On Thu, Aug 5, 2021 at 10:43 PM Hugh Dickins <hughd@xxxxxxxxxx> wrote: > > 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. The other question is if that optimization is worth it nowadays or not. I bet not too many users build NUMA=n kernel nowadays even though the kernel is actually running on a non-NUMA machine. Some small devices may run NUMA=n kernel, but I don't think they actually use THP. So such code complexity could be removed from this point of view too. > > > > > 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. Yeah, probably. > > Hugh
