Re: [PATCH v2 15/16] mm/slab: remove __GFP_NO_OBJ_EXT usage from alloc_slab_obj_exts()

[Hao Ge <hao.ge@linux.dev>]

Hi Vlastimil and Suren

On 2026/6/15 19:11, Vlastimil Babka (SUSE) wrote:
> On 6/15/26 07:38, Suren Baghdasaryan wrote:
>> On Fri, Jun 12, 2026 at 4:30 AM Hao Li <hao.li@linux.dev> wrote:
>>> On Fri, Jun 12, 2026 at 12:17:45PM +0200, Vlastimil Babka (SUSE) wrote:
>>>> On 6/12/26 08:54, Hao Li wrote:
>>>>> On Wed, Jun 10, 2026 at 05:40:17PM +0200, Vlastimil Babka (SUSE) wrote:
>>>>>> __GFP_NO_OBJ_EXT has limited scope within the slab allocator itself and
>>>>>> gfp flags are a scarce resource, unlike slab's alloc_flags.
>>>>>>
>>>>>> Introduce SLAB_ALLOC_NO_RECURSE alloc flag that has the same intent as
>>>>>> __GFP_NO_OBJ_EXT but a more generic name, meaning that a kmalloc()
>>>>>> family function should not recurse into another kmalloc*() for the
>>>>>> purposes of allocating auxiliary structures (obj_ext arrays or sheaves).
>>>>>>
>>>>>> First, replace the __GFP_NO_OBJ_EXT for allocating obj_ext arrays in
>>>>>> alloc_slab_obj_exts(). Make use of the newly added kmalloc_flags()
>>>>>> function, where we can pass alloc_flags with SLAB_ALLOC_NO_RECURSE
>>>>>> added. This will also pass through SLAB_ALLOC_TRYLOCK so we don't need
>>>>>> to special case kmalloc_nolock() anymore.
>>>>>>
>>>>>> Note that until now the kmalloc_nolock() ignored the incoming gfp flags
>>>>>> and hardcoded __GFP_ZERO | __GFP_NO_OBJ_EXT. But it's correct to pass on
>>>>>> the incoming gfp flags (only augmented with __GFP_ZERO), because if
>>>>>> alloc_flags contain SLAB_ALLOC_TRYLOCK, the incoming gfp flags have to
>>>>>> be also compatible with it.
>>>>>>
>>>>>> Signed-off-by: Vlastimil Babka (SUSE) <vbabka@kernel.org>
>>>>>> ---
>>>>>>   mm/slab.h |  1 +
>>>>>>   mm/slub.c | 13 +++++--------
>>>>>>   2 files changed, 6 insertions(+), 8 deletions(-)
>>>>>>
>>>>>> diff --git a/mm/slab.h b/mm/slab.h
>>>>>> index 45bfcfb35a9c..509f330654b8 100644
>>>>>> --- a/mm/slab.h
>>>>>> +++ b/mm/slab.h
>>>>>> @@ -21,6 +21,7 @@
>>>>>>   #define SLAB_ALLOC_DEFAULT        0x00 /* no flags */
>>>>>>   #define SLAB_ALLOC_TRYLOCK        0x01 /* a kmalloc_nolock() allocation */
>>>>>>   #define SLAB_ALLOC_NEW_SLAB       0x02 /* a flag for alloc_slab_obj_exts() */
>>>>>> +#define SLAB_ALLOC_NO_RECURSE     0x04 /* prevent kmalloc() recursion */
>>>>>>
>>>>>>   static inline bool alloc_flags_allow_spinning(const unsigned int alloc_flags)
>>>>>>   {
>>>>>> diff --git a/mm/slub.c b/mm/slub.c
>>>>>> index cbb38bd01e46..7dfbd0251aa2 100644
>>>>>> --- a/mm/slub.c
>>>>>> +++ b/mm/slub.c
>>>>>> @@ -2167,15 +2167,12 @@ int alloc_slab_obj_exts(struct slab *slab, struct kmem_cache *s,
>>>>>>
>>>>>>     gfp &= ~OBJCGS_CLEAR_MASK;
>>>>>>     /* Prevent recursive extension vector allocation */
>>>>>> -  gfp |= __GFP_NO_OBJ_EXT;
>>>>>> +  alloc_flags |= SLAB_ALLOC_NO_RECURSE;
>>>>>>
>>>>>>     sz = obj_exts_alloc_size(s, slab, gfp);
>>>>>>
>>>>> For the original calls to kmalloc_nolock and kmalloc_node, I notice a difference:
>>>>>
>>>>>> -  if (unlikely(!allow_spin))
>>>>>> -          vec = kmalloc_nolock(sz, __GFP_ZERO | __GFP_NO_OBJ_EXT,
