On Thu, Feb 06, 2020 at 01:23:21PM -0800, Mike Kravetz wrote: > On 2/6/20 12:39 PM, Matthew Wilcox wrote: > > On Wed, Feb 05, 2020 at 05:36:44PM -0800, Mike Kravetz wrote: > >> The value of min_free_kbytes is calculated in two routines: > >> 1) init_per_zone_wmark_min based on available memory > >> 2) set_recommended_min_free_kbytes may reserve extra space for > >> THP allocations > >> > >> In both of these routines, a user defined min_free_kbytes value will > >> be overwritten if the value calculated in the code is larger. No message > >> is logged if the user value is overwritten. > >> > >> Change code to never overwrite user defined value. However, do log a > >> message (once per value) showing the value calculated in code. > > > > But what if the user set min_free_kbytes to, say, half of system memory, > > and then hot-unplugs three quarters of their memory? I think the kernel > > should protect itself against such foolishness. > > I'm not sure what we should set it to in this case. Previously you said, > > >> I'm reluctant to suggest we do a more complex adjustment of the value > >> (eg figure out what the adjustment would have been, then apply some > >> fraction of that adjustment to keep the ratios in proportion) because > >> we don't really know why they adjusted it. > > So, I suspect you would suggest setting it to the default computed value? > But then, when do we start adjusting? What if they only remove a small > amount of memory? And, then add the same amount back in? I don't know about the default computed value ... we don't seem to have any protection against the user setting min_free_kbytes to double the amount of memory in the machine today. Which would presumably cause problems if I asked to maintain 32GB free at all times on my 16GB laptop? Maybe we should have such protection? > BTW - In the above scenario existing code would not change min_free_kbytes > because the user defined value is greater than value computed in code. True!
