Re: [PATCH v4] KSM: numa awareness sysfs knob

[Hugh Dickins <hughd@google.com>]

On Mon, 1 Oct 2012, Andrea Arcangeli wrote:
> On Sun, Sep 30, 2012 at 05:36:33PM -0700, Hugh Dickins wrote:
> > I'm all for the simplest solution, but here in ksm_migrate_page()
> > is not a good place for COW breaking - we don't want to get into
> > an indefinite number of page allocations, and the risk of failure.
> 
> Agreed, not a good place to break_cow.
> 
> > I was toying with the idea of leaving the new page in the old NUMAnode's
> > stable tree temporarily, until ksmd comes around again, and let that
> > clean it up.  Which would imply less reliance on get_kpfn_nid(),
> > and not skipping PageKsm in ksm_do_scan(), and...
> 
> There a break_cow could more easily run to cleanup the errors in the
> stable tree. It'd be one way to avoid altering migrate.

I get the feeling that you've thought this path through further
than I have, and are pretty sure that it will stay simple.

Whereas I was a little anxious about leaving a KSMnode behind in a
tree which ostensibly belongs to an already-offlined NUMAnode.

Don't let me push you into more far-reaching changes than are
necessary.  I didn't think we needed to change ->migrate, just
migrate_page_copy() in the anonymous case (the only one KSM affects).

Maybe that's bad practice, and maybe I'm wrong, that it would have to
percolate up a few levels of callers.  And I wouldn't want to think
through the memcg page charging in a hurry.

If you can see that another ksm_do_scan() break_cow() will sort it
all out simply and safely (together with extending the node pointer
or nid usage from unstable to stable tree, instead of get_kpfn_nid()),
please just go for that.

We can always be cleverer later - but I doubt it'll be a priority.

H(always good for saying the opposite in alternate mails)ugh

> 
> > But it's not all that simple, and I think we can do better.
> 
> Agreed.
> 
> > It's only just fully dawned on me that ksm_migrate_page() is actually
> > a very convenient place: no pagetable mangling required, because we
> > know that neither old nor new page is at this instant mapped into
> > userspace at all - don't we?  Instead there are swap-like migration
> > entries plugging all ptes until we're ready to put in the new page.
> 
> Yes.
> 
> > So I think what we really want to do is change the ksm_migrate_page()
> > interface a little, and probably the precise position it's called from,
> > to allow it to update mm/migrate.c's newpage - in the collision case
> 
> I agree your proposed modification to the ->migratepage protocol
> should be able to deal with that. We should notify the caller the
> "newpage" has been freed and we transferred all ownership to an
> "alternate_newpage". So then migrate will restore the ptes pointing to
> the alternate_newpage (not the allocated newpage). It should be also
> possible to get an hold on the alternate_newpage, before having to
> allocate newpage.
> 
> > when the new NUMAnode already has a stable copy of this page.  But when
> > it doesn't, just move KSMnode from old NUMAnode's stable tree to new.
> 
> Agreed, that is the easy case and doesn't require interface changes.
> 
> > How well the existing ksm.c primitives are suited to this, I've not
> > checked.  Probably not too well, but shouldn't be hard to add what's
> > needed.
> > 
> > What do you think?  Does that sound reasonable, Petr?
> 
> Sounds like a plan, I agree the modification to migrate is the best
> way to go here. Only cons: it's not the simplest solution.
> 
> > By the way, this is probably a good occasion to remind ourselves,
> > that page migration is still usually disabled on PageKsm pages:
> > ksm_migrate_page() is only being called for memory hotremove.  I had
> > been about to complain that calling remove_node_from_stable_tree()
> > from ksm_migrate_page() is also unsafe from a locking point of view;
> > until I remembered that MEM_GOING_OFFLINE has previously acquired
> > ksm_thread_mutex.
> > 
> > But page migration is much more important now than three years ago,
> > with compaction relying upon it, CMA and THP relying upon compaction,
> > and lumpy reclaim gone.
> 
> Agreed. AutoNUMA needs it too: AutoNUMA migrates all types of memory,
> not just anonymous memory, as long as the mapcount == 1.
> 
> If all users break_cow except one, then the KSM page can move around
> if it has left just one user, we don't need to wait this last user to
> break_cow (which may never happen) before can move it.
> 
> > Whilst it should not be mixed up in the NUMA patch itself, I think we
> > need now to relax that restriction.  I found re-reading my 62b61f611e
> > "ksm: memory hotremove migration only" was helpful.  Petr, is that
> > something you could take on also?  I _think_ it's just a matter of
> > protecting the stable tree(s) with an additional mutex (which ought
> > not to be contended, since ksm_thread_mutex is normally held above
> > it, except in migration); then removing a number of PageKsm refusals
> > (and the offlining arg to unmap_and_move() etc).  But perhaps there's
> > more to it, I haven't gone over it properly.
> 
> Removing the restriction sounds good. In addition to
> compaction/AutoNUMA etc.. KSM pages are marked MOVABLE so it's likely
> not good for the anti frag pageblock types.
> 
> So if I understand this correctly, there would be no way to trigger
> the stable tree corruption in current v4, without memory hotremove.
> 
> > Yes, I agree; but a few more comments I'll make against the v4 post.
> 
> Cool.
> 
> Thanks for the help!
> Andrea

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