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Subject[PATCH 9/11] ksm: enable KSM page migration
Migration of KSM pages is now safe: remove the PageKsm restrictions from
mempolicy.c and migrate.c.

But keep PageKsm out of __unmap_and_move()'s anon_vma contortions, which
are irrelevant to KSM: it looks as if that code was preventing hotremove
migration of KSM pages, unless they happened to be in swapcache.

There is some question as to whether enforcing a NUMA mempolicy migration
ought to migrate KSM pages, mapped into entirely unrelated processes; but
moving page_mapcount > 1 is only permitted with MPOL_MF_MOVE_ALL anyway,
and it seems reasonable to assume that you wouldn't set MADV_MERGEABLE on
any area where this is a worry.

Signed-off-by: Hugh Dickins <hughd@google.com>
---
mm/mempolicy.c | 3 +--
mm/migrate.c | 21 +++------------------
2 files changed, 4 insertions(+), 20 deletions(-)

--- mmotm.orig/mm/mempolicy.c 2013-01-24 12:28:38.848127553 -0800
+++ mmotm/mm/mempolicy.c 2013-01-25 14:38:49.596208731 -0800
@@ -496,9 +496,8 @@ static int check_pte_range(struct vm_are
/*
* vm_normal_page() filters out zero pages, but there might
* still be PageReserved pages to skip, perhaps in a VDSO.
- * And we cannot move PageKsm pages sensibly or safely yet.
*/
- if (PageReserved(page) || PageKsm(page))
+ if (PageReserved(page))
continue;
nid = page_to_nid(page);
if (node_isset(nid, *nodes) == !!(flags & MPOL_MF_INVERT))
--- mmotm.orig/mm/migrate.c 2013-01-25 14:37:03.832206218 -0800
+++ mmotm/mm/migrate.c 2013-01-25 14:38:49.596208731 -0800
@@ -731,20 +731,6 @@ static int __unmap_and_move(struct page
lock_page(page);
}

- /*
- * Only memory hotplug's offline_pages() caller has locked out KSM,
- * and can safely migrate a KSM page. The other cases have skipped
- * PageKsm along with PageReserved - but it is only now when we have
- * the page lock that we can be certain it will not go KSM beneath us
- * (KSM will not upgrade a page from PageAnon to PageKsm when it sees
- * its pagecount raised, but only here do we take the page lock which
- * serializes that).
- */
- if (PageKsm(page) && !offlining) {
- rc = -EBUSY;
- goto unlock;
- }
-
/* charge against new page */
mem_cgroup_prepare_migration(page, newpage, &mem);

@@ -771,7 +757,7 @@ static int __unmap_and_move(struct page
* File Caches may use write_page() or lock_page() in migration, then,
* just care Anon page here.
*/
- if (PageAnon(page)) {
+ if (PageAnon(page) && !PageKsm(page)) {
/*
* Only page_lock_anon_vma_read() understands the subtleties of
* getting a hold on an anon_vma from outside one of its mms.
@@ -851,7 +837,6 @@ uncharge:
mem_cgroup_end_migration(mem, page, newpage,
(rc == MIGRATEPAGE_SUCCESS ||
rc == MIGRATEPAGE_BALLOON_SUCCESS));
-unlock:
unlock_page(page);
out:
return rc;
@@ -1156,7 +1141,7 @@ static int do_move_page_to_node_array(st
goto set_status;

/* Use PageReserved to check for zero page */
- if (PageReserved(page) || PageKsm(page))
+ if (PageReserved(page))
goto put_and_set;

pp->page = page;
@@ -1318,7 +1303,7 @@ static void do_pages_stat_array(struct m

err = -ENOENT;
/* Use PageReserved to check for zero page */
- if (!page || PageReserved(page) || PageKsm(page))
+ if (!page || PageReserved(page))
goto set_status;

err = page_to_nid(page);

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