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

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Message-ID: <CA+55aFwTcw2rTjPM_EOjPeKNWwdfNVazx+O=YAFXPWw1h==Tgw@mail.gmail.com>
Date:	Thu, 19 Mar 2015 11:09:15 -0700
From:	Linus Torvalds <torvalds@...ux-foundation.org>
To:	Mel Gorman <mgorman@...e.de>
Cc:	Dave Chinner <david@...morbit.com>, Ingo Molnar <mingo@...nel.org>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Aneesh Kumar <aneesh.kumar@...ux.vnet.ibm.com>,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
	Linux-MM <linux-mm@...ck.org>, xfs@....sgi.com,
	ppc-dev <linuxppc-dev@...ts.ozlabs.org>
Subject: Re: [PATCH 4/4] mm: numa: Slow PTE scan rate if migration failures occur

On Thu, Mar 19, 2015 at 7:10 AM, Mel Gorman <mgorman@...e.de> 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
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