Re: [PATCH v3 01/17] mm: support madvise(MADV_FREE)

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.