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

[Dev Jain <dev.jain@arm.com>]

On 04/11/25 3:54 pm, Vlastimil Babka wrote:
> 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.

Thanks for explaining.

>>> 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,