Re: [PATCH v2] mm: vmscan: fix not scanning anonymous pages when detecting file refaults

[Kuo-Hsin Yang]

On Fri, Jun 28, 2019 at 10:32:01AM -0400, Johannes Weiner wrote:
> On Fri, Jun 28, 2019 at 07:16:27PM +0800, Kuo-Hsin Yang wrote:
> > Commit 2a2e48854d70 ("mm: vmscan: fix IO/refault regression in cache
> > workingset transition") introduced actual_reclaim parameter.  When file
> > refaults are detected, inactive_list_is_low() may return different
> > values depends on the actual_reclaim parameter.  Vmscan would only scan
> > active/inactive file lists at file thrashing state when the following 2
> > conditions are satisfied.
> > 
> > 1) inactive_list_is_low() returns false in get_scan_count() to trigger
> >    scanning file lists only.
> > 2) inactive_list_is_low() returns true in shrink_list() to allow
> >    scanning active file list.
> > 
> > This patch makes the return value of inactive_list_is_low() independent
> > of actual_reclaim and rename the parameter back to trace.
> 
> This is not. The root cause for the problem you describe isn't the
> patch you point to. The root cause is our decision to force-scan the
> file LRU based on relative inactive:active size alone, without taking
> file thrashing into account at all. This is a much older problem.
> 
> After the referenced patch, we're taking thrashing into account when
> deciding whether to deactivate active file pages or not. To solve the
> problem pointed out here, we can extend that same principle to the
> decision whether to force-scan files and skip the anon LRUs.
> 
> The patch you're pointing to isn't the culprit. On the contrary, it
> provides the infrastructure to solve a much older problem.
> 
> > Fixes: 2a2e48854d70 ("mm: vmscan: fix IO/refault regression in cache workingset transition")
> 
> Please replace this line with the two Fixes: lines that I provided
> earlier in this thread.

Thanks for your clarification, I will update the changelog.