Re: [PATCH 2/7] ksm: treat unstable nid like in stable tree

[Will Huck]

Hi Hugh,
On 03/02/2013 10:57 AM, Hugh Dickins wrote:

How ksm treat a ksm forked page? IIUC, it's not merged in ksm stable 
tree. It will just be ignore?

> On Sat, 2 Mar 2013, Ric Mason wrote:
> > On 03/02/2013 04:03 AM, Hugh Dickins wrote:
> > > On Fri, 1 Mar 2013, Ric Mason wrote:
> > > > I think the ksm implementation for num awareness  is buggy.
> > > Sorry, I just don't understand your comments below,
> > > but will try to answer or question them as best I can.
> > >
> > > > For page migratyion stuff, new page is allocated from node *which page is
> > > > migrated to*.
> > > Yes, by definition.
> > >
> > > > - when meeting a page from the wrong NUMA node in an unstable tree
> > > >       get_kpfn_nid(page_to_pfn(page)) *==* page_to_nid(tree_page)
> > > I thought you were writing of the wrong NUMA node case,
> > > but now you emphasize "*==*", which means the right NUMA node.
> > Yes, I mean the wrong NUMA node. During page migration, new page has already
> > been allocated in new node and old page maybe freed.  So tree_page is the
> > page in new node's unstable tree, page is also new node page, so
> > get_kpfn_nid(page_to_pfn(page)) *==* page_to_nid(tree_page).
> I don't understand; but here you seem to be describing a case where two
> pages from the same NUMA node get merged (after both have been migrated
> from another NUMA node?), and there's nothing wrong with that,
> so I won't worry about it further.
>
> > > >      - meeting a page which is ksm page before migration
> > > >        get_kpfn_nid(stable_node->kpfn) != NUMA(stable_node->nid) can't
> > > > capture
> > > > them since stable_node is for tree page in current stable tree. They are
> > > > always equal.
> > > When we meet a ksm page in the stable tree before it's migrated to another
> > > NUMA node, yes, it will be on the right NUMA node (because we were careful
> > > only to merge pages from the right NUMA node there), and that test will not
> > > capture them.  It's for capturng a ksm page in the stable tree after it has
> > > been migrated to another NUMA node.
> > ksm page migrated to another NUMA node still not freed, why? Who take page
> > count of it?
> The old page, the one which used to be a ksm page on the old NUMA node,
> should be freed very soon: since it was isolated from lru, and its page
> count checked, I cannot think of anything to hold a reference to it,
> apart from migration itself - so it just needs to reach putback_lru_page(),
> and then may rest awhile on __lru_cache_add()'s pagevec before being freed.
>
> But I don't see where I said the old page was still not freed.
>
> > If not  freed, since new page is allocated in new node, it is
> > the copy of current ksm page, so current ksm doesn't change,
> > get_kpfn_nid(stable_node->kpfn) *==* NUMA(stable_node->nid).
> But ksm_migrate_page() did
> 		VM_BUG_ON(stable_node->kpfn != page_to_pfn(oldpage));
> 		stable_node->kpfn = page_to_pfn(newpage);
> without changing stable_node->nid.
>
> Hugh
>
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