[PATCH] uidgid: make sure we fit into one cacheline

[Christian Brauner <brauner@kernel.org>]

When I expanded uidgid mappings I intended for a struct uid_gid_map to
fit into a single cacheline on x86 as they tend to be pretty
performance sensitive (idmapped mounts etc). But a 4 byte hole was added
that brought it over 64 bytes. Fix that by adding the static extent
array and the extent counter into a substruct. C's type punning for
unions guarantees that we can access ->nr_extents even if the last
written to member wasn't within the same object. This is also what we
rely on in struct_group() and friends. This of course relies on
non-strict aliasing which we don't do.

99) If the member used to read the contents of a union object is not the
    same as the member last used to store a value in the object, the
    appropriate part of the object representation of the value is
    reinterpreted as an object representation in the new type as
    described in 6.2.6 (a process sometimes called "type punning").

Link: https://www.open-std.org/jtc1/sc22/wg14/www/docs/n2310.pdf
Signed-off-by: Christian Brauner <brauner@kernel.org>
---
Before:

struct uid_gid_map {
        u32                        nr_extents;           /*     0     4 */

        /* XXX 4 bytes hole, try to pack */

        union {
                struct uid_gid_extent extent[5];         /*     8    60 */
                struct {
                        struct uid_gid_extent * forward; /*     8     8 */
                        struct uid_gid_extent * reverse; /*    16     8 */
                };                                       /*     8    16 */
        };                                               /*     8    64 */

        /* size: 72, cachelines: 2, members: 2 */
        /* sum members: 68, holes: 1, sum holes: 4 */
        /* last cacheline: 8 bytes */
};

After:

struct uid_gid_map {
        union {
                struct {
                        struct uid_gid_extent extent[5]; /*     0    60 */
                        u32        nr_extents;           /*    60     4 */
                };                                       /*     0    64 */
                struct {
                        struct uid_gid_extent * forward; /*     0     8 */
                        struct uid_gid_extent * reverse; /*     8     8 */
                };                                       /*     0    16 */
        };                                               /*     0    64 */

        /* size: 64, cachelines: 1, members: 1 */
};
---
 include/linux/user_namespace.h | 6 ++++--
 kernel/user.c                  | 6 +++---
 2 files changed, 7 insertions(+), 5 deletions(-)

diff --git a/include/linux/user_namespace.h b/include/linux/user_namespace.h
index 6030a8235617..3625096d5f85 100644
--- a/include/linux/user_namespace.h
+++ b/include/linux/user_namespace.h
@@ -21,9 +21,11 @@ struct uid_gid_extent {
 };
 
 struct uid_gid_map { /* 64 bytes -- 1 cache line */
-	u32 nr_extents;
 	union {
-		struct uid_gid_extent extent[UID_GID_MAP_MAX_BASE_EXTENTS];
+		struct {
+			struct uid_gid_extent extent[UID_GID_MAP_MAX_BASE_EXTENTS];
+			u32 nr_extents;
+		};
 		struct {
 			struct uid_gid_extent *forward;
 			struct uid_gid_extent *reverse;
diff --git a/kernel/user.c b/kernel/user.c
index aa1162deafe4..f46b1d41163b 100644
--- a/kernel/user.c
+++ b/kernel/user.c
@@ -36,33 +36,33 @@ EXPORT_SYMBOL_GPL(init_binfmt_misc);
  */
 struct user_namespace init_user_ns = {
 	.uid_map = {
-		.nr_extents = 1,
 		{
 			.extent[0] = {
 				.first = 0,
 				.lower_first = 0,
 				.count = 4294967295U,
 			},
+			.nr_extents = 1,
 		},
 	},
 	.gid_map = {
-		.nr_extents = 1,
 		{
 			.extent[0] = {
 				.first = 0,
 				.lower_first = 0,
 				.count = 4294967295U,
 			},
+			.nr_extents = 1,
 		},
 	},
 	.projid_map = {
-		.nr_extents = 1,
 		{
 			.extent[0] = {
 				.first = 0,
 				.lower_first = 0,
 				.count = 4294967295U,
 			},
+			.nr_extents = 1,
 		},
 	},
 	.ns.count = REFCOUNT_INIT(3),

---
base-commit: 698e7d1680544ef114203b0cf656faa0c1216ebc
change-id: 20240910-work-uid_gid_map-cce46aee1b76