On 2023-10-23 18:22, andrey.konovalov@xxxxxxxxx wrote: > From: Andrey Konovalov <andreyknvl@xxxxxxxxxx> > > Currently, the stack depot grows indefinitely until it reaches its > capacity. Once that happens, the stack depot stops saving new stack > traces. > > This creates a problem for using the stack depot for in-field testing > and in production. > > For such uses, an ideal stack trace storage should: > > 1. Allow saving fresh stack traces on systems with a large uptime while > limiting the amount of memory used to store the traces; > 2. Have a low performance impact. > > Implementing #1 in the stack depot is impossible with the current > keep-forever approach. This series targets to address that. Issue #2 is > left to be addressed in a future series. > > This series changes the stack depot implementation to allow evicting > unneeded stack traces from the stack depot. The users of the stack depot > can do that via new stack_depot_save_flags(STACK_DEPOT_FLAG_GET) and > stack_depot_put APIs. > > Internal changes to the stack depot code include: > > 1. Storing stack traces in fixed-frame-sized slots; the slot size is > controlled via CONFIG_STACKDEPOT_MAX_FRAMES (vs precisely-sized > slots in the current implementation); > 2. Keeping available slots in a freelist (vs keeping an offset to the next > free slot); > 3. Using a read/write lock for synchronization (vs a lock-free approach > combined with a spinlock). > > This series also integrates the eviction functionality in the tag-based > KASAN modes. > > Despite wasting some space on rounding up the size of each stack record, > with CONFIG_STACKDEPOT_MAX_FRAMES=32, the tag-based KASAN modes end up > consuming ~5% less memory in stack depot during boot (with the default > stack ring size of 32k entries). The reason for this is the eviction of > irrelevant stack traces from the stack depot, which frees up space for > other stack traces. > > For other tools that heavily rely on the stack depot, like Generic KASAN > and KMSAN, this change leads to the stack depot capacity being reached > sooner than before. However, as these tools are mainly used in fuzzing > scenarios where the kernel is frequently rebooted, this outcome should > be acceptable. > > There is no measurable boot time performance impact of these changes for > KASAN on x86-64. I haven't done any tests for arm64 modes (the stack > depot without performance optimizations is not suitable for intended use > of those anyway), but I expect a similar result. Obtaining and copying > stack trace frames when saving them into stack depot is what takes the > most time. > > This series does not yet provide a way to configure the maximum size of > the stack depot externally (e.g. via a command-line parameter). This will > be added in a separate series, possibly together with the performance > improvement changes. > > --- > > 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 > 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 > 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 Tested-by: Anders Roxell <anders.roxell@xxxxxxxxxx> Applied this patchset to linux-next tag next-20231023 and built an arm64 kernel and that booted fine in QEMU. Cheers, Anders
