Re: [PATCH v4 4/5] page_idle: Drain all LRU pagevec before idle tracking

[Joel Fernandes <joel@joelfernandes.org>]

On Tue, Aug 06, 2019 at 01:44:02PM +0200, Michal Hocko wrote:
[snip]
> > > > This operation even if expensive is only done once during the access of the
> > > > page_idle file. Did you have a better fix in mind?
> > > 
> > > Can we set the idle bit also for non-lru pages as long as they are
> > > reachable via pte?
> > 
> > Not at the moment with the current page idle tracking code. PageLRU(page)
> > flag is checked in page_idle_get_page().
> 
> yes, I am aware of the current code. I strongly suspect that the PageLRU
> check was there to not mark arbitrary page looked up by pfn with the
> idle bit because that would be unexpected. But I might be easily wrong
> here.

Yes, quite possible.

> > Even if we could set it for non-LRU, the idle bit (page flag) would not be
> > cleared if page is not on LRU because page-reclaim code (page_referenced() I
> > believe) would not clear it.
> 
> Yes, it is either reclaim when checking references as you say but also
> mark_page_accessed. I believe the later might still have the page on the
> pcp LRU add cache. Maybe I am missing something something but it seems
> that there is nothing fundamentally requiring the user mapped page to be
> on the LRU list when seting the idle bit.
> 
> That being said, your big hammer approach will work more reliable but if
> you do not feel like changing the underlying PageLRU assumption then
> document that draining should be removed longterm.

Yes, at the moment I am in preference of keeping the underlying assumption
same. I am Ok with adding of a comment on the drain call that it is to be
removed longterm.

thanks,

 - Joel