - gfp_t gfp, bool new_slab);
> -void mod_objcg_state(struct obj_cgroup *objcg, struct pglist_data *pgdat,
> - enum node_stat_item idx, int nr);
> +int alloc_slab_obj_exts(struct slab *slab, struct kmem_cache *s,
> + gfp_t gfp, bool new_slab);
>
> -static inline void memcg_free_slab_cgroups(struct slab *slab)
> +static inline void free_slab_obj_exts(struct slab *slab)
> {
> - kfree(slab_objcgs(slab));
> + struct slabobj_ext *obj_exts;
> +
> + if (!memcg_kmem_enabled() && is_kmem_only_obj_ext())
> + return;

Hm, not sure I understand this. I kmem is disabled and is_kmem_only_obj_ext()
is true, shouldn't slab->memcg_data == NULL (always)?

> +
> + obj_exts = slab_obj_exts(slab);
> + kfree(obj_exts);
> slab->memcg_data = 0;
> }
>
> +static inline void prepare_slab_obj_exts_hook(struct kmem_cache *s, gfp_t flags, void *p)
> +{
> + struct slab *slab;
> +
> + /* If kmem is the only extension then the vector will be created conditionally */
> + if (is_kmem_only_obj_ext())
> + return;
> +
> + slab = virt_to_slab(p);
> + if (!slab_obj_exts(slab))
> + WARN(alloc_slab_obj_exts(slab, s, flags, false),
> + "%s, %s: Failed to create slab extension vector!\n",
> + __func__, s->name);
> +}

This looks a bit crypric: the action is wrapped into WARN() and the rest is a set
of (semi-)static checks. Can we, please, invert it? E.g. something like:

if (slab_alloc_tracking_enabled()) {
slab = virt_to_slab(p);
if (!slab_obj_exts(slab))
WARN(alloc_slab_obj_exts(slab, s, flags, false),
"%s, %s: Failed to create slab extension vector!\n",
__func__, s->name);
}

The rest looks good to me.

Thank you!