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 -- To unsubscribe, send a message with 'unsubscribe linux-mm' in the body to majordomo@xxxxxxxxx. For more info on Linux MM, see: http://www.linux-mm.org/ . Don't email: <a href=mailto:"dont@xxxxxxxxx";> email@xxxxxxxxx </a>