Re: [LSF/MM TOPIC]: userfaultfd (was: [LSF/MM TOPIC] NUMA remote THP vs NUMA local non-THP under MADV_HUGEPAGE)

[Mike Rapoport <rppt@linux.ibm.com>]

Hi Peter,

On Wed, Jan 30, 2019 at 05:23:02PM +0800, Peter Xu wrote:
> On Wed, Jan 30, 2019 at 10:13:36AM +0200, Mike Rapoport wrote:
> > 
> > If we are to discuss userfaultfd, I'd like also to bring the subject of COW
> > mappings.
> > The pages populated with UFFDIO_COPY cannot be COW-shared between related
> > processes which unnecessarily increases memory footprint of a migrated
> > process tree.
> > I've posted a patch [1] a (real) while ago, but nobody reacted and I've put
> > this aside.
> > Maybe it's time to discuss it again :)
> 
> Hi, Mike,
> 
> It's interesting to know such a work...
> 
> Since I really don't have much context on this, so sorry if I'm going
> to ask a silly question... but I'd say when reading this I'm thinking
> of KSM.  I think KSM does not suite in this case since when doing
> UFFDIO_COPY_COW it'll contain hinting information while KSM was only
> scanning over the pages between processes which seems to be O(N*N) if
> assuming there're two processes.  However, would it make any sense to
> provide a general interface to scan for same pages between any two
> processes within specific range and merge them if found (rather than a
> specific interface for userfaultfd only)?  Then it might even be used
> by KSM admins (just as an example) when the admin knows exactly that
> memory range (addr1, len) of process A should very probably has many
> same contents as the memory range (addr2, len) of process B?

I haven't really thought about using KSM in our case. Our goal was to make
the VM layout of the migrated processes as close as possible to the
original, including the COW sharing between parent process and its
descendants. For that UFFDIO_COPY_COW seems to be more natural fit than
KSM.

> Thanks,
> 
> -- 
> Peter Xu
> 

-- 
Sincerely yours,
Mike.