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Date: Mon, 28 Jan 2002 19:43:01 -0200 (BRST)
From: Rik van Riel <riel@conectiva.com.br>
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To: Rick Stevens <rstevens@vitalstream.com>
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Subject: Re: Note describing poor dcache utilization under high memory pressure
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On Mon, 28 Jan 2002, Rick Stevens wrote:
> Daniel Phillips wrote:
> [snip]
  [page table COW description]

> Perhaps I'm missing this, but I read that as the child gets a reference
> to the parent's memory.  If the child attempts a write, then new memory
> is allocated, data copied and the write occurs to this new memory.  As
> I read this, it's only invoked on a child write.
>
> Would this not leave a hole where the parent could write and, since the
> child shares that memory, the new data would be read by the child?  Sort
> of a hidden shm segment?  If so, I think we've got problems brewing.
> Now, if a parent write causes the same behaviour as a child write, then
> my point is moot.

Daniel and I discussed this issue when Daniel first came up with
the idea of doing page table COW.  He seemed a bit confused by
fork semantics when we first discussed this idea, too ;)

You're right though, both parent and child need to react in the
same way, preferably _without_ having to walk all of the parent's
page tables and mark them read-only ...

kind regards,

Rik
-- 
"Linux holds advantages over the single-vendor commercial OS"
    -- Microsoft's "Competing with Linux" document

http://www.surriel.com/		http://distro.conectiva.com/

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