[PATCH v3 00/10] stackdepot: store compressed frames in stack records

[Matt Fleming <matt@readmodwrite.com>]

From: Matt Fleming <mfleming@cloudflare.com>

Hi,

Cloudflare runs KASAN on pre-production servers, and on some of them
stack depot keeps running out of pools even with stack_depot_max_pools
raised to 65,536. Once the depot is full, new persistent traces get no
handle and later KASAN reports lose the allocation and free history.

v2 of this series [1] tackled that with a trie that shares common frame
prefixes. Marco asked how much of the saving came from compressed frames
alone [2]. It turns out we can get a decent saving with just frame
compression and none of the trie complexity.

When every frame of a persistent record can be compressed with the arch
hooks, the record stores 32-bit frames instead of unsigned longs.
Evictable records are always allocated at maximum size, so they stay
uncompressed.

On our production KASAN hosts this reduced stack depot pools per 1000
stacks by 41.5% on x86-64 and 27.1% on arm64. arm64 saves less because
we run a lot of BPF programs, and their code lives outside the +/-2 GiB
range around _text that can be compressed.

Compressed records aren't contiguous unsigned long arrays, so callers
can no longer borrow stack depot's storage. Patches 1-7 move every
stack_depot_fetch() user to stack_depot_fetch_into() or the print
helpers and then remove stack_depot_fetch(). Patch 8 adds the arch
hooks, patch 9 stores compressed frames, and patch 10 teaches the GDB
helper to decode them.

Changes in v3:
- Drop the trie backend, the stackdepot.trie_enabled boot parameter and
  the trie KUnit tests.
- Drop STACK_DEPOT_FLAG_COUNTABLE. page_owner reads traces with
  stack_depot_fetch_into() instead.
- Remove stack_depot_fetch() now that it has no users.
- stack_depot_print() and stack_depot_snprint() decompress one frame at
  a time instead of copying 16-frame chunks.
- Store compressed frames directly in hash records, using the u16 size
  and u16 flags header layout from v2's COUNTABLE patch.
- The GDB helper decodes compressed frames instead of rejecting trie
  handles.
- v2's patch 1 landed as 8f56ea8e0706 ("stackdepot: stop preallocating
  after the final pool").

Thanks to Caleb, who wrote v1 and v2.

[1] https://lore.kernel.org/all/20260908-stackdepot-trie-v2-0-1996d5cef732@cloudflare.com/
[2] https://lore.kernel.org/r/CANpmjNMP=NdD8ZE9rPwikUBcLwc8V=J648X05YGPn41yAZ5hmw@mail.gmail.com/

Caleb Kan (6):
  stackdepot: add caller-owned stack trace fetching
  mm/kmemleak: use stack_depot_fetch_into() for leak reports
  kmsan: use stack_depot_fetch_into() for origin reports
  mm/slub: use stack_depot_fetch_into() for owner tracking
  drm/locking: print deadlock backtraces with stack_depot_snprint()
  stackdepot: add architecture hooks for compact frame storage

Matt Fleming (4):
  mm/page_owner: Read stack traces with stack_depot_fetch_into()
  stackdepot: Remove stack_depot_fetch()
  stackdepot: Store compressed frames in stack records
  scripts/gdb: Decode compressed stack depot frames

 arch/arm64/include/asm/stackdepot.h |  37 ++++
 arch/um/include/asm/Kbuild          |   1 +
 arch/x86/include/asm/stackdepot.h   |  37 ++++
 drivers/gpu/drm/drm_modeset_lock.c  |   5 +-
 include/asm-generic/Kbuild          |   1 +
 include/asm-generic/stackdepot.h    |  19 ++
 include/linux/stackdepot.h          |  45 +++-
 lib/Kconfig.debug                   |  16 ++
 lib/stackdepot.c                    | 164 ++++++++++++---
 lib/tests/Makefile                  |   1 +
 lib/tests/stackdepot_kunit.c        | 312 ++++++++++++++++++++++++++++
 mm/kmemleak.c                       |   4 +-
 mm/kmsan/kmsan_test.c               |   4 +-
 mm/kmsan/report.c                   |  28 +--
 mm/page_owner.c                     |   8 +-
 mm/slub.c                           |  12 +-
 scripts/gdb/linux/constants.py.in   |   5 +
 scripts/gdb/linux/stackdepot.py     |  23 +-
 18 files changed, 650 insertions(+), 72 deletions(-)
 create mode 100644 arch/arm64/include/asm/stackdepot.h
 create mode 100644 arch/x86/include/asm/stackdepot.h
 create mode 100644 include/asm-generic/stackdepot.h
 create mode 100644 lib/tests/stackdepot_kunit.c

-- 
2.43.0