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-rw-r--r--mm/memory.c27
1 files changed, 26 insertions, 1 deletions
diff --git a/mm/memory.c b/mm/memory.c
index 423e0e7..d14b251 100644
--- a/mm/memory.c
+++ b/mm/memory.c
@@ -1785,7 +1785,6 @@ gotten:
page_table = pte_offset_map_lock(mm, pmd, address, &ptl);
if (likely(pte_same(*page_table, orig_pte))) {
if (old_page) {
- page_remove_rmap(old_page, vma);
if (!PageAnon(old_page)) {
dec_mm_counter(mm, file_rss);
inc_mm_counter(mm, anon_rss);
@@ -1807,6 +1806,32 @@ gotten:
lru_cache_add_active(new_page);
page_add_new_anon_rmap(new_page, vma, address);
+ if (old_page) {
+ /*
+ * Only after switching the pte to the new page may
+ * we remove the mapcount here. Otherwise another
+ * process may come and find the rmap count decremented
+ * before the pte is switched to the new page, and
+ * "reuse" the old page writing into it while our pte
+ * here still points into it and can be read by other
+ * threads.
+ *
+ * The critical issue is to order this
+ * page_remove_rmap with the ptp_clear_flush above.
+ * Those stores are ordered by (if nothing else,)
+ * the barrier present in the atomic_add_negative
+ * in page_remove_rmap.
+ *
+ * Then the TLB flush in ptep_clear_flush ensures that
+ * no process can access the old page before the
+ * decremented mapcount is visible. And the old page
+ * cannot be reused until after the decremented
+ * mapcount is visible. So transitively, TLBs to
+ * old page will be flushed before it can be reused.
+ */
+ page_remove_rmap(old_page, vma);
+ }
+
/* Free the old page.. */
new_page = old_page;
ret |= VM_FAULT_WRITE;
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