On Tue, 15 Dec 2020 12:54:36 +0100 SeongJae Park [off-list ref] wrote:

From: SeongJae Park <redacted>

Even somehow the initial monitoring target regions are well constructed
to fulfill the assumption (pages in same region have similar access
frequencies), the data access pattern can be dynamically changed.  This
will result in low monitoring quality.  To keep the assumption as much
as possible, DAMON adaptively merges and splits each region based on
their access frequency.

For each ``aggregation interval``, it compares the access frequencies of
adjacent regions and merges those if the frequency difference is small.
Then, after it reports and clears the aggregated access frequency of
each region, it splits each region into two or three regions if the
total number of regions will not exceed the user-specified maximum
number of regions after the split.

In this way, DAMON provides its best-effort quality and minimal overhead
while keeping the upper-bound overhead that users set.

Signed-off-by: SeongJae Park <redacted>
Reviewed-by: Leonard Foerster <redacted>
---
 include/linux/damon.h |  41 +++++---
 mm/damon/core.c       | 220 ++++++++++++++++++++++++++++++++++++++++--
 2 files changed, 240 insertions(+), 21 deletions(-)

[...]

[...]

+
+/*
+ * Split a region in two
+ *
+ * r		the region to be split
+ * sz_r		size of the first sub-region that will be made
+ */
+static void damon_split_region_at(struct damon_ctx *ctx,
+				  struct damon_region *r, unsigned long sz_r)
+{
+	struct damon_region *new;
+
+	new = damon_new_region(r->ar.start + sz_r, r->ar.end);

Coverity Static Analysis Security Testing (SAST) by Synopsys, Inc. found that
'damon_new_region()' could return NULL in case of memory allocation failure,
but NULL check for 'new' is missed here.  I will add the check in the next
version.

+	r->ar.end = new->ar.start;
+
+	damon_insert_region(new, r, damon_next_region(r));
+}

[...]


Thanks,
SeongJae Park