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

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Date:	Wed, 18 Mar 2015 10:31:28 -0700
From:	Linus Torvalds <torvalds@...ux-foundation.org>
To:	Dave Chinner <david@...morbit.com>
Cc:	Mel Gorman <mgorman@...e.de>, 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 Wed, Mar 18, 2015 at 9:08 AM, Linus Torvalds
<torvalds@...ux-foundation.org> wrote:
>
> So why am I wrong? Why is testing for dirty not the same as testing
> for writable?
>
> I can see a few cases:
>
>  - your load has lots of writable (but not written-to) shared memory

Hmm. I tried to look at the xfsprog sources, and I don't see any
MAP_SHARED activity.  It looks like it's just using pread64/pwrite64,
and the only MAP_SHARED is for the xfsio mmap test thing, not for
xfsrepair.

So I don't see any shared mappings, but I don't know the code-base.

>  - something completely different that I am entirely missing

So I think there's something I'm missing. For non-shared mappings, I
still have the idea that pte_dirty should be the same as pte_write.
And yet, your testing of 3.19 shows that it's a big difference.
There's clearly something I'm completely missing.

                          Linus
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