Re: [PATCH v2 1/3] pagemap: add mmap-exclusive bit for marking pages mapped only here

[Konstantin Khlebnikov]

On 12.05.2015 15:05, Mark Williamson wrote:
> Hi Konstantin,
>
> I hope you won't mind me thinking out loud here on the idea of adding
> a flag to the v2 pagemap fields...  From a kernel PoV, I agree that
> this seems like the cleanest approach.  However, with my application
> developer hat on:
>
>   1. I was hoping we'd be able to backport a compatible fix to older
> kernels that might adopt the pagemap permissions change.  Using the V2
> format flags rules out doing this for kernels that are too old to have
> soft-dirty, I think.
>
>   2. From our software's PoV, I feel it's worth noting that it doesn't
> strictly fix ABI compatibility, though I realise that's probably not
> your primary concern here.  We'll need to modify our code to write the
> clear_refs file but that change is OK for us if it's the preferred
> solution.
>
> In the patches I've been playing with, I was considering putting the
> Exclusive flag in the now-unused PFN field of the pagemap entries.
> Since we're specifically trying to work around for the lack of PFN
> information, would there be any appetite for mirroring this flag
> unconditionally into the now-empty PFN field (i.e. whether using v1 or
> v2 flags) when accessed by an unprivileged process?
>
> I realise it's ugly from a kernel PoV and I feel a little bad for
> suggesting it - but it would address points 1 and 2 for us (our
> existing code just looks for changes in the pagemap entry, so sticking
> the flag in there would cause it to do the right thing).
>
> I'm sorry to raise application-specific issues at this point; I
> appreciate that your primary concern is to improve the kernel and
> technically I like the approach that you've taken!  I'll try and
> provide more code-oriented feedback once I've tried out the changes.

I prefer to backport v2 format (except soft-dirty bit and clear_refs)
into older kernels. Page-shift bits are barely used so nobody will see
the difference.

--
Konstantin
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