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References: <20210923104803.2620285-1-elver@google.com> <20210923104803.2620285-5-elver@google.com>
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From:   Alexander Potapenko <glider@google.com>
Date:   Thu, 23 Sep 2021 17:46:51 +0200
Message-ID: <CAG_fn=XGFY4kWSzTa4kX4Y0CPOpvQfhBzgZFK184ZptzyC6-CA@mail.gmail.com>
Subject: Re: [PATCH v3 5/5] kfence: add note to documentation about skipping
 covered allocations
To:     Marco Elver <elver@google.com>
Cc:     Andrew Morton <akpm@linux-foundation.org>,
        Dmitry Vyukov <dvyukov@google.com>,
        Jann Horn <jannh@google.com>,
        Aleksandr Nogikh <nogikh@google.com>,
        Taras Madan <tarasmadan@google.com>,
        LKML <linux-kernel@vger.kernel.org>,
        Linux Memory Management List <linux-mm@kvack.org>,
        kasan-dev <kasan-dev@googlegroups.com>
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On Thu, Sep 23, 2021 at 12:48 PM 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>
Acked-by: Alexander Potapenko <glider@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 recentl=
y freed objects are reused
>  first, and the chances of detecting use-after-frees of recently freed ob=
jects
>  is increased.
>
> +If pool utilization reaches 75% (default) or above, to reduce the risk o=
f the
> +pool eventually being fully occupied by allocated objects yet ensure div=
erse
> +coverage of allocations, KFENCE limits currently covered allocations of =
the
> +same source from further filling up the pool. The "source" of an allocat=
ion 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 sour=
ce
> +filling up the pool permanently, which is the most common risk for the p=
ool
> +becoming full and the sampled allocation rate dropping to zero. The thre=
shold
> +at which to start limiting currently covered allocations can be configur=
ed via
> +the boot parameter ``kfence.skip_covered_thresh`` (pool usage%).
> +
>  Interface
>  ---------
>
> --
> 2.33.0.464.g1972c5931b-goog
>


--=20
Alexander Potapenko
Software Engineer

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