[PATCH 1/5] bpf: Add an arena flag to retain allocated pages

[Andrea Righi <arighi@nvidia.com>]

Add BPF_F_ARENA_NO_FREE to retain arena pages until the map is
destroyed.

Make BPF page-free operations no-ops in both sleepable and non-sleepable
programs when the flag is set. Programs can manage reusable objects
within the retained pages without releasing their backing memory.

This provides a page-retention policy independently of how userspace
accesses the arena. It also supplies stable backing for external
consumers that must not lose pages while they are still using them.

Signed-off-by: Andrea Righi <arighi@nvidia.com>
---
 include/uapi/linux/bpf.h       | 3 +++
 kernel/bpf/arena.c             | 9 ++++++---
 tools/include/uapi/linux/bpf.h | 3 +++
 3 files changed, 12 insertions(+), 3 deletions(-)

diff --git a/include/uapi/linux/bpf.h b/include/uapi/linux/bpf.h
index e0ed44b1bbcb7..75101d602bd32 100644
--- a/include/uapi/linux/bpf.h
+++ b/include/uapi/linux/bpf.h
@@ -1500,6 +1500,9 @@ enum {
 
 /* Enable BPF ringbuf overwrite mode */
 	BPF_F_RB_OVERWRITE	= (1U << 19),
+
+	/* Keep arena pages allocated until the map is destroyed. */
+	BPF_F_ARENA_NO_FREE	= (1U << 20),
 };
 
 /* Flags for BPF_PROG_QUERY. */
diff --git a/kernel/bpf/arena.c b/kernel/bpf/arena.c
index 0ff707707da04..343e84139ec29 100644
--- a/kernel/bpf/arena.c
+++ b/kernel/bpf/arena.c
@@ -283,7 +283,8 @@ static struct bpf_map *arena_map_alloc(union bpf_attr *attr)
 	    /* BPF_F_MMAPABLE must be set */
 	    !(attr->map_flags & BPF_F_MMAPABLE) ||
 	    /* No unsupported flags present */
-	    (attr->map_flags & ~(BPF_F_SEGV_ON_FAULT | BPF_F_MMAPABLE | BPF_F_NO_USER_CONV)))
+	    (attr->map_flags & ~(BPF_F_SEGV_ON_FAULT | BPF_F_MMAPABLE |
+				 BPF_F_NO_USER_CONV | BPF_F_ARENA_NO_FREE)))
 		return ERR_PTR(-EINVAL);
 
 	if (attr->map_extra & ~PAGE_MASK)
@@ -1134,7 +1135,8 @@ __bpf_kfunc void bpf_arena_free_pages(void *p__map, void *ptr__ign, u32 page_cnt
 	struct bpf_map *map = p__map;
 	struct bpf_arena *arena = container_of(map, struct bpf_arena, map);
 
-	if (map->map_type != BPF_MAP_TYPE_ARENA || !page_cnt || !ptr__ign)
+	if (map->map_type != BPF_MAP_TYPE_ARENA || !page_cnt || !ptr__ign ||
+	    (map->map_flags & BPF_F_ARENA_NO_FREE))
 		return;
 	arena_free_pages(arena, (long)ptr__ign, page_cnt, true);
 }
@@ -1144,7 +1146,8 @@ void bpf_arena_free_pages_non_sleepable(void *p__map, void *ptr__ign, u32 page_c
 	struct bpf_map *map = p__map;
 	struct bpf_arena *arena = container_of(map, struct bpf_arena, map);
 
-	if (map->map_type != BPF_MAP_TYPE_ARENA || !page_cnt || !ptr__ign)
+	if (map->map_type != BPF_MAP_TYPE_ARENA || !page_cnt || !ptr__ign ||
+	    (map->map_flags & BPF_F_ARENA_NO_FREE))
 		return;
 	arena_free_pages(arena, (long)ptr__ign, page_cnt, false);
 }
diff --git a/tools/include/uapi/linux/bpf.h b/tools/include/uapi/linux/bpf.h
index e0ed44b1bbcb7..75101d602bd32 100644
--- a/tools/include/uapi/linux/bpf.h
+++ b/tools/include/uapi/linux/bpf.h
@@ -1500,6 +1500,9 @@ enum {
 
 /* Enable BPF ringbuf overwrite mode */
 	BPF_F_RB_OVERWRITE	= (1U << 19),
+
+	/* Keep arena pages allocated until the map is destroyed. */
+	BPF_F_ARENA_NO_FREE	= (1U << 20),
 };
 
 /* Flags for BPF_PROG_QUERY. */
-- 
2.56.0