[RFC PATCH 1/7] mm/slab: refactor freelist shuffle

Harry Yoo
2025-04-24T08:18:07Z

shuffle_freelist() function returns false when 1) it can't shuffle
the freelist due to lack of a random sequence or,
2) when there is a single object.

In a later patch, I'd like to return an error in shuffle_freelist() when
setup_object() fails. But with current code, it'll be hard to determine
whether it should initialize the objects anyway or let allocate_slab()
return an error.

To address this, decouple the shuffle eligibility checks into
should_shuffle_freelist() function and call it before shuffle_freelist().
Change the return type of shuffle_freelist() to void for now.

Signed-off-by: Harry Yoo <harry.yoo@oracle.com>
---
mm/slub.c | 26 +++++++++++++++-----------
1 file changed, 15 insertions(+), 11 deletions(-)

diff --git a/mm/slub.c b/mm/slub.c
index dac149df1be1..95a9f04b5904 100644
--- a/mm/slub.c
+++ b/mm/slub.c
@@ -2534,17 +2534,21 @@ static void *next_freelist_entry(struct kmem_cache *s,
return (char *)start + idx;
}

+static bool should_shuffle_freelist(struct kmem_cache *s, struct slab *slab)
+{
+ if (slab->objects < 2 || !s->random_seq)
+ return false;
+ return true;
+}
+
/* Shuffle the single linked freelist based on a random pre-computed sequence */
-static bool shuffle_freelist(struct kmem_cache *s, struct slab *slab)
+static void shuffle_freelist(struct kmem_cache *s, struct slab *slab)
{
void *start;
void *cur;
void *next;
unsigned long idx, pos, page_limit, freelist_count;

- if (slab->objects < 2 || !s->random_seq)
- return false;
-
freelist_count = oo_objects(s->oo);
pos = get_random_u32_below(freelist_count);

@@ -2564,8 +2568,6 @@ static bool shuffle_freelist(struct kmem_cache *s, struct slab *slab)
cur = next;
}
set_freepointer(s, cur, NULL);
-
- return true;
}
#else
static inline int init_cache_random_seq(struct kmem_cache *s)
@@ -2573,10 +2575,13 @@ static inline int init_cache_random_seq(struct kmem_cache *s)
return 0;
}
static inline void init_freelist_randomization(void) { }
-static inline bool shuffle_freelist(struct kmem_cache *s, struct slab *slab)
+static inline bool should_shuffle_freelist(struct kmem_cache *s,
+ struct slab *slab)
{
return false;
}
+static inline void shuffle_freelist(struct kmem_cache *s, struct slab *slab)
+{ }
#endif /* CONFIG_SLAB_FREELIST_RANDOM */

static __always_inline void account_slab(struct slab *slab, int order,
@@ -2606,7 +2611,6 @@ static struct slab *allocate_slab(struct kmem_cache *s, gfp_t flags, int node)
gfp_t alloc_gfp;
void *start, *p, *next;
int idx;
- bool shuffle;

flags &= gfp_allowed_mask;

@@ -2648,9 +2652,9 @@ static struct slab *allocate_slab(struct kmem_cache *s, gfp_t flags, int node)

setup_slab_debug(s, slab, start);

- shuffle = shuffle_freelist(s, slab);
-
- if (!shuffle) {
+ if (should_shuffle_freelist(s, slab)) {
+ shuffle_freelist(s, slab);
+ } else {
start = fixup_red_left(s, start);
start = setup_object(s, start);
slab->freelist = start;
--
2.43.0