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

[Minchan Kim <minchan-DgEjT+Ai2ygdnm+yROfE0A@public.gmane.org>]

On Fri, Nov 13, 2015 at 01:16:54AM -0500, Daniel Micay wrote:
> On 13/11/15 01:15 AM, Minchan Kim wrote:
> > On Thu, Nov 12, 2015 at 12:21:30AM -0500, Daniel Micay wrote:
> >>> I also think that the kernel should commit to either zeroing the page
> >>> or leaving it unchanged in response to MADV_FREE (even if the decision
> >>> of which to do is made later on).  I think that your patch series does
> >>> this, but only after a few of the patches are applied (the swap entry
> >>> freeing), and I think that it should be a real guaranteed part of the
> >>> semantics and maybe have a test case.
> >>
> >> This would be a good thing to test because it would be required to add
> >> MADV_FREE_UNDO down the road. It would mean the same semantics as the
> >> MEM_RESET and MEM_RESET_UNDO features on Windows, and there's probably
> >> value in that for the sake of migrating existing software too.
> > 
> > So, do you mean that we could implement MADV_FREE_UNDO with "read"
> > opearation("just access bit marking) easily in future?
> > 
> > If so, it would be good reason to change MADV_FREE from dirty bit to
> > access bit. Okay, I will look at that.
> 
> I just meant testing that the data is either zero or the old data if
> it's read before it's written to. Not having it stay around once there
> is a read. Not sure if that's what Andy meant.

Either zero of old data is gauranteed.
Now:

        MADV_FREE(range)
        A = read from the range
        ...
        ...
        B = read from the range


        A and B could have different value. But value should be old or zero.

But Andy want more strict ABI so he suggested access bit instead of dirty bit.

        MADV_FREE(range)
        A = read from the range
        ...
        ...
        B = read from the range

        A and B cannot have different value.

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?