On 14.05.2015 21:50, Mark Williamson wrote:
Hi Konstantin,
On Wed, May 13, 2015 at 11:51 AM, Konstantin Khlebnikov
<khlebnikov@xxxxxxxxxxxxxx> wrote:
On 12.05.2015 15:05, Mark Williamson wrote:
<snip>
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.
<snip>
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.
My concern was whether a change to format would be acceptable to
include in the various -stable kernels; they are already including the
additional protections on pagemap, so we're starting to need our
fallback mode in distributions. Do you think that such a patch would
be acceptable there?
(As an application vendor we're likely to be particularly stuck with
what the commercial distributions decide to ship, which is why I'm
trying to keep an eye on this)
I appreciate that this is a slightly administrative concern! I
definitely like the technical approach of this code and it seems to
work fine for us.
I cannot guarantee that v2 format will be accepted into stable kernels
and into distributives. I'm not the gate keeper.
As a fallback probably you should invent some kind of suid helper
which gives you access to required information without exposing pfn.
For example: it gets pids and memory ranges as arguments and prints
bitmap of CoWed pages into stdout.
--
Konstantin
--
To unsubscribe, send a message with 'unsubscribe linux-mm' in
the body to majordomo@xxxxxxxxx. For more info on Linux MM,
see: http://www.linux-mm.org/ .
Don't email: <a href=mailto:"dont@xxxxxxxxx";> email@xxxxxxxxx </a>