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

["Vlastimil Babka (SUSE)" <vbabka@kernel.org>]

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.

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