[PATCH v5 5/6] mm/slab: allow freeptr_offset to be used with ctor

[Suren Baghdasaryan <surenb@google.com>]

There is no real reason to prevent freeptr_offset usage when a slab
cache has a ctor. The only real limitation is that any field unioned
with the free pointer and initialized by ctor will be overwritten since
free pointer is set after @ctor invocation. Document this limitation
and enable usage of freeptr_offset with ctor.

Suggested-by: Vlastimil Babka <vbabka@suse.cz>
Signed-off-by: Suren Baghdasaryan <surenb@google.com>
---
 include/linux/slab.h | 5 +++--
 mm/slub.c            | 2 +-
 2 files changed, 4 insertions(+), 3 deletions(-)

diff --git a/include/linux/slab.h b/include/linux/slab.h
index 681b685b6c4e..6bad744bef5e 100644
--- a/include/linux/slab.h
+++ b/include/linux/slab.h
@@ -305,8 +305,9 @@ struct kmem_cache_args {
 	 * Using %0 as a value for @freeptr_offset is valid. If @freeptr_offset
 	 * is specified, %use_freeptr_offset must be set %true.
 	 *
-	 * Note that @ctor currently isn't supported with custom free pointers
-	 * as a @ctor requires an external free pointer.
+	 * Note that fields unioned with free pointer cannot be initialized by
+	 * @ctor since free pointer is set after @ctor invocation, so those
+	 * values will be overwritten.
 	 */
 	unsigned int freeptr_offset;
 	/**
diff --git a/mm/slub.c b/mm/slub.c
index 870a1d95521d..f62c829b7b6b 100644
--- a/mm/slub.c
+++ b/mm/slub.c
@@ -5462,7 +5462,7 @@ 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)))) {
 		/*
-- 
2.47.0.338.g60cca15819-goog