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kfence: add note to documentation about skipping covered allocations

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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.

Link: https://lkml.kernel.org/r/20210923104803.2620285-5-elver@google.com
Signed-off-by: Marco Elver <elver@google.com>
Reviewed-by: Dmitry Vyukov <dvyukov@google.com>
Acked-by: Alexander Potapenko <glider@google.com>
Cc: Aleksandr Nogikh <nogikh@google.com>
Cc: Jann Horn <jannh@google.com>
Cc: Taras Madan <tarasmadan@google.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>
This commit is contained in:
Marco Elver 2021-11-05 13:45:37 -07:00 committed by Linus Torvalds
parent 08f6b10630
commit 5cc906b4b4
1 changed files with 11 additions and 0 deletions
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11
Documentation/dev-tools/kfence.rst
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@ -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
---------