mm,ksm: swapoff might need to copy

Before establishing that KSM page migration was the cause of my
WARN_ON_ONCE(page_mapped(page))s, I suspected that they came from the
lack of a ksm_might_need_to_copy() in swapoff's unuse_pte() - which in
many respects is equivalent to faulting in a page.

In fact I've never caught that as the cause: but in theory it does at
least need the KSM_RUN_UNMERGE check in ksm_might_need_to_copy(), to
avoid bringing a KSM page back in when it's not supposed to be.

I intended to copy how it's done in do_swap_page(), but have a strong
aversion to how "swapcache" ends up being used there: rework it with
"page != swapcache".

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>
Acked-by: Johannes Weiner <hannes@cmpxchg.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

1 files changed, 14 insertions, 1 deletions

diff --git a/mm/swapfile.c b/mm/swapfile.c
index 9b51266413cd..c72c648f750c 100644
--- a/mm/swapfile.c
+++ b/mm/swapfile.c
@@ -874,11 +874,17 @@ unsigned int count_swap_pages(int type, int free)
 static int unuse_pte(struct vm_area_struct *vma, pmd_t *pmd,
                 unsigned long addr, swp_entry_t entry, struct page *page)
 {
+        struct page *swapcache;
         struct mem_cgroup *memcg;
         spinlock_t *ptl;
         pte_t *pte;
         int ret = 1;
 
+        swapcache = page;
+        page = ksm_might_need_to_copy(page, vma, addr);
+        if (unlikely(!page))
+                return -ENOMEM;
+
         if (mem_cgroup_try_charge_swapin(vma->vm_mm, page,
                                          GFP_KERNEL, &memcg)) {
                 ret = -ENOMEM;
@@ -897,7 +903,10 @@ static int unuse_pte(struct vm_area_struct *vma, pmd_t *pmd,
         get_page(page);
         set_pte_at(vma->vm_mm, addr, pte,
                    pte_mkold(mk_pte(page, vma->vm_page_prot)));
-        page_add_anon_rmap(page, vma, addr);
+        if (page == swapcache)
+                page_add_anon_rmap(page, vma, addr);
+        else /* ksm created a completely new copy */
+                page_add_new_anon_rmap(page, vma, addr);
         mem_cgroup_commit_charge_swapin(page, memcg);
         swap_free(entry);
         /*
@@ -908,6 +917,10 @@ static int unuse_pte(struct vm_area_struct *vma, pmd_t *pmd,
 out:
         pte_unmap_unlock(pte, ptl);
 out_nolock:
+        if (page != swapcache) {
+                unlock_page(page);
+                put_page(page);
+        }
         return ret;
 }