Re: [PATCH 1/2] mm/kmemleak: report leaks only after N consecutive unreferenced scans

[Catalin Marinas <catalin.marinas@arm.com>]

On Fri, Jun 26, 2026 at 08:52:02AM -0700, Breno Leitao wrote:
> 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.
> 
> Signed-off-by: Breno Leitao <leitao@debian.org>

It looks like a good addition to me. All objects require a second pass
initially to get their checksum updated but that's not sufficient when
they are moved between nodes without having their content changed (list
in a linked list).

Reviewed-by: Catalin Marinas <catalin.marinas@arm.com>

> diff --git a/mm/kmemleak.c b/mm/kmemleak.c
> index 7c7ba17ce7af0..5b14ccb36f95b 100644
> --- a/mm/kmemleak.c
> +++ b/mm/kmemleak.c
> @@ -151,6 +151,8 @@ struct kmemleak_object {
>  	int min_count;
>  	/* the total number of pointers found pointing to this object */
>  	int count;
> +	/* consecutive scans the object has been seen unreferenced */
> +	unsigned int unref_scans;
>  	/* checksum for detecting modified objects */
>  	u32 checksum;
>  	depot_stack_handle_t trace_handle;
> @@ -232,6 +234,9 @@ static unsigned long max_percpu_addr;
>  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;
> +module_param(min_unref_scans, uint, 0644);

0644 is fine. Not sure why kmemleak_verbose was 0600.

-- 
Catalin