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Date:   Tue, 24 Oct 2023 11:32:43 +0200
From:   Anders Roxell <anders.roxell@linaro.org>
To:     andrey.konovalov@linux.dev
Cc:     Marco Elver <elver@google.com>,
        Alexander Potapenko <glider@google.com>,
        Andrey Konovalov <andreyknvl@gmail.com>,
        Dmitry Vyukov <dvyukov@google.com>,
        Vlastimil Babka <vbabka@suse.cz>, kasan-dev@googlegroups.com,
        Evgenii Stepanov <eugenis@google.com>,
        Oscar Salvador <osalvador@suse.de>,
        Andrew Morton <akpm@linux-foundation.org>, linux-mm@kvack.org,
        linux-kernel@vger.kernel.org,
        Andrey Konovalov <andreyknvl@google.com>
Subject: Re: [PATCH v3 00/19] stackdepot: allow evicting stack traces
Message-ID: <20231024093243.GA3298341@mutt>
References: <cover.1698077459.git.andreyknvl@google.com>
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On 2023-10-23 18:22, andrey.konovalov@linux.dev wrote:
> From: Andrey Konovalov <andreyknvl@google.com>
> 
> 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@linaro.org>

Applied this patchset to linux-next tag next-20231023 and built an arm64
kernel and that
booted fine in QEMU.

Cheers,
Anders