[PATCH 0/2] mm: preserve nolock context through memcg cleanup

[Karl Mehltretter]

The no-lock allocation and free APIs can still enter regular locking
through two memcg cleanup paths.

Patch 1 handles rollback after a post-allocation charge failure. If
kmalloc_nolock() obtains an object but its memcg charge fails,
memcg_slab_post_alloc_hook() frees the object through the regular SLUB
path. Preserve the selected allocation mode by using kfree_nolock()
when the allocation flags disallow spinning.

Patch 2 handles the final reference to a killed object cgroup.
kfree_nolock() and free_pages_nolock() can drop that reference in the
caller's context, after which obj_cgroup_release() takes regular locks.
Queue the objcg on a lockless list and finish its teardown from normal
irq_work. On PREEMPT_RT that work runs in irq_workd task context.

These paths were found while discussing the following RFC and during
the subsequent investigation of no-lock allocation and free paths:

https://lore.kernel.org/r/20260919171443.90512-1-kmehltretter@xxxxxxxxx

Based on 40288c9206c17. Each patch's forced cleanup test passed on
x86-64 release and lockdep PREEMPT_RT QEMU, arm64 and ARM32 SMP QEMU,
and Pi 400 hardware. The objcg test accounted for all 256 queued and
deferred releases. The series also passed non-RT builds and arena tests.

Karl Mehltretter (2):
  mm/slub: preserve no-lock freeing after memcg charge failure
  mm/memcontrol: defer final objcg release from no-lock frees

 include/linux/memcontrol.h |  1 +
 mm/memcontrol.c            | 29 ++++++++++++++++++++++++++---
 mm/slub.c                  |  5 ++++-
 3 files changed, 31 insertions(+), 4 deletions(-)


base-commit: 40288c9206c17eb66a603262e06a58d300d0f279
-- 
2.53.0