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

[Hush Bensen]

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.
>
> > --Andy
> Thanks,
> Pavel
>
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