Re: [PATCH 1/2] mm/slab: skip get_from_any_partial() if !allow_spin

[Vlastimil Babka <vbabka@suse.cz>]

On 2/6/26 18:13, Harry Yoo wrote:
> 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, return NULL and fall back
> to slab allocation if !allow_spin.
> 
> Fixes: af92793e52c3 ("slab: Introduce kmalloc_nolock() and kfree_nolock().")
> Cc: stable@vger.kernel.org
> Signed-off-by: Harry Yoo <harry.yoo@oracle.com>
> ---
>  mm/slub.c | 8 ++++++++
>  1 file changed, 8 insertions(+)
> 
> diff --git a/mm/slub.c b/mm/slub.c
> index 102fb47ae013..d46464654c15 100644
> --- a/mm/slub.c
> +++ b/mm/slub.c
> @@ -3789,6 +3789,14 @@ static void *get_from_any_partial(struct kmem_cache *s, struct partial_context *
>  	enum zone_type highest_zoneidx = gfp_zone(pc->flags);
>  	unsigned int cpuset_mems_cookie;
>  
> +	/*
> +	 * read_mems_allow_begin() accesses current->mems_allowed_seq,
> +	 * a seqcount_spinlock_t that is not NMI-safe. Skip allocation
> +	 * when GFP flags indicate spinning is not allowed.
> +	 */
> +	if (!gfpflags_allow_spinning(pc->flags))
> +		return NULL;

I think it would be less restrictive to just continue, but skip the
read_mems_allowed_retry() part in the do-while loop, so just make it one
iteration for !allow_spin. If lockdep doesn't like even the
read_mems_allowed_begin() (not clear to me), skip it too?

> +
>  	/*
>  	 * The defrag ratio allows a configuration of the tradeoffs between
>  	 * inter node defragmentation and node local allocations. A lower