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

[Vlastimil Babka (SUSE)]

On 6/9/26 15:35, Usama Arif wrote:
> On Tue, 09 Jun 2026 11:17:45 +0200 "Vlastimil Babka (SUSE)" <vbabka@xxxxxxxxxx> 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

Thanks for the heads up. I missed it because my series is based on
slab/for-next and that commit is in mm-unstable. My patch 15 actually
modifies the TODO comment that is meanwhile resolved by Hao Ge's patch.

Which means my patch 15/15 can't be used as-is, and at worst I will drop it.
But I'd encourage Hao Ge with Suren to find some way to avoid the gfp flag
usage too, because it's now quite a niche use case (preventing false
positive CONFIG_MEM_ALLOC_PROFILING_DEBUG warnings, IIUC?) to take a
valuable gfp flag bit, IMHO.

>> 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@xxxxxxxxxx>
>> ---
>> 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@xxxxxxxxxx>
>> 
>>