Hi, Kame.
On Thu, 2010-03-25 at 19:12 +0900, KAMEZAWA Hiroyuki wrote:
> On Thu, 25 Mar 2010 18:59:25 +0900 (JST)
> KOSAKI Motohiro <kosaki.motohiro@xxxxxxxxxxxxxx> wrote:
>
> > > > > > Kosaki-san,
> > > > > >
> > > > > > IIUC, the race in memory-hotunplug was fixed by this patch [2/11].
> > > > > >
> > > > > > But, this behavior of unmap_and_move() requires access to _freed_
> > > > > > objects (spinlock). Even if it's safe because of SLAB_DESTROY_BY_RCU,
> > > > > > it't not good habit in general.
> > > > > >
> > > > > > After direct compaction, page-migration will be one of "core" code of
> > > > > > memory management. Then, I agree to patch [1/11] as our direction for
> > > > > > keeping sanity and showing direction to more updates. Maybe adding
> > > > > > refcnt and removing RCU in futuer is good.
> > > > >
> > > > > But Christoph seems oppose to remove SLAB_DESTROY_BY_RCU. then refcount
> > > > > is meaningless now.
> > > >
> > > > Christoph is opposed to removing it because of cache-hotness issues more
> > > > so than use-after-free concerns. The refcount is needed with or without
> > > > SLAB_DESTROY_BY_RCU.
> > > >
> > >
> > > I wonder a code which the easiest to be read will be like following.
> > > ==
> > >
> > > if (PageAnon(page)) {
> > > struct anon_vma anon = page_lock_anon_vma(page);
> > > /* to take this lock, this page must be mapped. */
> > > if (!anon_vma)
> > > goto uncharge;
> > > increase refcnt
> > > page_unlock_anon_vma(anon);
> > > }
> > > ....
> > > ==
> >
> > This seems very good and acceptable to me. This refcnt usage
> > obviously reduce rcu-lock holding time.
> >
> > I still think no refcount doesn't cause any disaster. but I agree
> > this is forward step patch.
> >
>
> BTW, by above change and the change in patch [2/11],
> "A page turnd to be SwapCache and free unmapped but not freed"
> page will be never migrated.
>
> Mel, could you change the check as this ??
>
> if (PageAnon(page)) {
> rcu_read_lock();
> if (!page_mapcount(page)) {
> rcu_read_unlock();
> if (!PageSwapCache(page))
> goto uncharge;
> /* unmapped swap cache can be migrated */
Which case do we have PageAnon && (page_mapcount == 0) && PageSwapCache ?
With looking over code which add_to_swap_cache, I found somewhere.
1) shrink_page_list
I think this case doesn't matter by isolate_lru_xxx.
2) shmem_swapin
It seems to be !PageAnon
3) shmem_writepage
It seems to be !PageAnon.
4) do_swap_page
page_add_anon_rmap increases _mapcount before setting page->mapping to anon_vma.
So It doesn't matter.
I think following codes in unmap_and_move seems to handle 3) case.
---
* Corner case handling:
* 1. When a new swap-cache page is read into, it is added to the LRU
* and treated as swapcache but it has no rmap yet.
...
if (!page->mapping) {
if (!PageAnon(page) && page_has_private(page)) {
....
}
goto skip_unmap;
}
---
Do we really check PageSwapCache in there?
Do I miss any case?
--
Kind regards,
Minchan Kim
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