The size is avoidably too big and I wrote a hack to demonstrate it
does not have to be.
stock:
# cat /sys/kernel/slab/mm_struct/slab_size
1408
# cat /sys/kernel/slab/mm_struct/objs_per_slab
23
patched:
# cat /sys/kernel/slab/mm_struct/slab_size
1344
# cat /sys/kernel/slab/mm_struct/objs_per_slab
24
At the beginning of the struct there is a field annotated with
____cacheline_aligned_in_smp (aka 64 on x86-64).
Apart from making free space up to the next field, this has a side
effect of forcing the size of the struct to be a multiple of 64. While
normally expected, it is actively harmful for mm_struct.
Allocations come from a dedicated slab:
mm_size = sizeof(struct mm_struct) + cpumask_size() + mm_cid_size();
mm_cachep = kmem_cache_create_usercopy("mm_struct",
mm_size, ARCH_MIN_MMSTRUCT_ALIGN,
SLAB_HWCACHE_ALIGN|SLAB_PANIC|SLAB_ACCOUNT,
offsetof(struct mm_struct, saved_auxv),
sizeof_field(struct mm_struct, saved_auxv),
NULL);
As in, the actual allocated size is bigger then sizeof(struct
mm_struct) and SLAB_HWCACHE_ALIGN already provides the expected round
up.
Instead, the following hack:
diff --git a/include/linux/mm_types.h b/include/linux/mm_types.h
index 36c5b43999e6..33de9495a8d3 100644
--- a/include/linux/mm_types.h
+++ b/include/linux/mm_types.h
@@ -685,7 +685,8 @@ struct mm_struct {
* 0, the &struct mm_struct is freed.
*/
atomic_t mm_count;
- } ____cacheline_aligned_in_smp;
+ };
+ char __padbuf[SMP_CACHE_BYTES - __alignof__(atomic_t)];
struct maple_tree mm_mt;
#ifdef CONFIG_MMU
.. provides the necessary padding without inducing the sizeof problem.
Now the patch looks like crap and a cheap shot failed to produce a way
to do it I would be happy with. Additionally I have epsilon interest
in arguing how to do it nicely.
That is to say I'm reporting this for an interested party to fix in a
whatever (non)hacky way they see fit.
ptype /o struct mm_struct :
stock:
/* offset | size */ type = struct mm_struct {
/* 0 | 1344 */ struct {
/* 0 | 64 */ struct {
/* 0 | 4 */ atomic_t mm_count;
/* XXX 60-byte padding */
/* total size (bytes): 64 */
};
/* 64 | 16 */ struct maple_tree {
[snip]
/* 1288 | 32 */ struct {
/* 1288 | 16 */ struct list_head {
/* 1288 | 8 */ struct list_head *next;
/* 1296 | 8 */ struct list_head *prev;
/* total size (bytes): 16 */
} list;
/* 1304 | 8 */ unsigned long bitmap;
/* 1312 | 8 */ struct mem_cgroup *memcg;
/* total size (bytes): 32 */
} lru_gen;
/* XXX 24-byte padding */
/* total size (bytes): 1344 */
};
/* 1344 | 0 */ unsigned long cpu_bitmap[];
/* total size (bytes): 1344 */
}
patched:
/* offset | size */ type = struct mm_struct {
/* 0 | 1320 */ struct {
/* 0 | 4 */ struct {
/* 0 | 4 */ atomic_t mm_count;
/* total size (bytes): 4 */
};
/* 4 | 60 */ char __padbuf[60];
/* 64 | 16 */ struct maple_tree {
[snip]
/* 1288 | 32 */ struct {
/* 1288 | 16 */ struct list_head {
/* 1288 | 8 */ struct list_head *next;
/* 1296 | 8 */ struct list_head *prev;
/* total size (bytes): 16 */
} list;
/* 1304 | 8 */ unsigned long bitmap;
/* 1312 | 8 */ struct mem_cgroup *memcg;
/* total size (bytes): 32 */
} lru_gen;
/* total size (bytes): 1320 */
};
/* 1320 | 0 */ unsigned long cpu_bitmap[];
/* total size (bytes): 1320 */
}
Cheers,
--
Mateusz Guzik <mjguzik gmail.com>
