Re: [PATCH net-next 0/6] mptcp: misc. features for v6.18

[Matthieu Baerts <matttbe@kernel.org>]

2 Sept 2025 23:18:56 Catalin Marinas <catalin.marinas@arm.com>:

> On Tue, Sep 02, 2025 at 08:50:19PM +0200, Matthieu Baerts wrote:
>> Hi Catalin,
>>
>> 2 Sept 2025 20:25:19 Catalin Marinas <catalin.marinas@arm.com>:
>>
>>> On Tue, Sep 02, 2025 at 08:27:59AM -0700, Jakub Kicinski wrote:
>>>> On Tue, 2 Sep 2025 16:51:47 +0200 Matthieu Baerts wrote:
>>>>> It is unclear why a second scan is needed and only the second one caught
>>>>> something. Was it the same with the strange issues you mentioned in
>>>>> driver tests? Do you think I should re-add the second scan + cat?
>>>>
>>>> Not sure, cc: Catalin, from experience it seems like second scan often
>>>> surfaces issues the first scan missed.
>>>
>>> It's some of the kmemleak heuristics to reduce false positives. It does
>>> a checksum of the object during scanning and only reports a leak if the
>>> checksum is the same in two consecutive scans.
>>
>> Thank you for the explanation!
>>
>> Does that mean a scan should be triggered at the end of the tests,
>> then wait 5 second for the grace period, then trigger another scan
>> and check the results?
>>
>> Or wait 5 seconds, then trigger two consecutive scans?
>
> The 5 seconds is the minimum age of an object before it gets reported as
> a leak. It's not related to the scanning process. So you could do two
> scans in succession and wait 5 seconds before checking for leaks.
>
> However, I'd go with the first option - do a scan, wait 5 seconds and do
> another. That's mostly because at the end of the scan kmemleak prints if
> it found new unreferenced objects. It might not print the message if a
> leaked object is younger than 5 seconds. In practice, though, the scan
> may take longer, depending on how loaded your system is.
>
> The second option works as well but waiting between them has a better
> chance of removing false positives if, say, some objects are moved
> between lists and two consecutive scans do not detect the list_head
> change (and update the object's checksum).

Thank you for this very nice reply, that's very clear!

I will then adapt our CI having CONFIG_DEBUG_KMEMLEAK_DEFAULT_OFF
to do a manual scan at the very end, wait 5 seconds and do another.

Cheers,
Matt