Re: [PATCH v4 6/7] mm/slab: Let a bucket set handle __GFP_ACCOUNT

A bucket set holds one row of caches, cloned from KMALLOC_NORMAL, and an
allocation of any other kmalloc type falls back to the general caches.
Extend this to handle __GFP_ACCOUNT, so that a single bucket user can
isolate either GFP_KERNEL or GFP_KERNEL_ACCOUNT allocations, as is
needed for skb data, where AF_UNIX uses:

	sk->sk_allocation = GFP_KERNEL_ACCOUNT;

The coverage is selected at bucket creation time:

	b = kmem_buckets_create_types(name, flags, 0, INT_MAX, NULL,
				      BIT(KMEM_BUCKET_NORMAL) |
				      BIT(KMEM_BUCKET_CGROUP));

The prior kmem_buckets_create() function keeps its name and defaults
to only KMEM_BUCKET_NORMAL, leaving existing users as-is.

Only the accounted type is offered. Nothing wants a reclaimable or
no-obj-ext row, and of the twelve places passing GFP_DMA to an skb
allocator, all rare hardware: b44, b43legacy, prestera and s390 ctcm.

The choice is made at creation rather than every set getting every type
because the rows, when populated, are not free. Each holds 13 caches, and
a cache is a 1208 byte struct plus an unconditional per-cpu allocation,
a node struct, and an entry in /proc/slabinfo and under /sys/kernel/slab.

KMEM_BUCKET_CGROUP collapses to KMEM_BUCKET_NORMAL without CONFIG_MEMCG,
exactly as KMALLOC_CGROUP does, so NR_KMEM_BUCKET_TYPES is 1 there and a
bucket set is the same single row it is today. Where the type is asked for
but the system is not creating caches of it (under "cgroup.memory=nokmem")
the row is aliased to the normal one, as new_kmalloc_cache() does for the
general caches, so those allocations stay isolated rather than falling
back to the general caches.

Built and tests pass (and skip as expected) on ARCH=x86_64 defconfig
with GCC 16.2.0 in all combinations of CONFIG_SLAB_BUCKETS=y/n and
CONFIG_MEMCG=y/n/y+"cgroup.memory=nokmem".

Assisted-by: LLM
Signed-off-by: Kees Cook <kees@kernel.org>
---
Cc: Vlastimil Babka <vbabka@kernel.org>
Cc: Harry Yoo <harry@kernel.org>
Cc: Andrew Morton <akpm@linux-foundation.org>
Cc: Hao Li <hao.li@linux.dev>
Cc: Christoph Lameter <cl@gentwo.org>
Cc: David Rientjes <rientjes@google.com>
Cc: Roman Gushchin <roman.gushchin@linux.dev>
Cc: <redacted>
Cc: Pedro Falcato <pfalcato@suse.de>
Cc: Kuniyuki Iwashima <kuniyu@google.com>
Cc: <redacted>
---
 include/linux/slab.h   |  45 ++++++++++++-
 mm/slab.h              |  23 ++++++-
 lib/tests/slub_kunit.c |  65 +++++++++++++++---
 mm/slab_common.c       | 145 ++++++++++++++++++++++++++++++++---------
 4 files changed, 235 insertions(+), 43 deletions(-)

diff --git a/lib/tests/slub_kunit.c b/lib/tests/slub_kunit.c
index 823607e06248..58f800582170 100644
--- a/lib/tests/slub_kunit.c
+++ b/lib/tests/slub_kunit.c

@@ -723,15 +723,63 @@ static void test_kmem_buckets_type_fallback(struct kunit *test)
 				      "expected a DMA cache, got %s", c->name);
 	}
 
-	/* Nor can one that has to be accounted. */
-	if (IS_ENABLED(CONFIG_MEMCG) && !mem_cgroup_kmem_disabled()) {