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

On Tue, Aug 06, 2019 at 01:44:02PM +0200, Michal Hocko wrote:
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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.

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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