[PATCH RFC hotfixes 0/2] mm/slab: fix unbounded recursion in free path with memalloc profiling

[Harry Yoo (Oracle)]

This is a follow-up fix after the recent discussion [1].
Based on slab/for-next (b0b6ec46e025f) and available at
git.kernel.org [2].

Instead preventing cycles by bumping up the allocation size of obj_exts
arrays, it introduces a new kmalloc type called KMALLOC_NO_OBJ_EXT and
disallow formation of cycles between kmalloc types when allocating
obj_exts arrays. obj_exts arrays of normal kmalloc caches are served
from KMALLOC_NO_OBJ_EXT caches (that don't have obj_exts), and all other
obj_exts arrays are served from normal kmalloc caches.

I tried to reuse SLAB_ALLOC_NO_RECURSE to make kmalloc_slab() select
KMALLOC_NO_OBJ_EXT, but it was not great because it does not allow
sheaves for those caches. So I introduced a new slab alloc flag
SLAB_ALLOC_NO_OBJ_EXT.

To avoid huge confusion, I had to decouple "disallowing sheaves"
semantics from SLAB_NO_OBJ_EXT and introduced SLAB_NO_SHEAVES.

While this cannot be directly backported to v6.18 and v6.12 due to lack
of SLAB_ALLOC_* flags and kmalloc_flags(), I don't this will be
particularily challenging to backport it. Instead of a new slab alloc
flag, we can use __GFP_NO_OBJ_EXT to select KMALLOC_NO_OBJ_EXT as
kmalloc caches don't have sheaves in v6.18 anyway.

[1] https://lore.kernel.org/linux-mm/9a139365-28e6-4f1e-b35b-7f6091e9aa14@xxxxxxxxxx

[2] https://git.kernel.org/pub/scm/linux/kernel/git/harry/linux.git/log/?h=kmalloc-no-objext-rfc-v1r3

Signed-off-by: Harry Yoo (Oracle) <harry@xxxxxxxxxx>
---
Harry Yoo (Oracle) (2):
      mm/slab: decouple SLAB_NO_SHEAVES from SLAB_NO_OBJ_EXT
      mm/slab: prevent unbounded recursion in free path with new kmalloc type

 include/linux/slab.h | 16 +++++++--
 mm/slab.h            | 17 ++++++++--
 mm/slab_common.c     | 18 +++++++++-
 mm/slub.c            | 93 ++++++++++++++++++++++------------------------------
 4 files changed, 85 insertions(+), 59 deletions(-)
---
base-commit: b0b6ec46e025fd46c344915a42bc535d9b15a1fb
change-id: 20260702-kmalloc-no-objext-2619c1e06083

Best regards,
-- 
Harry Yoo (Oracle) <harry@xxxxxxxxxx>