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). -- Catalin
