On Tue 30-05-17 13:19:22, Mike Rapoport wrote:

On Tue, May 30, 2017 at 09:44:08AM +0200, Michal Hocko wrote:

quoted

On Wed 24-05-17 17:27:36, Mike Rapoport wrote:

quoted

On Wed, May 24, 2017 at 01:18:00PM +0200, Michal Hocko wrote:

[...]

quoted

quoted

Why cannot khugepaged simply skip over all VMAs which have userfault
regions registered? This would sound like a less error prone approach to
me.

khugepaged does skip over VMAs which have userfault. We could register the
regions with userfault before populating them to avoid collapses in the
transition period.

Why cannot you register only post-copy regions and "manually" copy the
pre-copy parts?

We can register only post-copy regions, but this will cause VMA
fragmentation. Now we register the entire VMA with userfaultfd, no matter
how many pages were dirtied there since the pre-dump. If we register only
post-copy regions, we will split out the VMAs for those regions.

Is this really a problem, though?

quoted

quoted

But then we'll have to populate these regions with
UFFDIO_COPY which adds quite an overhead.

How big is the performance impact?

I don't have the numbers handy, but for each post-copy range it means that
instead of memcpy() we will use ioctl(UFFDIO_COPY).

It would be good to measure that though. You are proposing a new user
API and the THP api is quite convoluted already so there better be a
very good reason to add a new API. So far I can only see that it would
be more convinient to add another madvise command and that is rather
insufficient justification IMHO. Also do you expect somebody else would
use new madvise? What would be the usecase?
-- 
Michal Hocko
SUSE Labs

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