[PATCH slab/for-next-fixes v2 0/3] 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].
See patch 2 for the detailed description on the bug.

Based on slab/for-next-fixes (72bb229f916) and will be soon 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-v2r2

[ "v2r2" because after exposing v2r1 I adjusted some of Vlastimil's
  feedback and fixed a memory leak due to bootstrapping twice when
  kmalloc caches are aliased ]

Signed-off-by: Harry Yoo (Oracle) <harry@xxxxxxxxxx>
---
Changes in v2:
- Fix a memory leak when kmalloc caches are aliased (patch 1).
  This is included as part of this series as patch 3 makes it
  easier to trigger the leak.

- Fix a warning in MEM_ALLOC_PROFILING_DEBUG by dropping
  mark_obj_codetag_empty() for obj_exts. Sashiko raised this and
  I reproduced it and confirmed that the warnings are gone.
  https://sashiko.dev/#/patchset/20260702-kmalloc-no-objext-v1-0-167175008538%40kernel.org

- Added R-b from Vlastimil on patch 2, thanks!
- Adjusted Vlastimil's feedback on RFC v1
- Link to v1: https://lore.kernel.org/r/20260702-kmalloc-no-objext-v1-0-167175008538@xxxxxxxxxx

---
Harry Yoo (Oracle) (3):
      mm/slab: fix a memory leak due to bootstrapping sheaves twice
      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            |  15 +++++--
 mm/slab_common.c     |  18 ++++++++-
 mm/slub.c            | 109 +++++++++++++++++++++------------------------------
 4 files changed, 88 insertions(+), 70 deletions(-)
---
base-commit: 72bb229f9161a1efcd5df32141b69fcc6ae81a13
change-id: 20260702-kmalloc-no-objext-2619c1e06083

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