Ping Hugh, :-)
On 03/06/2013 06:18 PM, Ric Mason wrote:
Hi Hugh,
On 03/06/2013 01:05 PM, Hugh Dickins wrote:
On Wed, 6 Mar 2013, Ric Mason wrote:
[ I've deleted the context because that was about the unstable tree,
and here you have moved to asking about a case in the stable tree. ]
I think I can basically understand you, please correct me if something
wrong.
For ksm page:
If one ksm page(in old node) migrate to another(new) node(ksm page is
treated as old page, one new page allocated in another node now),
since we can't get right lock in this time, we can't move stable node
to its new tree at this time, stable node still in old node and
stable_node->nid still store old node value. If ksmd scan and compare
another page in old node and search stable tree will figure out that
stable node relevant ksm page is migrated to new node, stable node
will be erased from old node's stable tree and link to migrate_nodes
list. What's the life of new page in new node? new page will be scaned
by ksmd, it will search stable tree in new node and if doesn't find
matched stable node, the new node is deleted from migrate_node list
and add to new node's table tree as a leaf, if find stable node in
stable tree, they will be merged. But in special case, the stable node
relevant ksm page can also migrated, new stable node will replace the
stable node which relevant page migrated this time.
For unstable tree page:
If search in unstable tree and find the tree page which has equal
content is migrated, just stop search and return, nothing merged. The
new page in new node for this migrated unstable tree page will be
insert to unstable tree in new node.
For the case of a ksm page is migrated to a different NUMA node and
migrate
its stable node to the right tree and collide with an existing
stable node.
get_kpfn_nid(stable_node->kpfn) != NUMA(stable_node->nid) can
capture nothing
That's not so: as I've pointed out before, ksm_migrate_page() updates
stable_node->kpfn for the new page on the new NUMA node; but it cannot
(get the right locking to) move the stable_node to its new tree at
that time.
It's moved out once ksmd notices that it's in the wrong NUMA node tree -
perhaps when one its rmap_items reaches the head of
cmp_and_merge_page(),
or perhaps here in stable_tree_search() when it matches another page
coming in to cmp_and_merge_page().
You may be concentrating on the case when that "another page" is a ksm
page migrated from a different NUMA node; and overlooking the case of
when the matching ksm page in this stable tree has itself been migrated.
since stable_node is the node in the right stable tree, nothing
happen to it
before this check. Did you intend to check
get_kpfn_nid(page_node->kpfn) !=
NUMA(page_node->nid) ?
Certainly not: page_node is usually NULL. But I could have checked
get_kpfn_nid(stable_node->kpfn) != nid: I was duplicating the test
from cmp_and_merge_page(), but here we do have local variable nid.
Hugh
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