Re: [PATCH v3 00/19] stackdepot: allow evicting stack traces

[Marco Elver <elver@google.com>]

On Mon, 23 Oct 2023 at 18:22, <andrey.konovalov@linux.dev> wrote:
[...]
> ---
>
> Changes v2->v3:
> - Fix null-ptr-deref by using the proper number of entries for
>   initializing the stack table when alloc_large_system_hash()
>   auto-calculates the number (see patch #12).
> - Keep STACKDEPOT/STACKDEPOT_ALWAYS_INIT Kconfig options not configurable
>   by users.
> - Use lockdep_assert_held_read annotation in depot_fetch_stack.
> - WARN_ON invalid flags in stack_depot_save_flags.
> - Moved "../slab.h" include in mm/kasan/report_tags.c in the right patch.
> - Various comment fixes.
>
> Changes v1->v2:
> - Rework API to stack_depot_save_flags(STACK_DEPOT_FLAG_GET) +
>   stack_depot_put.
> - Add CONFIG_STACKDEPOT_MAX_FRAMES Kconfig option.
> - Switch stack depot to using list_head's.
> - Assorted minor changes, see the commit message for each path.
>
> Andrey Konovalov (19):
>   lib/stackdepot: check disabled flag when fetching
>   lib/stackdepot: simplify __stack_depot_save
>   lib/stackdepot: drop valid bit from handles
>   lib/stackdepot: add depot_fetch_stack helper
>   lib/stackdepot: use fixed-sized slots for stack records

1. I know fixed-sized slots are need for eviction to work, but have
you evaluated if this causes some excessive memory waste now? Or is it
negligible?

If it turns out to be a problem, one way out would be to partition the
freelist into stack size classes; e.g. one for each of stack traces of
size 8, 16, 32, 64.

>   lib/stackdepot: fix and clean-up atomic annotations
>   lib/stackdepot: rework helpers for depot_alloc_stack
>   lib/stackdepot: rename next_pool_required to new_pool_required
>   lib/stackdepot: store next pool pointer in new_pool
>   lib/stackdepot: store free stack records in a freelist
>   lib/stackdepot: use read/write lock

2. I still think switching to the percpu_rwsem right away is the right
thing, and not actually a downside. I mentioned this before, but you
promised a follow-up patch, so I trust that this will happen. ;-)

>   lib/stackdepot: use list_head for stack record links
>   kmsan: use stack_depot_save instead of __stack_depot_save
>   lib/stackdepot, kasan: add flags to __stack_depot_save and rename
>   lib/stackdepot: add refcount for records
>   lib/stackdepot: allow users to evict stack traces
>   kasan: remove atomic accesses to stack ring entries
>   kasan: check object_size in kasan_complete_mode_report_info
>   kasan: use stack_depot_put for tag-based modes
>
>  include/linux/stackdepot.h |  59 ++++--
>  lib/Kconfig                |  10 +
>  lib/stackdepot.c           | 418 ++++++++++++++++++++++++-------------
>  mm/kasan/common.c          |   7 +-
>  mm/kasan/generic.c         |   9 +-
>  mm/kasan/kasan.h           |   2 +-
>  mm/kasan/report_tags.c     |  27 +--
>  mm/kasan/tags.c            |  24 ++-
>  mm/kmsan/core.c            |   7 +-
>  9 files changed, 365 insertions(+), 198 deletions(-)

Acked-by: Marco Elver <elver@google.com>

The series looks good in its current state. However, see my 2
higher-level comments above.