[PATCH 6.18 686/752] mm/slab: do not access current->mems_allowed_seq if !allow_spin

Sasha Levin
2026-02-28T17:58:22Z

From: Harry Yoo <harry.yoo@oracle.com>

[ Upstream commit 144080a5823b2dbd635acb6decf7ab23182664f3 ]

Lockdep complains when get_from_any_partial() is called in an NMI
context, because current->mems_allowed_seq is seqcount_spinlock_t and
not NMI-safe:

================================
WARNING: inconsistent lock state
6.19.0-rc5-kfree-rcu+ #315 Tainted: G N
--------------------------------
inconsistent {INITIAL USE} -> {IN-NMI} usage.
kunit_try_catch/9989 [HC1[1]:SC0[0]:HE0:SE1] takes:
ffff889085799820 (&____s->seqcount#3){.-.-}-{0:0}, at: ___slab_alloc+0x58f/0xc00
{INITIAL USE} state was registered at:
lock_acquire+0x185/0x320
kernel_init_freeable+0x391/0x1150
kernel_init+0x1f/0x220
ret_from_fork+0x736/0x8f0
ret_from_fork_asm+0x1a/0x30
irq event stamp: 56
hardirqs last enabled at (55): [<ffffffff850a68d7>] _raw_spin_unlock_irq+0x27/0x70
hardirqs last disabled at (56): [<ffffffff850858ca>] __schedule+0x2a8a/0x6630
softirqs last enabled at (0): [<ffffffff81536711>] copy_process+0x1dc1/0x6a10
softirqs last disabled at (0): [<0000000000000000>] 0x0

other info that might help us debug this:
Possible unsafe locking scenario:

CPU0
----
lock(&____s->seqcount#3);
<Interrupt>
lock(&____s->seqcount#3);

*** DEADLOCK ***

According to Documentation/locking/seqlock.rst, seqcount_t is not
NMI-safe and seqcount_latch_t should be used when read path can interrupt
the write-side critical section. In this case, do not access
current->mems_allowed_seq and avoid retry.

Fixes: af92793e52c3 ("slab: Introduce kmalloc_nolock() and kfree_nolock().")
Cc: stable@vger.kernel.org
Signed-off-by: Harry Yoo <harry.yoo@oracle.com>
Link: https://patch.msgid.link/20260210081900.329447-2-harry.yoo@oracle.com
Signed-off-by: Vlastimil Babka <vbabka@suse.cz>
Signed-off-by: Sasha Levin <sashal@kernel.org>
---
mm/slub.c | 13 +++++++++++--
1 file changed, 11 insertions(+), 2 deletions(-)

diff --git a/mm/slub.c b/mm/slub.c
index bc6156801e8e6..4e2a3f7656099 100644
--- a/mm/slub.c
+++ b/mm/slub.c
@@ -3594,6 +3594,7 @@ static struct slab *get_any_partial(struct kmem_cache *s,
enum zone_type highest_zoneidx = gfp_zone(pc->flags);
struct slab *slab;
unsigned int cpuset_mems_cookie;
+ bool allow_spin = gfpflags_allow_spinning(pc->flags);

/*
* The defrag ratio allows a configuration of the tradeoffs between
@@ -3618,7 +3619,15 @@ static struct slab *get_any_partial(struct kmem_cache *s,
return NULL;

do {
- cpuset_mems_cookie = read_mems_allowed_begin();
+ /*
+ * read_mems_allowed_begin() accesses current->mems_allowed_seq,
+ * a seqcount_spinlock_t that is not NMI-safe. Do not access
+ * current->mems_allowed_seq and avoid retry when GFP flags
+ * indicate spinning is not allowed.
+ */
+ if (allow_spin)
+ cpuset_mems_cookie = read_mems_allowed_begin();
+
zonelist = node_zonelist(mempolicy_slab_node(), pc->flags);
for_each_zone_zonelist(zone, z, zonelist, highest_zoneidx) {
struct kmem_cache_node *n;
@@ -3640,7 +3649,7 @@ static struct slab *get_any_partial(struct kmem_cache *s,
}
}
}
- } while (read_mems_allowed_retry(cpuset_mems_cookie));
+ } while (allow_spin && read_mems_allowed_retry(cpuset_mems_cookie));
#endif /* CONFIG_NUMA */
return NULL;
}
--
2.51.0