[PATCH] fs/ceph: set superblock s_magic for IMA fsmagic matching: up to 60x speedup

[Dennis Marttinen]

The CephFS kernel driver forgets to set the filesystem magic signature in
its superblock. As a result, IMA policy rules based on fsmagic matching do
not apply as intended. This causes a major performance regression in Talos
Linux [1] when mounting CephFS volumes, such as when deploying Rook Ceph
[2]. Talos Linux ships a hardened kernel with the following IMA policy
(irrelevant lines omitted):

# cat /sys/kernel/security/integrity/ima/policy
[...]
dont_measure fsmagic=0xc36400 # CEPH_SUPER_MAGIC
[...]
measure func=FILE_CHECK mask=^MAY_READ euid=0
measure func=FILE_CHECK mask=^MAY_READ uid=0
[...]

Currently, IMA compares 0xc36400 == 0x0 for CephFS files, resulting in all
files opened with O_RDONLY or O_RDWR getting measured with SHA512 on every
open(2):

# cat /data/cephfs/test-file
# tail -1 /sys/kernel/security/integrity/ima/ascii_runtime_measurements
10 69990c87e8af323d47e2d6ae4... ima-ng sha512:<hash> /data/cephfs/test-file

Since O_WRONLY is rare, this results in an order of magnitude lower
performance than expected for practically all file operations. Properly
setting CEPH_SUPER_MAGIC in the CephFS superblock resolves the regression.

Tests performed on a 3x replicated Ceph v19.3.0 cluster across three
i5-7200U nodes each equipped with one Micron 7400 MAX M.2 disk (BlueStore)
and Gigabit ethernet, on Talos Linux v1.10.2:

FS-Mark 3.3
Test: 500 Files, Empty
Files/s > Higher Is Better
6.12.27-talos . 16.6  |====
+twelho patch . 208.4 |====================================================

FS-Mark 3.3
Test: 500 Files, 1KB Size
Files/s > Higher Is Better
6.12.27-talos . 15.6  |=======
+twelho patch . 118.6 |====================================================

FS-Mark 3.3
Test: 500 Files, 32 Sub Dirs, 1MB Size
Files/s > Higher Is Better
6.12.27-talos . 12.7 |===============
+twelho patch . 44.7 |=====================================================

IO500 [3] 2fcd6d6 results (benchmarks within variance omitted):

| IO500 benchmark   | 6.12.27-talos  | +twelho patch  | Speedup   |
|-------------------|----------------|----------------|-----------|
| mdtest-easy-write | 0.018524 kIOPS | 1.135027 kIOPS | 6027.33 % |
| mdtest-hard-write | 0.018498 kIOPS | 0.973312 kIOPS | 5161.71 % |
| ior-easy-read     | 0.064727 GiB/s | 0.155324 GiB/s | 139.97 %  |
| mdtest-hard-read  | 0.018246 kIOPS | 0.780800 kIOPS | 4179.29 % |

This applies outside of synthetic benchmarks as well, for example, the time
to rsync a 55 MiB directory with ~12k of mostly small files drops from an
unusable 10m5s to a reasonable 26s (23x the throughput).

[1]: https://www.talos.dev/
[2]: https://www.talos.dev/v1.10/kubernetes-guides/configuration/ceph-with-rook/
[3]: https://github.com/IO500/io500

Signed-off-by: Dennis Marttinen <twelho at welho.tech>
---
It took me a year to hunt this down: profiling distributed filesystems is
non-trivial. Since the regression is associated with IMA use, I received a
hint to CC the folks associated with IMA code. The patch targets the 6.12
kernel series currently used by Talos Linux, but should apply on top of
master as well. Please note that this is an independent contribution -
I am not affiliated with any company or organization.

 fs/ceph/super.c | 1 +
 1 file changed, 1 insertion(+)

diff --git a/fs/ceph/super.c b/fs/ceph/super.c
index 73f321b52895e..9549f97233a9e 100644
--- a/fs/ceph/super.c
+++ b/fs/ceph/super.c
@@ -1217,6 +1217,7 @@ static int ceph_set_super(struct super_block *s, struct fs_context *fc)
 	s->s_time_min = 0;
 	s->s_time_max = U32_MAX;
 	s->s_flags |= SB_NODIRATIME | SB_NOATIME;
+	s->s_magic = CEPH_SUPER_MAGIC;
 
 	ceph_fscrypt_set_ops(s);
 

base-commit: adc218676eef25575469234709c2d87185ca223a
-- 
2.49.0