bpf: Use kmalloc_size_roundup() to adjust size_index

Commit d52b593 ("bpf: Adjust size_index according to the value of
KMALLOC_MIN_SIZE") uses KMALLOC_MIN_SIZE to adjust size_index, but as
reported by Nathan, the adjustment is not enough, because
__kmalloc_minalign() also decides the minimal alignment of slab object
as shown in new_kmalloc_cache() and its value may be greater than
KMALLOC_MIN_SIZE (e.g., 64 bytes vs 8 bytes under a riscv QEMU VM).

Instead of invoking __kmalloc_minalign() in bpf subsystem to find the
maximal alignment, just using kmalloc_size_roundup() directly to get the
corresponding slab object size for each allocation size. If these two
sizes are unmatched, adjust size_index to select a bpf_mem_cache with
unit_size equal to the object_size of the underlying slab cache for the
allocation size.

Fixes: 822fb26 ("bpf: Add a hint to allocated objects.")
Reported-by: Nathan Chancellor <[email protected]>
Closes: https://lore.kernel.org/bpf/[email protected]/
Signed-off-by: Hou Tao <[email protected]>
Tested-by: Emil Renner Berthing <[email protected]>
Link: https://lore.kernel.org/r/[email protected]
Signed-off-by: Alexei Starovoitov <[email protected]>

Author: Hou Tao

kernel/bpf/memalloc.c

@@ -965,37 +965,31 @@ void notrace *bpf_mem_cache_alloc_flags(struct bpf_mem_alloc *ma, gfp_t flags)
 	return !ret ? NULL : ret + LLIST_NODE_SZ;
 }
 
-/* Most of the logic is taken from setup_kmalloc_cache_index_table() */
 static __init int bpf_mem_cache_adjust_size(void)
 {
-	unsigned int size, index;
+	unsigned int size;
 
-	/* Normally KMALLOC_MIN_SIZE is 8-bytes, but it can be
-	 * up-to 256-bytes.
+	/* Adjusting the indexes in size_index() according to the object_size
+	 * of underlying slab cache, so bpf_mem_alloc() will select a
+	 * bpf_mem_cache with unit_size equal to the object_size of
+	 * the underlying slab cache.
+	 *
+	 * The maximal value of KMALLOC_MIN_SIZE and __kmalloc_minalign() is
+	 * 256-bytes, so only do adjustment for [8-bytes, 192-bytes].
 	 */
-	size = KMALLOC_MIN_SIZE;
-	if (size <= 192)
-		index = size_index[(size - 1) / 8];
-	else
-		index = fls(size - 1) - 1;
-	for (size = 8; size < KMALLOC_MIN_SIZE && size <= 192; size += 8)
-		size_index[(size - 1) / 8] = index;
+	for (size = 192; size >= 8; size -= 8) {
+		unsigned int kmalloc_size, index;
 
-	/* The minimal alignment is 64-bytes, so disable 96-bytes cache and
-	 * use 128-bytes cache instead.
-	 */
-	if (KMALLOC_MIN_SIZE >= 64) {
-		index = size_index[(128 - 1) / 8];
-		for (size = 64 + 8; size <= 96; size += 8)
-			size_index[(size - 1) / 8] = index;
-	}
+		kmalloc_size = kmalloc_size_roundup(size);
+		if (kmalloc_size == size)
+			continue;
 
-	/* The minimal alignment is 128-bytes, so disable 192-bytes cache and
-	 * use 256-bytes cache instead.
-	 */
-	if (KMALLOC_MIN_SIZE >= 128) {
-		index = fls(256 - 1) - 1;
-		for (size = 128 + 8; size <= 192; size += 8)
+		if (kmalloc_size <= 192)
+			index = size_index[(kmalloc_size - 1) / 8];
+		else
+			index = fls(kmalloc_size - 1) - 1;
+		/* Only overwrite if necessary */
+		if (size_index[(size - 1) / 8] != index)
 			size_index[(size - 1) / 8] = index;
 	}