> And now I am thinking if we use access bit, we could implment MADV_FREE_UNDO > easily when we need it. Maybe, that's what you want. Right? Yes, but why the access bit instead of the dirty bit for that? It could always be made more strict (i.e. access bit) in the future, while going the other way won't be possible. So I think the dirty bit is really the more conservative choice since if it turns out to be a mistake it can be fixed without a backwards incompatible change.
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