diff options
authorHugh Dickins <hughd@google.com>2013-02-22 16:36:07 -0800
committerLinus Torvalds <torvalds@linux-foundation.org>2013-02-23 17:50:23 -0800
commit5117b3b835f288314a2d4e5512bc1747e3a7c8ed (patch)
parentbc56620b493496b8a6962080b644ccc537f4d526 (diff)
mm,ksm: FOLL_MIGRATION do migration_entry_wait
In "ksm: remove old stable nodes more thoroughly" I said that I'd never seen its WARN_ON_ONCE(page_mapped(page)). True at the time of writing, but it soon appeared once I tried fuller tests on the whole series. It turned out to be due to the KSM page migration itself: unmerge_and_ remove_all_rmap_items() failed to locate and replace all the KSM pages, because of that hiatus in page migration when old pte has been replaced by migration entry, but not yet by new pte. follow_page() finds no page at that instant, but a KSM page reappears shortly after, without a fault. Add FOLL_MIGRATION flag, so follow_page() can do migration_entry_wait() for KSM's break_cow(). I'd have preferred to avoid another flag, and do it every time, in case someone else makes the same easy mistake; but did not find another transgressor (the common get_user_pages() is of course safe), and cannot be sure that every follow_page() caller is prepared to sleep - ia64's xencomm_vtop()? Now, THP's wait_split_huge_page() can already sleep there, since anon_vma locking was changed to mutex, but maybe that's somehow excluded. Signed-off-by: Hugh Dickins <hughd@google.com> Cc: Mel Gorman <mgorman@suse.de> Cc: Petr Holasek <pholasek@redhat.com> Cc: Andrea Arcangeli <aarcange@redhat.com> Cc: Izik Eidus <izik.eidus@ravellosystems.com> Cc: Johannes Weiner <hannes@cmpxchg.org> Signed-off-by: Andrew Morton <akpm@linux-foundation.org> Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>
3 files changed, 20 insertions, 3 deletions
diff --git a/include/linux/mm.h b/include/linux/mm.h
index 6124f1db50f..e7c3f9a0111 100644
--- a/include/linux/mm.h
+++ b/include/linux/mm.h
@@ -1651,6 +1651,7 @@ static inline struct page *follow_page(struct vm_area_struct *vma,
#define FOLL_SPLIT 0x80 /* don't return transhuge pages, split them */
#define FOLL_HWPOISON 0x100 /* check page is hwpoisoned */
#define FOLL_NUMA 0x200 /* force NUMA hinting page fault */
+#define FOLL_MIGRATION 0x400 /* wait for page to replace migration entry */
typedef int (*pte_fn_t)(pte_t *pte, pgtable_t token, unsigned long addr,
void *data);
diff --git a/mm/ksm.c b/mm/ksm.c
index 0320327b8a6..d61cba6fa1d 100644
--- a/mm/ksm.c
+++ b/mm/ksm.c
@@ -364,7 +364,7 @@ static int break_ksm(struct vm_area_struct *vma, unsigned long addr)
do {
- page = follow_page(vma, addr, FOLL_GET);
+ page = follow_page(vma, addr, FOLL_GET | FOLL_MIGRATION);
if (IS_ERR_OR_NULL(page))
if (PageKsm(page))
diff --git a/mm/memory.c b/mm/memory.c
index 5d2ef1217d0..ec8ba011fa7 100644
--- a/mm/memory.c
+++ b/mm/memory.c
@@ -1548,8 +1548,24 @@ split_fallthrough:
ptep = pte_offset_map_lock(mm, pmd, address, &ptl);
pte = *ptep;
- if (!pte_present(pte))
- goto no_page;
+ if (!pte_present(pte)) {
+ swp_entry_t entry;
+ /*
+ * KSM's break_ksm() relies upon recognizing a ksm page
+ * even while it is being migrated, so for that case we
+ * need migration_entry_wait().
+ */
+ if (likely(!(flags & FOLL_MIGRATION)))
+ goto no_page;
+ if (pte_none(pte) || pte_file(pte))
+ goto no_page;
+ entry = pte_to_swp_entry(pte);
+ if (!is_migration_entry(entry))
+ goto no_page;
+ pte_unmap_unlock(ptep, ptl);
+ migration_entry_wait(mm, pmd, address);
+ goto split_fallthrough;
+ }
if ((flags & FOLL_NUMA) && pte_numa(pte))
goto no_page;
if ((flags & FOLL_WRITE) && !pte_write(pte))