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mm/slab: handle the !allow_spin case in kfree_rcu_sheaf()

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Teach kfree_rcu_sheaf() how to handle the !allow_spin case. Try to get
an empty sheaf from pcs->spare or the barn even when spinning is not
allowed. Unlike __pcs_replace_full_main(), try harder to allocate
an empty sheaf because the fallback path will be more expensive than
kfree_nolock().

Now that slab has internal alloc_flags to describe context, introduce
free_flags analogously and convert free_flags to alloc_flags when
allocating memory in the free path.

When trylock fails or the kernel observes non-NULL pcs->rcu_free after
lock acquisition, free the sheaf instead of putting it to the barn.
This is rare and not worth complicating the code.

Since call_rcu() cannot be called in an unknown context,
kfree_rcu_sheaf() fails when the rcu sheaf becomes full.

Link: https://lore.kernel.org/linux-mm/872bd673-3d45-4111-8a41-31185db3ece5@kernel.org
Reviewed-by: Vlastimil Babka (SUSE) <vbabka@kernel.org>
Signed-off-by: Harry Yoo (Oracle) <harry@kernel.org>
Link: https://patch.msgid.link/20260729-kfree_rcu_nolock-v5-2-a28cdcda9673@kernel.org
Reviewed-by: Shengming Hu <hu.shengming@zte.com.cn>
Signed-off-by: Vlastimil Babka (SUSE) <vbabka@kernel.org>
This commit is contained in:
Harry Yoo (Oracle) 2026-07-29 17:20:10 +09:00 committed by Vlastimil Babka (SUSE)
parent b0c4582862
commit f3521fbec2
3 changed files with 44 additions and 10 deletions
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18
mm/slab.h
View File

@ -24,11 +24,27 @@
#define SLAB_ALLOC_NO_RECURSE 0x04 /* prevent kmalloc() recursion */
#define SLAB_ALLOC_NO_OBJ_EXT 0x08 /* prevent obj_exts array allocation */
#define SLAB_FREE_DEFAULT 0x00 /* no flags */
#define SLAB_FREE_NOLOCK 0x01 /* spinning not allowed */
static inline unsigned int to_alloc_flags(unsigned int free_flags)
{
if (free_flags & SLAB_FREE_NOLOCK)
return SLAB_ALLOC_NOLOCK;
else
return SLAB_ALLOC_DEFAULT;
}
static inline bool alloc_flags_allow_spinning(const unsigned int alloc_flags)
{
return !(alloc_flags & SLAB_ALLOC_NOLOCK);
}
static inline bool free_flags_allow_spinning(const unsigned int free_flags)
{
return !(free_flags & SLAB_FREE_NOLOCK);
}
void *__kmalloc_flags_noprof(DECL_TOKEN_PARAMS(size, token), gfp_t flags,
unsigned int alloc_flags, int node)
__assume_kmalloc_alignment __alloc_size(1);
@ -436,7 +452,7 @@ static inline bool is_kmalloc_normal(struct kmem_cache *s)
return !(s->flags & (SLAB_CACHE_DMA|SLAB_ACCOUNT|SLAB_RECLAIM_ACCOUNT|SLAB_NO_OBJ_EXT));
}
bool __kfree_rcu_sheaf(struct kmem_cache *s, void *obj);
bool __kfree_rcu_sheaf(struct kmem_cache *s, void *obj, unsigned int free_flags);
void flush_all_rcu_sheaves(void);
void flush_rcu_sheaves_on_cache(struct kmem_cache *s);

2
mm/slab_common.c
View File

@ -1622,7 +1622,7 @@ static bool kfree_rcu_sheaf(void *obj)
s = slab->slab_cache;
if (likely(!IS_ENABLED(CONFIG_NUMA) || slab_nid(slab) == numa_mem_id()))
return __kfree_rcu_sheaf(s, obj);
return __kfree_rcu_sheaf(s, obj, SLAB_FREE_DEFAULT);
return false;
}

