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

[Hugh Dickins]

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 :)

> 
> 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.

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
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
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