[RFC PATCH V3 1/7] mm/slab: allow specifying freepointer offset when using constructor

[Harry Yoo <harry.yoo@oracle.com>]

When a slab cache has a constructor, the free pointer is placed after the
object because certain fields must not be overwritten even after the
object is freed.

However, some fields that the constructor does not care can safely be
overwritten. Allow specifying the free pointer offset within the object,
reducing the overall object size when some fields can be reused for the
free pointer.

Signed-off-by: Harry Yoo <harry.yoo@oracle.com>
---
 mm/slab_common.c | 2 +-
 mm/slub.c        | 6 ++++--
 2 files changed, 5 insertions(+), 3 deletions(-)

diff --git a/mm/slab_common.c b/mm/slab_common.c
index 932d13ada36c..2c2ed2452271 100644
--- a/mm/slab_common.c
+++ b/mm/slab_common.c
@@ -231,7 +231,7 @@ static struct kmem_cache *create_cache(const char *name,
 	err = -EINVAL;
 	if (args->use_freeptr_offset &&
 	    (args->freeptr_offset >= object_size ||
-	     !(flags & SLAB_TYPESAFE_BY_RCU) ||
+	     (!(flags & SLAB_TYPESAFE_BY_RCU) && !args->ctor) ||
 	     !IS_ALIGNED(args->freeptr_offset, __alignof__(freeptr_t))))
 		goto out;
 
diff --git a/mm/slub.c b/mm/slub.c
index 462a39d57b3a..64705cb3734f 100644
--- a/mm/slub.c
+++ b/mm/slub.c
@@ -7781,7 +7781,8 @@ static int calculate_sizes(struct kmem_cache_args *args, struct kmem_cache *s)
 	s->inuse = size;
 
 	if (((flags & SLAB_TYPESAFE_BY_RCU) && !args->use_freeptr_offset) ||
-	    (flags & SLAB_POISON) || s->ctor ||
+	    (flags & SLAB_POISON) ||
+	    (s->ctor && !args->use_freeptr_offset) ||
 	    ((flags & SLAB_RED_ZONE) &&
 	     (s->object_size < sizeof(void *) || slub_debug_orig_size(s)))) {
 		/*
@@ -7802,7 +7803,8 @@ static int calculate_sizes(struct kmem_cache_args *args, struct kmem_cache *s)
 		 */
 		s->offset = size;
 		size += sizeof(void *);
-	} else if ((flags & SLAB_TYPESAFE_BY_RCU) && args->use_freeptr_offset) {
+	} else if (((flags & SLAB_TYPESAFE_BY_RCU) || s->ctor) &&
+			args->use_freeptr_offset) {
 		s->offset = args->freeptr_offset;
 	} else {
 		/*
-- 
2.43.0