Re: [PATCH] slab: prevent infinite loop in kmalloc_nolock() with debugging

[Vlastimil Babka <vbabka@suse.cz>]

On 11/4/25 6:26 AM, Dev Jain wrote:
> 
> On 03/11/25 5:54 pm, Vlastimil Babka wrote:
>> In review of a followup work, Harry noticed a potential infinite loop.
>> Upon closed inspection, it already exists for kmalloc_nolock() on a
>> cache with debugging enabled, since commit af92793e52c3 ("slab:
>> Introduce kmalloc_nolock() and kfree_nolock().")
>>
>> When alloc_single_from_new_slab() fails to trylock node list_lock, we
>> keep retrying to get partial slab or allocate a new slab. If we indeed
>> interrupted somebody holding the list_lock, the trylock fill fail
> 
> Hi Vlastimil,
> 
> I see that we always take n->list_lock spinlock by disabling irqs. So
> how can we interrupt someone holding the list_lock?

From a NMI or e.g. a kprobe->bpf hook, which are the use cases for
kmalloc_nolock(). The word "interrupt" thus doesn't mean IRQ, but I'm
not sure which word would be better. "Preempt" would be perhaps even
more potentially misleading.

> If we are already in a path holding list_lock, and trigger a slab
> allocation
> and recursively end up in the same path again, we can get the situation
> you mention, is that possible?

There shouldn't be such recursion in the code itself, in the absence of
NMI/kprobe/etc.
>> deterministically and we end up allocating and defer-freeing slabs
>> indefinitely with no progress.
>>
>> To fix it, fail the allocation if spinning is not allowed. This is
>> acceptable in the restricted context of kmalloc_nolock(), especially
>> with debugging enabled.
>>
>> Reported-by: Harry Yoo <harry.yoo@oracle.com>
>> Closes: https://lore.kernel.org/all/aQLqZjjq1SPD3Fml@hyeyoo/
>> Fixes: af92793e52c3 ("slab: Introduce kmalloc_nolock() and
>> kfree_nolock().")
>> Signed-off-by: Vlastimil Babka <vbabka@suse.cz>
>> ---
>> as we discussed in the linked thread, 6.18 hotfix to be included in
>> slab/for-next-fixes
>> ---
>>   mm/slub.c | 6 +++++-
>>   1 file changed, 5 insertions(+), 1 deletion(-)
>>
>> diff --git a/mm/slub.c b/mm/slub.c
>> index d4367f25b20d..f1a5373eee7b 100644
>> --- a/mm/slub.c
>> +++ b/mm/slub.c
>> @@ -4666,8 +4666,12 @@ static void *___slab_alloc(struct kmem_cache
>> *s, gfp_t gfpflags, int node,
>>       if (kmem_cache_debug(s)) {
>>           freelist = alloc_single_from_new_slab(s, slab, orig_size,
>> gfpflags);
>>   -        if (unlikely(!freelist))
>> +        if (unlikely(!freelist)) {
>> +            /* This could cause an endless loop. Fail instead. */
>> +            if (!allow_spin)
>> +                return NULL;
>>               goto new_objects;
>> +        }
>>             if (s->flags & SLAB_STORE_USER)
>>               set_track(s, freelist, TRACK_ALLOC, addr,
>>
>> ---
>> base-commit: 6146a0f1dfae5d37442a9ddcba012add260bceb0
>> change-id: 20251103-fix-nolock-loop-854e0101672f
>>
>> Best regards,