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Breno Leitao e776db8e71 mm: kmemleak: report leaks only after N consecutive unreferenced scans

kmemleak reports an object the first scan it is found unreferenced.  Its
mark phase runs without stopping the rest of the kernel and without a
write barrier, so a live object whose only reference is briefly invisible
during a concurrent RCU update -- e.g.  a VMA moved between maple tree
nodes, or a page-cache xa_node -- can be seen as unreferenced for that one
scan.  Because an object is flagged as reported only once, such a
transient race turns into a permanent false positive.

Track how many consecutive scans each object has been seen unreferenced
and only report it once that reaches min_unref_scans, a new module
parameter.  It defaults to 1, leaving the behaviour unchanged; setting it
higher (e.g.  2) still reports a genuine leak, one scan later, while an
object referenced again before the threshold restarts its run and is never
reported.

min_unref_scans can be set at boot with kmemleak.min_unref_scans=<n> or at
run-time via /sys/module/kmemleak/parameters/min_unref_scans.

Link: https://lore.kernel.org/20260713-catalin_pto-v1-2-5b93b1131089@debian.org
Signed-off-by: Breno Leitao <leitao@debian.org>
Reviewed-by: Catalin Marinas <catalin.marinas@arm.com>
Cc: David Hildenbrand <david@kernel.org>
Cc: Geert Uytterhoeven <geert@linux-m68k.org>
Cc: Jonathan Corbet <corbet@lwn.net>
Cc: Liam R. Howlett <liam@infradead.org>
Cc: Lorenzo Stoakes <ljs@kernel.org>
Cc: Michal Hocko <mhocko@suse.com>
Cc: Mike Rapoport <rppt@kernel.org>
Cc: Shuah Khan <shuah@kernel.org>
Cc: Suren Baghdasaryan <surenb@google.com>
Cc: Vlastimil Babka <vbabka@kernel.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>