On Wed, 2021-04-14 at 08:51 -0700, Andi Kleen wrote:
> >    2) It will not scan PTE tables under non-leaf PMD entries that
> > do not
> >       have the accessed bit set, when
> >       CONFIG_HAVE_ARCH_PARENT_PMD_YOUNG=y.
> 
> This assumes  that workloads have reasonable locality. Could there
> be a worst case where only one or two pages in each PTE are used,
> so this PTE skipping trick doesn't work?

Databases with large shared memory segments shared between
many processes come to mind as a real-world example of a
worst case scenario.

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