[209/262] kfence: add note to documentation about skipping covered allocations

Commit Message

Andrew Morton Nov. 5, 2021, 8:45 p.m. UTC

From: Marco Elver <[e-mail]>
Subject: kfence: add note to documentation about skipping covered allocations

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/[e-mail]
Signed-off-by: Marco Elver <[e-mail]>
Reviewed-by: Dmitry Vyukov <[e-mail]>
Acked-by: Alexander Potapenko <[e-mail]>
Cc: Aleksandr Nogikh <[e-mail]>
Cc: Jann Horn <[e-mail]>
Cc: Taras Madan <[e-mail]>
Signed-off-by: Andrew Morton <[e-mail]>
---

 Documentation/dev-tools/kfence.rst |   11 +++++++++++
 1 file changed, 11 insertions(+)

Patch

--- a/Documentation/dev-tools/kfence.rst~kfence-add-note-to-documentation-about-skipping-covered-allocations
+++ a/Documentation/dev-tools/kfence.rst
@@ -269,6 +269,17 @@  tail of KFENCE's freelist, so that the l
 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
 ---------