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

Suren Baghdasaryan
2022-09-02T00:24:00Z

On Thu, Sep 1, 2022 at 4:36 PM Roman Gushchin <roman.gushchin@linux.dev> wrote:
>
> 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!

Hi Roman,

>
> 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.

Yes, I agree. I wanted to mention here that the current dependency is
incorrect and should be reworked. Having both depending on
SLAB_OBJ_EXT seems like the right approach.

>
> >
> > 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?

To save space by avoiding padding, however, all members today will be
pointers, so it's meaningless and we can safely drop it.

>
> > +
> > /*
> > * 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)?

So, the logic was to skip freeing when the only possible objects in
slab->memcg_data are "struct obj_cgroup" and kmem is disabled.
Otherwise there are other objects stored in slab->memcg_data which
have to be freed. Did I make it more complicated than it should have
been?

>
> > +
> > + 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);
> }

Yeah, this is much more readable. Thanks for the suggestion and for
reviewing the code!

>
> The rest looks good to me.
>
> Thank you!
>
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