34
mm/slub.c
View File

@ -2789,7 +2789,8 @@ static inline struct slab_sheaf *alloc_empty_sheaf(struct kmem_cache *s,
return __alloc_empty_sheaf(s, gfp, alloc_flags, s->sheaf_capacity);
}
static void free_empty_sheaf(struct kmem_cache *s, struct slab_sheaf *sheaf)
static void __free_empty_sheaf(struct kmem_cache *s, struct slab_sheaf *sheaf,
unsigned int free_flags)
{
/*
* If the sheaf was created with SLAB_ALLOC_NO_RECURSE flag then its
@ -2801,11 +2802,20 @@ static void free_empty_sheaf(struct kmem_cache *s, struct slab_sheaf *sheaf)
mark_obj_codetag_empty(sheaf);
VM_WARN_ON_ONCE(sheaf->size > 0);
kfree(sheaf);
if (unlikely(free_flags & SLAB_FREE_NOLOCK))
kfree_nolock(sheaf);
else
kfree(sheaf);
stat(s, SHEAF_FREE);
}
static void free_empty_sheaf(struct kmem_cache *s, struct slab_sheaf *sheaf)
{
__free_empty_sheaf(s, sheaf, SLAB_FREE_DEFAULT);
}
static unsigned int
refill_objects(struct kmem_cache *s, void **p, gfp_t gfp, unsigned int min,
unsigned int max);
@ -6042,10 +6052,11 @@ static void rcu_free_sheaf(struct rcu_head *head)
*/
static DEFINE_WAIT_OVERRIDE_MAP(kfree_rcu_sheaf_map, LD_WAIT_CONFIG);
bool __kfree_rcu_sheaf(struct kmem_cache *s, void *obj)
bool __kfree_rcu_sheaf(struct kmem_cache *s, void *obj, unsigned int free_flags)
{
struct slub_percpu_sheaves *pcs;
struct slab_sheaf *rcu_sheaf;
bool allow_spin = free_flags_allow_spinning(free_flags);
if (WARN_ON_ONCE(IS_ENABLED(CONFIG_PREEMPT_RT)))
return false;
@ -6058,9 +6069,10 @@ bool __kfree_rcu_sheaf(struct kmem_cache *s, void *obj)
pcs = this_cpu_ptr(s->cpu_sheaves);
if (unlikely(!pcs->rcu_free)) {
struct slab_sheaf *empty;
struct node_barn *barn;
unsigned int alloc_flags = to_alloc_flags(free_flags);
gfp_t gfp = allow_spin ? GFP_NOWAIT : __GFP_NOWARN;
/* Bootstrap or debug cache, fall back */
if (unlikely(!cache_has_sheaves(s))) {
@ -6080,7 +6092,7 @@ bool __kfree_rcu_sheaf(struct kmem_cache *s, void *obj)
goto fail;
}
empty = barn_get_empty_sheaf(barn, true);
empty = barn_get_empty_sheaf(barn, allow_spin);
if (empty) {
pcs->rcu_free = empty;
@ -6089,20 +6101,20 @@ bool __kfree_rcu_sheaf(struct kmem_cache *s, void *obj)
local_unlock(&s->cpu_sheaves->lock);
empty = alloc_empty_sheaf(s, GFP_NOWAIT, SLAB_ALLOC_DEFAULT);
empty = alloc_empty_sheaf(s, gfp, alloc_flags);
if (!empty)
goto fail;
if (!local_trylock(&s->cpu_sheaves->lock)) {
barn_put_empty_sheaf(barn, empty);
__free_empty_sheaf(s, empty, free_flags);
goto fail;
}
pcs = this_cpu_ptr(s->cpu_sheaves);
if (unlikely(pcs->rcu_free))
barn_put_empty_sheaf(barn, empty);
__free_empty_sheaf(s, empty, free_flags);
else
pcs->rcu_free = empty;
}
@ -6120,6 +6132,12 @@ bool __kfree_rcu_sheaf(struct kmem_cache *s, void *obj)
if (likely(rcu_sheaf->size < s->sheaf_capacity)) {
rcu_sheaf = NULL;
} else {
if (unlikely(!allow_spin)) {
/* call_rcu() cannot be called in an unknown context */
rcu_sheaf->size--;
local_unlock(&s->cpu_sheaves->lock);
goto fail;
}
pcs->rcu_free = NULL;
rcu_sheaf->node = numa_node_id();
}