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

To: Dave Chinner <david@xxxxxxxxxxxxx>
Subject: Re: [PATCH 4/4] mm: numa: Slow PTE scan rate if migration failures occur
From: Linus Torvalds <torvalds@xxxxxxxxxxxxxxxxxxxx>
Date: Wed, 18 Mar 2015 10:31:28 -0700
Cc: Mel Gorman <mgorman@xxxxxxx>, Ingo Molnar <mingo@xxxxxxxxxx>, Andrew Morton <akpm@xxxxxxxxxxxxxxxxxxxx>, Aneesh Kumar <aneesh.kumar@xxxxxxxxxxxxxxxxxx>, Linux Kernel Mailing List <linux-kernel@xxxxxxxxxxxxxxx>, Linux-MM <linux-mm@xxxxxxxxx>, xfs@xxxxxxxxxxx, ppc-dev <linuxppc-dev@xxxxxxxxxxxxxxxx>
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On Wed, Mar 18, 2015 at 9:08 AM, Linus Torvalds
<torvalds@xxxxxxxxxxxxxxxxxxxx> 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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