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