Re: [PATCH] mm: Save soft-dirty bits on swapped pages

From: Cyrill Gorcunov
Date: Thu Jul 25 2013 - 03:07:44 EST

On Wed, Jul 24, 2013 at 12:40:22PM -0700, Andy Lutomirski wrote:
> Hmm. So there are at least three kinds of memory:
> Anonymous pages: soft-dirty works
> Shared file-backed pages: soft-dirty does not work
> Private file-backed pages: soft-dirty works (but see below)
> Perhaps another bit should be allocated to expose to userspace either
> "soft-dirty", "soft-clean", or "soft-dirty unsupported"?

> There's another possible issue with private file-backed pages, though:
> how do you distinguish clean-and-not-cowed from cowed-but-soft-clean?
> (The former will reflect changes in the underlying file, I think, but
> the latter won't.)

When fault happens with cow allocation (on write) the pte get soft dirty
bit set (the code uses pte_mkdirty(entry) in __do_fault) and until we
explicitly clean the bit it remains set. Or you mean something else?
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