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    SubjectRe: [PATCH 2/2] mm,migration: Fix race between shift_arg_pages and rmap_walk by guaranteeing rmap_walk finds PTEs created within the temporary stack
    On Sun, 9 May 2010 20:21:45 +0100
    Mel Gorman <mel@csn.ul.ie> wrote:

    > On Thu, May 06, 2010 at 07:12:59PM -0700, Linus Torvalds wrote:
    > >
    > >
    > > On Fri, 7 May 2010, KAMEZAWA Hiroyuki wrote:
    > > >
    > > > IIUC, move_page_tables() may call "page table allocation" and it cannot be
    > > > done under spinlock.
    > >
    > > Bah. It only does a "alloc_new_pmd()", and we could easily move that out
    > > of the loop and pre-allocate the pmd's.
    > >
    > > If that's the only reason, then it's a really weak one, methinks.
    > >
    >
    > It turns out not to be easy to the preallocating of PUDs, PMDs and PTEs
    > move_page_tables() needs. To avoid overallocating, it has to follow the same
    > logic as move_page_tables duplicating some code in the process. The ugliest
    > aspect of all is passing those pre-allocated pages back into move_page_tables
    > where they need to be passed down to such functions as __pte_alloc. It turns
    > extremely messy.
    >
    > I stopped working on it about half way through as it was already too ugly
    > to live and would have similar cost to Kamezawa's much more straight-forward
    > approach of using move_vma().
    >
    > While using move_vma is straight-forward and solves the problem, it's
    > not as cheap as Andrea's solution. Andrea allocates a temporary VMA and
    > puts it on a list and very little else. It didn't show up any problems
    > in microbenchmarks. Calling move_vma does a lot more work particularly in
    > copy_vma and this slows down exec.
    >
    > With Kamezawa's patch, kernbench was fine on wall time but in System Time,
    > it slowed by up 1.48% in comparison to Andrea's slowing up by 0.64%[1].
    >
    > aim9 was slowed as well. Kamezawa's slowed by 2.77% where Andrea's reported
    > faster by 2.58%. While AIM9 is flaky and these figures are barely outside
    > the noise, calling move_vma() is obviously more expensive.
    >

    Thank you for testing.


    > While my solution at http://lkml.org/lkml/2010/4/30/198 is cheapest as it
    > does not touch exec() at all, is_vma_temporary_stack() could be broken in
    > the future if any of the assumptions it makes change.
    >
    > So what you have is an inverse relationship between magic and
    > performance. Mine has the most magic and is fastest. Kamezawa's has the
    > least magic but slowest and Andrea has the goldilocks factor. Which do
    > you prefer?
    >

    I like the fastest one ;)

    Thanks,
    -Kame



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