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

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

On Wed, 2013-01-02 at 21:10 -0800, Hugh Dickins wrote:
> On Tue, 1 Jan 2013, Simon Jeons wrote:
> > 
> > Hi Petr and Hugh,
> > 
> > One offline question, thanks for your clarify.
> 
> Perhaps not as offline as you intended :)

Hi Hugh,

Thanks for your detail explanation. :)

> 
> > 
> > How to understand age = (unsigned char)(ksm_scan.seqnr -
> > rmap_item->address);? It used for what?
> 
> 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?

> 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? 

> 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