Messages in this thread | | | Date | Wed, 2 Jan 2013 21:10:48 -0800 (PST) | From | Hugh Dickins <> | Subject | Re: [PATCH v7 1/2] KSM: numa awareness sysfs knob |
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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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