mm: kmemleak: default min_unref_scans to 2 for verbose auto-scan

Patch series "mm: kmemleak: default min_unref_scans to 2 for verbose
kernels", v2.

When CONFIG_DEBUG_KMEMLEAK_VERBOSE is set, which means the host is in
auto scan mode, set min_unref_scans to 2, avoiding false positives.

CONFIG_DEBUG_KMEMLEAK_VERBOSE depends on CONFIG_DEBUG_KMEMLEAK_AUTO_SCAN,
so a kernel built with it already runs the scan thread periodically and
the user has asked for detailed leak reports.

The confirming second scan comes for free there, so default
min_unref_scans to 2 in that case and keep it at 1 everywhere else.
CONFIG_DEBUG_KMEMLEAK_VERBOSE defaults to n, so nothing changes for
kernels that do not opt in.

The other two patches bring the documentation and the selftest comments
in line with the new conditional default.

PS: A similar patch (v1 of this patchset) is applied to Meta's kernel,
in real production hosts.


This patch (of 3):

min_unref_scans defers reporting an object as leaked until it has stayed
unreferenced for that many consecutive scans, filtering out objects that
are only transiently unreferenced during a scan.

It defaults to 1, which reports on the first unreferenced scan.

CONFIG_DEBUG_KMEMLEAK_VERBOSE depends on CONFIG_DEBUG_KMEMLEAK_AUTO_SCAN,
so a kernel built with it runs the scan thread periodically and the user
has opted into detailed leak reporting.  A second confirming scan then
happens on its own.

Default min_unref_scans to 2 there to suppress transient false positives,
and keep it at 1 otherwise, where a manually triggered scan is expected to
report immediately.  The value stays writable through the module
parameter.

CONFIG_DEBUG_KMEMLEAK_VERBOSE defaults to n, so this does not change the
default for kernels that do not opt in.

Link: https://lore.kernel.org/[email protected]
Link: https://lore.kernel.org/[email protected]
Signed-off-by: Breno Leitao <[email protected]>
Acked-by: Catalin Marinas <[email protected]>
Cc: David Hildenbrand <[email protected]>
Cc: Jonathan Corbet <[email protected]>
Cc: Liam R. Howlett <[email protected]>
Cc: Lorenzo Stoakes <[email protected]>
Cc: Michal Hocko <[email protected]>
Cc: Mike Rapoport <[email protected]>
Cc: Shuah Khan <[email protected]>
Cc: Suren Baghdasaryan <[email protected]>
Cc: Vlastimil Babka <[email protected]>
Signed-off-by: Andrew Morton <[email protected]>

mm/kmemleak.c

@@ -237,7 +237,8 @@ static struct task_struct *scan_thread;
 /* used to avoid reporting of recently allocated objects */
 static unsigned long jiffies_min_age;
 /* consecutive scans an object must stay unreferenced before reporting */
-static unsigned int min_unref_scans = 1;
+static unsigned int min_unref_scans =
+	IS_ENABLED(CONFIG_DEBUG_KMEMLEAK_VERBOSE) ? 2 : 1;
 module_param(min_unref_scans, uint, 0644);
 static unsigned long jiffies_last_scan;
 /* delay between automatic memory scannings */