Re: [PATCH v3 14/15] mm/slab: remove __GFP_NO_OBJ_EXT usage from alloc_slab_obj_exts()

[Hao Li]

On Wed, Jun 17, 2026 at 04:36:58PM +0200, Vlastimil Babka (SUSE) wrote:
> On 6/17/26 15:56, Harry Yoo wrote:
> > 
> > 
> > On 6/15/26 8:54 PM, 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_NOLOCK 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_NOLOCK, the incoming gfp flags have to
> >> be also compatible with it. However, we might have added __GFP_THISNODE
> >> for opportunistic slab allocation, as pointed out by Hao Li, and
> >> __GFP_COMP by allocate_slab() as pointed out by Shengming Hu. Solve this
> >> by adding both flags to OBJCGS_CLEAR_MASK as it makes sense to strip
> >> them anyway for non-kmalloc_nolock() allocations of sheaves or obj_ext
> >> arrays as well.
> >> 
> >> To avoid recursion of sheaf -> obj_ext -> sheaf -> ... allocations at
> >> this patch, until the next patch converts sheaves to
> >> SLAB_ALLOC_NO_RECURSE, use both gfp and alloc_flags for obj_ext. The
> >> next patch will remove the gfp part.
> >> 
> >> Link: https://patch.msgid.link/20260610-slab_alloc_flags-v2-15-7190909db118@xxxxxxxxxx
> >> Signed-off-by: Vlastimil Babka (SUSE) <vbabka@xxxxxxxxxx>
> >> ---
> > 
> > Looks good to me,
> > Reviewed-by: Harry Yoo (Oracle) <harry@xxxxxxxxxx>
> 
> Thanks!
>  
> > With some comments below.
> > 
> > I was worried that perhaps replacing SLAB_ALLOC_NO_RECURSE with
> > __GFP_NO_OBJ_EXT will create a cycle of
> > 
> > alloc_slab_obj_exts(SLAB_ALLOC_DEFAULT)
> > -> kmalloc_flags(SLAB_ALLOC_NO_RECURSE)
> > -> alloc_from_pcs(SLAB_ALLOC_NO_RECURSE)
> > -> refill_objects(SLAB_ALLOC_DEFAULT)
> > -> new_slab(SLAB_ALLOC_DEFAULT)
> > -> account_slab(SLAB_ALLOC_DEFAULT)
> > -> alloc_slab_obj_exts(SLAB_ALLOC_DEFAULT)
> > 
> > with __GFP_NO_OBJ_EXT, it would have been passed to refill_objects(),
> > but SLAB_ALLOC_NO_RECURSE is not. However this cycle does not exist
> > because alloc_slab_obj_exts() clears __GFP_ACCOUNT (as part of
> > OBJCG_CLEAR_MASK) and memory profiling itself does not invoke
> > alloc_slab_obj_exts() when allocating new slabs if SLAB_ACCOUNT is not
> > set (which is interesting, by the way).
> 
> Hm yeah I think we should propagate alloc_flags to refill_objects() etc, to 
> avoid later surprise. But can be done as a later cleanup.
>  
> > Also alloc_slab_obj_exts() propagating SLAB_ALLOC_NEW_SLAB to
> > kmalloc_flags() is little bit confusing because it does not have any
> > effect due to SLAB_ALLOC_NO_RECURSE.
> 
> OK let's address this one by this fixup:

Both the patch and the fix looks good to me.
Reviewed-by: Hao Li <hao.li@xxxxxxxxx>

-- 
Thanks,
Hao