A later patch will introduce bpf_selem_unlink_lockless() to handle
rqspinlock errors. bpf_selem_unlink_lockless() will allow an selem
to be partially unlinked from map or local storage. Therefore,
bpf_selem_free() needs to be decoupled from map and local storage
as SDATA(selem)->smap or selem->local_storage may be NULL.
Decoupling from local storage is already done when local storage
migrated from BPF memory allocator to kmalloc_nolock(). This patch
prepares to decouple from map.

Currently, map is still needed in bpf_selem_free() to:

  1. Uncharge memory
    a. map->ops->map_local_storage_uncharge
    b. map->elem_size
  2. Infer how memory should be freed
    a. map->use_kmalloc_nolock
  3. Free special fields
    a. map->record

The dependency of 1.a will be addressed by a later patch by returning
the amount of memory to uncharge directly to the owner who calls
bpf_local_storage_destroy().

The dependency of 3.a will be addressed by a later patch by freeing
special fields under b->lock, when the map is still alive.

This patch handles 1.b and 2.a by simply saving the informnation in
bpf_local_storage_elem.

Signed-off-by: Amery Hung <redacted>
---
 include/linux/bpf_local_storage.h | 4 +++-
 kernel/bpf/bpf_local_storage.c    | 4 +++-
 2 files changed, 6 insertions(+), 2 deletions(-)

diff --git a/include/linux/bpf_local_storage.h b/include/linux/bpf_local_storage.h
index 70b35dfc01c9..20918c31b7e5 100644
--- a/include/linux/bpf_local_storage.h
+++ b/include/linux/bpf_local_storage.h

@@ -80,7 +80,9 @@ struct bpf_local_storage_elem {
 						 * after raw_spin_unlock
 						 */
 	};
-	/* 8 bytes hole */
+	u16 size;
+	bool use_kmalloc_nolock;
+	/* 4 bytes hole */
 	/* The data is stored in another cacheline to minimize
 	 * the number of cachelines access during a cache hit.
 	 */

diff --git a/kernel/bpf/bpf_local_storage.c b/kernel/bpf/bpf_local_storage.c
index 667b468652d1..62201552dca6 100644
--- a/kernel/bpf/bpf_local_storage.c
+++ b/kernel/bpf/bpf_local_storage.c

@@ -97,6 +97,8 @@ bpf_selem_alloc(struct bpf_local_storage_map *smap, void *owner,
 			if (swap_uptrs)
 				bpf_obj_swap_uptrs(smap->map.record, SDATA(selem)->data, value);
 		}
+		selem->size = smap->elem_size;
+		selem->use_kmalloc_nolock = smap->use_kmalloc_nolock;
 		return selem;
 	}
 

@@ -219,7 +221,7 @@ void bpf_selem_free(struct bpf_local_storage_elem *selem,
 
 	smap = rcu_dereference_check(SDATA(selem)->smap, bpf_rcu_lock_held());
 
-	if (!smap->use_kmalloc_nolock) {
+	if (!selem->use_kmalloc_nolock) {
 		/*
 		 * No uptr will be unpin even when reuse_now == false since uptr
 		 * is only supported in task local storage, where

-- 
2.47.3