Re: [PATCH 4/4] mm: numa: Slow PTE scan rate if migration failures occur

[Linus Torvalds <torvalds@xxxxxxxxxxxxxxxxxxxx>]

On Thu, Mar 19, 2015 at 7:10 AM, Mel Gorman <mgorman@xxxxxxx> wrote:
> -       if (!pmd_dirty(pmd))
> +       /* See similar comment in do_numa_page for explanation */
> +       if (!(vma->vm_flags & VM_WRITE))

Yeah, that would certainly be a whole lot more obvious than all the
"if this particular pte/pmd looks like X" tests.

So that, together with scanning rate improvements (this *does* seem to
be somewhat chaotic, so it's quite possible that the current scanning
rate thing is just fairly unstable) is likely the right thing. I'd
just like to _understand_ why that write/dirty bit makes such a
difference. I thought I understood what was going on, and was happy,
and then Dave come with his crazy numbers.

Damn you Dave, and damn your numbers and "facts" and stuff. Sometimes
I much prefer ignorant bliss.

                           Linus