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From:   Dmitry Vyukov <dvyukov@google.com>
Date:   Tue, 21 Sep 2021 12:51:21 +0200
Message-ID: <CACT4Y+aUUNFvVsA86D280e4JqaQ4UdesMnG-+DVc=9v59_ZsJA@mail.gmail.com>
Subject: Re: [PATCH v2 5/5] kfence: add note to documentation about skipping
 covered allocations
To:     Marco Elver <elver@google.com>
Cc:     Andrew Morton <akpm@linux-foundation.org>,
        Alexander Potapenko <glider@google.com>,
        Jann Horn <jannh@google.com>,
        Aleksandr Nogikh <nogikh@google.com>,
        Taras Madan <tarasmadan@google.com>,
        linux-kernel@vger.kernel.org, linux-mm@kvack.org,
        kasan-dev@googlegroups.com
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On Tue, 21 Sept 2021 at 12:10, Marco Elver <elver@google.com> wrote:
>
> Add a note briefly mentioning the new policy about "skipping currently
> covered allocations if pool close to full." Since this has a notable
> impact on KFENCE's bug-detection ability on systems with large uptimes,
> it is worth pointing out the feature.
>
> Signed-off-by: Marco Elver <elver@google.com>

Reviewed-by: Dmitry Vyukov <dvyukov@google.com>

> ---
> v2:
> * Rewrite.
> ---
>  Documentation/dev-tools/kfence.rst | 11 +++++++++++
>  1 file changed, 11 insertions(+)
>
> diff --git a/Documentation/dev-tools/kfence.rst b/Documentation/dev-tools/kfence.rst
> index 0fbe3308bf37..d45f952986ae 100644
> --- a/Documentation/dev-tools/kfence.rst
> +++ b/Documentation/dev-tools/kfence.rst
> @@ -269,6 +269,17 @@ tail of KFENCE's freelist, so that the least recently freed objects are reused
>  first, and the chances of detecting use-after-frees of recently freed objects
>  is increased.
>
> +If pool utilization reaches 75% (default) or above, to reduce the risk of the
> +pool eventually being fully occupied by allocated objects yet ensure diverse
> +coverage of allocations, KFENCE limits currently covered allocations of the
> +same source from further filling up the pool. The "source" of an allocation is
> +based on its partial allocation stack trace. A side-effect is that this also
> +limits frequent long-lived allocations (e.g. pagecache) of the same source
> +filling up the pool permanently, which is the most common risk for the pool
> +becoming full and the sampled allocation rate dropping to zero. The threshold
> +at which to start limiting currently covered allocations can be configured via
> +the boot parameter ``kfence.skip_covered_thresh`` (pool usage%).
> +
>  Interface
>  ---------
>
> --
> 2.33.0.464.g1972c5931b-goog
>