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SubjectRe: Subtle MM bug
   Date: 	Wed, 10 Jan 2001 11:03:21 -0800 (PST)
From: Linus Torvalds <torvalds@transmeta.com>

"Feel free to try" is definitely the open source motto.

I basically came to the conclusion that it sucks when I
gave it a go.

In my scheme I tried to save space by using very small descriptors to
keep track of anonymous areas in processes. This was essentially a
vma->vm_anon pointer that kept track of the pages for you.

After trying to fight this for a few days I determined that this
doesn't work at all because of how COW dups the pages around on you.
Also it was a devil to work out anonymous pages created due to writes
to private mmaps of a file, as soon as one of these were made for the
first time on a vma you had to cook up one of the anon descriptors.

Yeah, I got the anon descriptor down to 2 pointers and an atomic
counter, but it didn't work so this achievement was worthless :-)

There are a few approaches that work, but they tend to take up too
much space to be considerable, as Linus mentioned.

Later,
David S. Miller
davem@redhat.com
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