>>>>>> -                               slab_nid(slab));
>>>>> kmalloc_nolock completely discarded `gfp` flags.
>>>>>
>>>>>> -  else
>>>>>> -          vec = kmalloc_node(sz, gfp | __GFP_ZERO, slab_nid(slab));
>>>>> while kmalloc_node preserved and passed it along.
>>>>>
>>>>>> +  /* This will use kmalloc_nolock() if alloc_flags say so */
>>>>>> +  vec = kmalloc_flags(sz, gfp | __GFP_ZERO, alloc_flags, slab_nid(slab));
>>>>> Now both paths are merged into kmalloc_flags, the gfp flags are
>>>>> unconditionally carried through. It seems this might carry some unwanted flags.
>>>>>
>>>>> I traced the call path and found that ___slab_alloc sets the __GFP_THISNODE
>>>>> for trynode_flags. If this flag propagates all the way into
>>>>> kmalloc_flags->...->__kmalloc_nolock_noprof, it will trigger the
>>>>> VM_WARN_ON_ONCE warning. Maybe we need to strip the original gfp if
>>>>> `!allow_spin`.
>>>> Thanks. This should do the job in a more generic way I hope?
>>>>
>>> Yeah, this is more elegant.
>>>
>>>> diff --git a/mm/slub.c b/mm/slub.c
>>>> index f9b8dc56bb57..0bf53f70c9be 100644
>>>> --- a/mm/slub.c
>>>> +++ b/mm/slub.c
>>>> @@ -2047,12 +2047,15 @@ static inline void dec_slabs_node(struct kmem_cache *s, int node,
>>>>   #endif /* CONFIG_SLUB_DEBUG */
>>>>
>>>>   /*
>>>> - * The allocated objcg pointers array is not accounted directly.
>>>> + * The allocated objcg pointers array or sheaf is not accounted directly.
>>>>    * Moreover, it should not come from DMA buffer and is not readily
>>>> - * reclaimable. So those GFP bits should be masked off.
>>>> + * reclaimable. Node restriction for the parent allocation also should
>>>> + * not apply to the slab's internal objects.
>>>> + * So those GFP bits should be masked off.
>>>>    */
>>>>   #define OBJCGS_CLEAR_MASK      (__GFP_DMA | __GFP_RECLAIMABLE | \
>>>> -                               __GFP_ACCOUNT | __GFP_NOFAIL)
>>>> +                               __GFP_ACCOUNT | __GFP_NOFAIL |
>>>> +                               __GFP_THISNODE )
>>> Good idea! Both code and comments make sense to me.
>> Makes sense. I see
>> https://git.kernel.org/pub/scm/linux/kernel/git/vbabka/slab.git/log/?h=slab/for-next
>> already implementing this and also keeping __GFP_NO_OBJ_EXT and
>> SLAB_ALLOC_NO_RECURSE both used. That version looks good to me, so
>> I'll wait for v3.
> OK.
>
>> At the end of this series, we end up with no users of __GFP_NO_OBJ_EXT
>> but we still keep it defined. I'm guessing you leave it because of the
>> new patch [1] which aliases __GFP_NO_OBJ_EXT? I will have to make that
> Yeah.
>
>> mechanism work without a GFP flag, possibly using a similar approach.
>> CC'ing Hao Ge to be in the loop of these changes. I'll work with him
>> on aliminating that __GFP_NO_OBJ_EXT alias.

Glad to work with you on this.

I'm still figuring out a proper solution.

Once I make some progress, I'll start a separate mail

thread for this to avoid disturbing too many people.

Thanks

Best Regards

Hao

> Good, then we can remove the flag completely.
>
>> [1] https://lore.kernel.org/all/20260604024008.46592-1-hao.ge@linux.dev/
>>
>>>>   #ifdef CONFIG_SLAB_OBJ_EXT
>>>>
>>>>
>>> --
>>> Thanks,
>>> Hao