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? 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. -- Mike Kravetz
