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 -- To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majord...@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/
