Re: [PATCH v7 1/2] KSM: numa awareness sysfs knob

[Simon Jeons <simon.jeons@gmail.com>]

On Fri, 2013-01-04 at 15:03 -0800, Hugh Dickins wrote:
> On Thu, 3 Jan 2013, Simon Jeons wrote:
> > On Wed, 2013-01-02 at 21:10 -0800, Hugh Dickins wrote:
> > > 
> > > As you can see, remove_rmap_item_from_tree uses it to decide whether
> > > or not it should rb_erase the rmap_item from the unstable_tree.
> > > 
> > > Every full scan of all the rmap_items, we increment ksm_scan.seqnr,
> > > forget the old unstable_tree (it would just be a waste of processing
> > > to remove every node one by one), and build up the unstable_tree afresh.
> > > 
> > 
> > When the rmap_items left over from the previous scan will be removed?
> 
> Removed from the unstable rbtree?  Not at all, it's simply restarted
> afresh, and the old rblinkages ignored.  Freed back to slab?  When the
> scan passes that mm+address and realizes that rmap_item is not wanted
> any more.  (Or when ksm is shut down with KSM_RUN_UNMERGE.)
> 

Make sense. Thanks Hugh. :)

> > 
> > > That works fine until we need to remove an rmap_item: then we have to be
> > > very sure to remove it from the unstable_tree if it's already been linked
> > > there during this scan, but ignore its rblinkage if that's just left over
> > > from the previous scan.
> > > 
> > > A single bit would be enough to decide this; but we got it troublesomely
> > > wrong in the early days of KSM (didn't always visit every rmap_item each
> > > scan), so it's convenient to use 8 bits (the low unsigned char, stored
> > 
> > When the scenario didn't always visit every rmap_item each scan can
> > occur? 
> 
> You're asking me about a stage of KSM development 3.5 years ago:
> I don't remember the details.
> 
> > 
> > > below the FLAGs and below the page-aligned address in the rmap_item -
> > > there's lots of them, best keep them as small as we can) and do a
> > > BUG_ON(age > 1) if we made a mistake.
> > > 
> > > We haven't hit that BUG_ON in over three years: if we need some more
> > > bits for something, we can cut the age down to one or two bits.
> > > 
> > > Hugh