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

From: Hush Bensen
Date: Thu Jul 25 2013 - 04:27:14 EST

On 07/25/2013 03:29 PM, Pavel Emelyanov wrote:
On 07/24/2013 11:40 PM, Andy Lutomirski wrote:
On Wed, Jul 24, 2013 at 12:04 PM, Cyrill Gorcunov <gorcunov@xxxxxxxxx> 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)
The shared file-backed pages case works, but unmap-map case doesn't

What's the meaning of unmap-map case?

preserve the soft-dirty bit. Just like the private file did. We'll
fix this case next.

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.)
There's a bit called PAGE_FILE bit in /proc/pagemap file introduced with
the 052fb0d635df5d49dfc85687d94e1a87bf09378d commit.

Plz, refer to Documentation/vm/pagemap.txt and soft-dirty.txt, all this
is described there pretty well.


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