2017-09-26 19:26 GMT+08:00 Michal Hocko <mhocko@xxxxxxxxxx>: > On Tue 26-09-17 19:06:37, Yafang Shao wrote: >> 2017-09-26 18:25 GMT+08:00 Michal Hocko <mhocko@xxxxxxxxxx>: >> > On Wed 20-09-17 06:43:35, Yafang Shao wrote: >> >> we can find the logic in domain_dirty_limits() that >> >> when dirty bg_thresh is bigger than dirty thresh, >> >> bg_thresh will be set as thresh * 1 / 2. >> >> if (bg_thresh >= thresh) >> >> bg_thresh = thresh / 2; >> >> >> >> But actually we can set vm background dirtiness bigger than >> >> vm dirtiness successfully. This behavior may mislead us. >> >> We'd better do this validity check at the beginning. >> > >> > This is an admin only interface. You can screw setting this up even >> > when you keep consistency between the background and direct limits. In >> > general we do not try to be clever for these knobs because we _expect_ >> > admins to do sane things. Why is this any different and why do we need >> > to add quite some code to handle one particular corner case? >> > >> >> Of course we expect admins to do the sane things, but not all admins >> are expert or faimilar with linux kernel source code. >> If we have to read the source code to know what is the right thing to >> do, I don't think this is a good interface, even for the admin. > > Well, it is kind of natural to setup background below the direct limit > in general so I am not sure what is so surprising here. Moreover setting > a non default drity limits already requires some expertise. It is not > like an arbitrary value will work just fine... > >> Anyway, there's no document on that direct limits should not less than >> background limits. > > Then improve the documentation. I have improved the kernel documentation as well, in order to make it more clear for the newbies. >> > To be honest I am not entirely sure this is worth the code and the >> > future maintenance burden. >> I'm not sure if this code is a burden for the future maintenance, but >> I think that if we don't introduce this code it is a burden to the >> admins. > > anytime we might need to tweak background vs direct limit we would have > to change these checks as well and that sounds like a maint. burden to > me. Would pls. show me some example ? > -- > 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>
