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SubjectRe: [PATCH] mm: Save soft-dirty bits on swapped pages
On Wed, Jul 24, 2013 at 12:04 PM, Cyrill Gorcunov <> wrote:
> On Wed, Jul 24, 2013 at 10:55:41PM +0400, Pavel Emelyanov wrote:
>> >
>> > Well, some part of information already lays in pte (such as 'file' bit,
>> > swap entries) so it looks natural i think to work on this level. but
>> > letme think if use page struct for that be more convenient...
>> It hardly will be. Consider we have a page shared between two tasks,
>> then first one "touches" it and soft-dirty is put onto his PTE and,
>> subsequently, the page itself. The we go and clear sofr-dirty for the
>> 2nd task. What should we do with the soft-dirty bit on the page?
> Indeed, this won't help. Well then, bippidy-boppidy-boo, our
> pants are metaphorically on fire (c)

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.)


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