kasan: allow sampling page_alloc allocations for HW_TAGS

As Hardware Tag-Based KASAN is intended to be used in production, its
performance impact is crucial.  As page_alloc allocations tend to be big,
tagging and checking all such allocations can introduce a significant
slowdown.

Add two new boot parameters that allow to alleviate that slowdown:

- kasan.page_alloc.sample, which makes Hardware Tag-Based KASAN tag only
  every Nth page_alloc allocation with the order configured by the second
  added parameter (default: tag every such allocation).

- kasan.page_alloc.sample.order, which makes sampling enabled by the first
  parameter only affect page_alloc allocations with the order equal or
  greater than the specified value (default: 3, see below).

The exact performance improvement caused by using the new parameters
depends on their values and the applied workload.

The chosen default value for kasan.page_alloc.sample.order is 3, which
matches both PAGE_ALLOC_COSTLY_ORDER and SKB_FRAG_PAGE_ORDER.  This is
done for two reasons:

1. PAGE_ALLOC_COSTLY_ORDER is "the order at which allocations are deemed
   costly to service", which corresponds to the idea that only large and
   thus costly allocations are supposed to sampled.

2. One of the workloads targeted by this patch is a benchmark that sends
   a large amount of data over a local loopback connection. Most multi-page
   data allocations in the networking subsystem have the order of
   SKB_FRAG_PAGE_ORDER (or PAGE_ALLOC_COSTLY_ORDER).

When running a local loopback test on a testing MTE-enabled device in sync
mode, enabling Hardware Tag-Based KASAN introduces a ~50% slowdown. 
Applying this patch and setting kasan.page_alloc.sampling to a value
higher than 1 allows to lower the slowdown.  The performance improvement
saturates around the sampling interval value of 10 with the default
sampling page order of 3.  This lowers the slowdown to ~20%.  The slowdown
in real scenarios involving the network will likely be better.

Enabling page_alloc sampling has a downside: KASAN misses bad accesses to
a page_alloc allocation that has not been tagged.  This lowers the value
of KASAN as a security mitigation.

However, based on measuring the number of page_alloc allocations of
different orders during boot in a test build, sampling with the default
kasan.page_alloc.sample.order value affects only ~7% of allocations.  The
rest ~93% of allocations are still checked deterministically.

Link: https://lkml.kernel.org/r/129da0614123bb85ed4dd61ae30842b2dd7c903f.1671471846.git.andreyknvl@google.com
Signed-off-by: Andrey Konovalov <andreyknvl@google.com>
Reviewed-by: Marco Elver <elver@google.com>
Cc: Alexander Potapenko <glider@google.com>
Cc: Andrey Ryabinin <ryabinin.a.a@gmail.com>
Cc: Dmitry Vyukov <dvyukov@google.com>
Cc: Evgenii Stepanov <eugenis@google.com>
Cc: Jann Horn <jannh@google.com>
Cc: Mark Brand <markbrand@google.com>
Cc: Peter Collingbourne <pcc@google.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>

1 files changed, 17 insertions, 0 deletions

diff --git a/Documentation/dev-tools/kasan.rst b/Documentation/dev-tools/kasan.rst
index 5c93ab915049..e66916a483cd 100644
--- a/Documentation/dev-tools/kasan.rst
+++ b/Documentation/dev-tools/kasan.rst
@@ -140,6 +140,23 @@ disabling KASAN altogether or controlling its features:
 - ``kasan.vmalloc=off`` or ``=on`` disables or enables tagging of vmalloc
   allocations (default: ``on``).
 
+- ``kasan.page_alloc.sample=<sampling interval>`` makes KASAN tag only every
+  Nth page_alloc allocation with the order equal or greater than
+  ``kasan.page_alloc.sample.order``, where N is the value of the ``sample``
+  parameter (default: ``1``, or tag every such allocation).
+  This parameter is intended to mitigate the performance overhead introduced
+  by KASAN.
+  Note that enabling this parameter makes Hardware Tag-Based KASAN skip checks
+  of allocations chosen by sampling and thus miss bad accesses to these
+  allocations. Use the default value for accurate bug detection.
+
+- ``kasan.page_alloc.sample.order=<minimum page order>`` specifies the minimum
+  order of allocations that are affected by sampling (default: ``3``).
+  Only applies when ``kasan.page_alloc.sample`` is set to a value greater
+  than ``1``.
+  This parameter is intended to allow sampling only large page_alloc
+  allocations, which is the biggest source of the performance overhead.
+
 Error reports
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