[PATCH bpf-next v2 0/4] bpf: arena: handle memory.max on fault-in with reclaim/OOM

Since commit e66fe1bc6d25 ("bpf: arena: Reintroduce memcg accounting"),
arena pages are charged to the memcg of the process that created the arena.
That accounting exposes two problems in the arena user page fault path.

1. The fault-in allocation runs under arena->spinlock, so it can only use the
   non-blocking allocator, which never reclaims. Once memory.current is at
   memory.max the allocation simply fails. Reaching memory.max is completely
   normal for a healthy application - e.g. reading a large file fills
   memory.current with page cache - so the process ends up killed for no real
   reason.

2. That failure is turned into VM_FAULT_SIGSEGV, which is misleading: the
   faulting address is a perfectly valid arena address. When we know it is an
   out-of-memory condition we can return VM_FAULT_OOM and let the memcg OOM
   path handle it properly.

Preallocate the page outside the lock (patch 2), the way do_anonymous_page()
does, so the allocation can sleep and go through reclaim and the OOM path.
This needs a sleepable allocator (patch 1), because can_alloc_pages() is a
conservative guess for BPF program context and always forces the non-blocking
allocator under PREEMPT_RT. patch 3&4 adds a selftest that faults an arena in
under a memory.max limit: without the fix the child gets SIGSEGV on a valid
address, with it the child is killed by the memcg OOM killer.


v1 -> v2:
   - Rebase on the separate deadlock fix (found by the Sashiko AI review),
     now applied to bpf-next.
   - Honor the map's NUMA node on fault-in.
   - Return VM_FAULT_SIGBUS for the non-recoverable faults (lock, range-tree
     and page-table failures); a scratch-page hole stays VM_FAULT_SIGSEGV
     only under BPF_F_SEGV_ON_FAULT. (Kumar Kartikeya Dwivedi)
   - Add read_cgroup_file() to cgroup_helpers instead of open-coding the
     /mnt/... path in the test. (Emil Tsalapatis)
   - Dump the cgroup memory stats on test failure to ease debugging.

v1:
https://lore.kernel.org/bpf/[email protected]/

Jiayuan Chen (4):
  bpf: Add a sleepable page allocator for map memory
  bpf: arena: allocate the fault-in page outside the lock
  selftests/bpf: Add read_cgroup_file() to cgroup_helpers
  selftests/bpf: Add a test for arena fault-in under memory.max

 include/linux/bpf.h                           |   1 +
 kernel/bpf/arena.c                            |  92 +++++++++---
 kernel/bpf/syscall.c                          |  21 ++-
 tools/testing/selftests/bpf/cgroup_helpers.c  |  67 +++++++++
 tools/testing/selftests/bpf/cgroup_helpers.h  |   4 +
 .../selftests/bpf/prog_tests/arena_memcg.c    | 139 ++++++++++++++++++
 .../testing/selftests/bpf/progs/arena_memcg.c |  24 +++
 7 files changed, 323 insertions(+), 25 deletions(-)
 create mode 100644 tools/testing/selftests/bpf/prog_tests/arena_memcg.c
 create mode 100644 tools/testing/selftests/bpf/progs/arena_memcg.c

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2.43.0