On Thu, 5 Aug 2021, Yang Shi wrote: > > By rereading the code, I think you are correct. Both cases do work > correctly without leaking. And the !CONFIG_NUMA case may carry the > huge page indefinitely. > > I think it is because khugepaged may collapse memory for another NUMA > node in the next loop, so it doesn't make too much sense to carry the > huge page, but it may be an optimization for !CONFIG_NUMA case. Yes, that is its intention. > > However, as I mentioned in earlier email the new pcp implementation > could cache THP now, so we might not need keep this convoluted logic > anymore. Just free the page if collapse is failed then re-allocate > THP. The carried THP might improve the success rate a little bit but I > doubt how noticeable it would be, may be not worth for the extra > complexity at all. It would be great if the new pcp implementation is good enough to get rid of khugepaged's confusing NUMA=y/NUMA=n differences; and all the *hpage stuff too, I hope. That would be a welcome cleanup. > > > Collapse failure is not uncommon and leaking huge pages gets noticed. After writing that, I realized how I'm almost always testing a NUMA=y kernel (though on non-NUMA machines), and seldom try the NUMA=n build. So did so to check no leak, indeed; but was surprised, when comparing vmstats, that the NUMA=n run had done 5 times as much thp_collapse_alloc as the NUMA=y run. I've merely made a note to look into that one day: maybe it was just a one-off oddity, or maybe the incrementing of stats is wrong down one path or the other. Hugh
