On Tue 23-01-18 21:08:36, Vinayak Menon wrote: > > > On 1/23/2018 8:25 PM, Michal Hocko wrote: > > [Please cc linux-api when proposing user interface] > > > > On Mon 22-01-18 11:10:14, Vinayak Menon wrote: > >> Based on Kirill's patch [1]. > >> > >> Currently, faultaround code produces young pte. This can screw up > >> vmscan behaviour[2], as it makes vmscan think that these pages are hot > >> and not push them out on first round. > >> > >> During sparse file access faultaround gets more pages mapped and all of > >> them are young. Under memory pressure, this makes vmscan swap out anon > >> pages instead, or to drop other page cache pages which otherwise stay > >> resident. > >> > >> Modify faultaround to produce old ptes if sysctl 'want_old_faultaround_pte' > >> is set, so they can easily be reclaimed under memory pressure. > >> > >> This can to some extend defeat the purpose of faultaround on machines > >> without hardware accessed bit as it will not help us with reducing the > >> number of minor page faults. > > So we just want to add a knob to cripple the feature? Isn't it better to > > simply disable it than to have two distinct implementation which is > > rather non-intuitive and I would bet that most users will be clueless > > about how to set it or when to touch it at all. So we will end up with > > random cargo cult hints all over internet giving you your performance > > back... > > > If you are talking about non-HW access bit systems, then yes it would be better to disable faultaround > when want_old_faultaround_pte is set to 1, like MInchan did here https://patchwork.kernel.org/patch/9115901/ > I can submit a patch for that. > > > I really dislike this new interface. If the fault around doesn't work > > for you then disable it. > > > Faultaround works well for me on systems with HW access bit. But > the benefit is reduced because of making the faultaround ptes young > [2]. Ideally they should be old as they are speculatively mapped and > not really accessed. But because of issues on certain architectures > they need to be made young[3][4]. This patch is trying to help the > other architectures which can tolerate old ptes, by fixing the vmscan > behaviour. And this is not a theoretical problem that I am trying to > fix. We have really seen the benefit of faultaround on arm mobile > targets, but the problem is the vmscan behaviour due to the young > pte workaround. And this patch helps in fixing that. Do you think > something more needs to be added in the documentation to make things > more clear on the flag usage ? No, I would either prefer auto-tuning or document that fault around can lead to this behavior and recommend to disable it rather than add a new knob. -- Michal Hocko SUSE Labs -- To unsubscribe, send a message with 'unsubscribe linux-mm' in the body to majordomo@xxxxxxxxx. For more info on Linux MM, see: http://www.linux-mm.org/ . Don't email: <a href=mailto:"dont@xxxxxxxxx";> email@xxxxxxxxx </a>
