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

[Vlastimil Babka <vbabka@suse.cz>]

On 2/9/26 04:18, Harry Yoo wrote:
> On Fri, Feb 06, 2026 at 11:19:01AM -0800, Alexei Starovoitov wrote:
>> On Fri, Feb 6, 2026 at 10:10 AM Vlastimil Babka <vbabka@suse.cz> wrote:
>> >
>> > 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,
> 
> Ack.
> 
>> > but skip the
>> > read_mems_allowed_retry() part in the do-while loop, so just make it one
>> > iteration for !allow_spin.
> 
> Makes sense.
> 
>> > If lockdep doesn't like even the
>> > read_mems_allowed_begin() (not clear to me), skip it too?
> 
> Yes, lockdep doesn't like read_mems_allowed_begin(), and thus
> we should skip both.
> 
>> 
>> +1
>> Just unconditional return NULL seems too restrictive.
> 
> Ack.
> 
> I'll do something like this:

Looks good!

> 
> diff --git a/mm/slub.c b/mm/slub.c
> index 102fb47ae013..cc686ab929fe 100644
> --- a/mm/slub.c
> +++ b/mm/slub.c
> @@ -3788,6 +3788,7 @@ static void *get_from_any_partial(struct kmem_cache *s, struct partial_context *
>  	struct zone *zone;
>  	enum zone_type highest_zoneidx = gfp_zone(pc->flags);
>  	unsigned int cpuset_mems_cookie;
> +	bool allow_spin = gfpflags_allow_spinning(pc->flags);
> 
>  	/*
>  	 * The defrag ratio allows a configuration of the tradeoffs between
> @@ -3812,7 +3813,15 @@ static void *get_from_any_partial(struct kmem_cache *s, struct partial_context *
>  		return NULL;
> 
>  	do {
> -		cpuset_mems_cookie = read_mems_allowed_begin();
> +		/*
> +		 * read_mems_allow_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;
> @@ -3836,7 +3845,7 @@ static void *get_from_any_partial(struct kmem_cache *s, struct partial_context *
>  				}
>  			}
>  		}
> -	} while (read_mems_allowed_retry(cpuset_mems_cookie));
> +	} while (allow_spin && read_mems_allowed_retry(cpuset_mems_cookie));
>  #endif	/* CONFIG_NUMA */
>  	return NULL;
>  }
> 
>