Re: [PATCH RFC 00/15] mm/slab: introduce alloc_flags and slab_alloc_context

[Usama Arif <usama.arif@linux.dev>]

On Tue, 09 Jun 2026 11:17:45 +0200 "Vlastimil Babka (SUSE)" <vbabka@kernel.org> wrote:

> This series is based on slab/for-next. If all goes well, it would
> hopefully go to slab/for-next soon after the 7.2 merge window, so any
> other work can be based on it to avoid conflicts, as it touches a lot
> parts of slab.
> 
> Git: https://git.kernel.org/pub/scm/linux/kernel/git/vbabka/linux.git/log/?h=b4/slab_alloc_flags
> 
> The slab implementation currently relies on gfp flags to convey
> some context information internally:
> 
> - The absence of both __GFP_RECLAIM flags is interpreted as "cannot spin
>   on locks", and intended to be used by kmalloc_nolock(). But false
>   positives are possible e.g. during early boot where gfp_allowed_mask
>   clears __GFP_RECLAIM from all allocations. This leads to unnecessary
>   allocation failures and workarounds such as fd3634312a04 ("debugobject:
>   Make it work with deferred page initialization - again").
> 
> - __GFP_NO_OBJ_EXT exists and takes up valuable bit in the gfp flags
>   space, only to prevent recursive kmalloc() allocations for obj_ext
>   arrays and sheaves.
> 

Hello Valstimil!

I think memory allocation profiling uses __GFP_NO_OBJ_EXT, and I dont see
it being removed in the series (hopefully I didnt miss it).

Adding Hao Ge in CC who did this in the commit:
mm/alloc_tag: replace fixed-size early PFN array with dynamic linked list


> The page allocator uses its internal alloc_flags to convey various
> context information, including ALLOC_TRYLOCK (meaning "cannot spin").
> This series copies that concept for the slab allocator, with its own
> slab-specific internal flags:
> 
> - SLAB_ALLOC_DEFAULT - no extra flags (the value is 0), but explicit
> - SLAB_ALLOC_TRYLOCK - do not spin on locks (used by kmalloc_nolock())
> - SLAB_ALLOC_NEW_SLAB - replacing existing 'bool new_slab' parameter
> 			for allocating obj_ext arrays
> - SLAB_ALLOC_NO_RECURSE - replacing usage of __GFP_NO_OBJ_EXT
> 
> To reduce the amount of parameters in various internal functions, we
> additionally introduce slab_alloc_context (also inspired by page
> allocator's alloc_context) for passing a number of existing arguments
> and the new alloc_flags:
> 
> /* Structure holding extra parameters for slab allocations */
> struct slab_alloc_context {
> 	unsigned long caller_addr;
> 	unsigned long orig_size;
> 	unsigned int alloc_flags;
> 	struct list_lru *lru;
> };
> 
> This also replaces the existing struct partial_context.
> 
> The last necessary piece is kmalloc_flags() which can take the
> alloc_flags in addition to gfp flags and is intended for the recursive
> allocations of sheaves and obj_ext arrays, so that both
> SLAB_ALLOC_TRYLOCK and SLAB_ALLOC_NO_RECURSE can be communicated.
> Internally it decides between kmalloc_nolock() and normal kmalloc()
> depending SLAB_ALLOC_TRYLOCK.
> 
> The rest of the series is gradually expanding the usage of both
> alloc_flags and slab_alloc_context as necessary, with bits of
> refactoring. Then, __GFP_NO_OBJ_EXT is removed completely.
> 
> Note that some usage of gfpflags_allow_spinning() relying on absence of
> __GFP_RECLAIM remains outside of slab (and page allocator) in memcg,
> page_owner and stackdepot code. These can thus yield false-positive
> decisions that spinning is not allowed, but should not result in
> important allocations failing anymore.
> 
> Signed-off-by: Vlastimil Babka (SUSE) <vbabka@kernel.org>
> ---
> Vlastimil Babka (SUSE) (15):
>       mm/slab: always zero only requested size on alloc
>       mm/slab: stop inlining __slab_alloc_node()
>       mm/slab: introduce slab_alloc_context
>       mm/slab: introduce alloc_flags and SLAB_ALLOC_TRYLOCK
>       mm/slab: add alloc_flags to slab_alloc_context
>       mm/slab: replace struct partial_context with slab_alloc_context
>       mm/slab: pass alloc_flags to new slab allocation
>       mm/slab: pass alloc_flags through slab_post_alloc_hook() chain
>       mm/slab: replace slab_alloc_node() parameters with slab_alloc_context
>       mm/slab: allow kmem_cache_alloc_bulk() with any gfp flags
>       mm/slab: pass slab_alloc_context to __do_kmalloc_node()
>       mm/slab: introduce kmalloc_flags()
>       mm/slab: remove __GFP_NO_OBJ_EXT usage from alloc_slab_obj_exts()
>       mm/slab: replace __GFP_NO_OBJ_EXT with SLAB_ALLOC_NO_RECURSE for sheaves
>       mm: remove the __GFP_NO_OBJ_EXT flag
> 
>  include/linux/gfp_types.h       |   7 -
>  include/linux/slab.h            |  14 +-
>  include/trace/events/mmflags.h  |  10 +-
>  lib/alloc_tag.c                 |   2 +-
>  mm/kfence/core.c                |   6 +-
>  mm/memcontrol.c                 |   5 +-
>  mm/slab.h                       |  16 +-
>  mm/slub.c                       | 423 ++++++++++++++++++++++++----------------
>  tools/include/linux/gfp_types.h |   7 -
>  9 files changed, 288 insertions(+), 202 deletions(-)
> ---
> base-commit: 500b2c9755301742bdbb61249511ac11a4665dae
> change-id: 20260601-slab_alloc_flags-25c782b0c57c
> 
> Best regards,
> --  
> Vlastimil Babka (SUSE) <vbabka@kernel.org>
> 
>