Re: [v4] mm: khugepaged: avoid overriding min_free_kbytes set by user

[Vijay Balakrishna]

On 9/17/2020 10:52 AM, Michal Hocko wrote:
> On Thu 17-09-20 10:27:16, Vijay Balakrishna wrote:
> > On 9/17/2020 2:28 AM, Michal Hocko wrote:
> > > On Wed 16-09-20 23:39:39, Vijay Balakrishna wrote:
> > > > set_recommended_min_free_kbytes need to honor min_free_kbytes set by the
> > > > user.  Post start-of-day THP enable or memory hotplug operations can
> > > > lose user specified min_free_kbytes, in particular when it is higher than
> > > > calculated recommended value.
> > >
> > > I was about to recommend a more detailed explanation when I have
> > > realized that this patch is not really needed after all. Unless I am
> > > missing something.
> > >
> > > init_per_zone_wmark_min ignores the newly calculated min_free_kbytes if
> > > it is lower than user_min_free_kbytes. So calculated min_free_kbytes >=
> > > user_min_free_kbytes.
> > >
> > > Except for value clamping when the value is reduced and this likely
> > > needs fixing. But set_recommended_min_free_kbytes should be fine.
> >
> > IIUC, after start-of-day if a user performs
> > - THP disable
> > - modifies min_free_bytes
> > - THP enable
> > above sequence currently wouldn't result in calling init_per_zone_wmark_min.
>
> I will not, but why do you think this matters? All we should care about
> is that auto-tuning shouldn't reduce user provided value [1] and that
> the memory hotplug should be consistent with the boot time heuristic.
> init_per_zone_wmark_min should make sure that the user value is not
> reduced and thp heuristic makes sure it will not reduce this value.
> So the property should be transitive with the existing code (modulo the
> problem I have highlighted).
>
> [1] one could argue that it shouldn't even increase the value strictly
> speaking because an admin might have a very good reason to decrease the
> value but this has never been the semantic and changing it now might be
> problematic

I made an attempt to address Kirill A. Shutemov's comment.  And incrased 
min_free_kbytes to see the issue in my testing and attempted a fix.  I'm 
ok leaving as it is.  Do not want introduce any changes that may cause 
regression.

Thanks,
Vijay