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

[Christoph Paasch <cpaasch@openai.com>]

On Tue, Sep 2, 2025 at 2:38 PM Matthieu Baerts <matttbe@kernel.org> wrote:
>
> 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.

FWIW - I am able to pretty reliably reproduce the kmemleak. However, I
also tried adding an inline kmemleak scan to the test harness (did it
once with, once without a sleep). When I do that the kmemleak
disappears :-)

(not saying that adding the scan isn't useful, just pointing out that
this particular leak seems to be related to how quickly we iterate
over the testcases)


Christoph