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

[Hao Li <hao.li@linux.dev>]

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,
Hao