[PATCH] mm: dup_mmap down new mmap_sem

One anomaly remains from when Andrea rationalized the responsibilities of
mmap_sem and page_table_lock: in dup_mmap we add vmas to the child holding its
page_table_lock, but not the mmap_sem which normally guards the vma list and
rbtree. Which could be an issue for unuse_mm: though since it just walks down
the list (today with page_table_lock, tomorrow not), it's probably okay. Will
need a memory barrier? Oh, keep it simple, Nick and I agreed, no harm in
taking child's mmap_sem here.

Signed-off-by: Hugh Dickins <[e-mail]>
Signed-off-by: Andrew Morton <[e-mail]>
Signed-off-by: Linus Torvalds <[e-mail]>

kernel/fork.c

···
 
 	down_write(&oldmm->mmap_sem);
 	flush_cache_mm(oldmm);
+	down_write(&mm->mmap_sem);
+
 	mm->locked_vm = 0;
 	mm->mmap = NULL;
 	mm->mmap_cache = NULL;
···
 		}
 
 		/*
-		 * Link in the new vma and copy the page table entries:
-		 * link in first so that swapoff can see swap entries.
-		 * Note that, exceptionally, here the vma is inserted
-		 * without holding mm->mmap_sem.
+		 * Link in the new vma and copy the page table entries.
 		 */
 		spin_lock(&mm->page_table_lock);
 		*pprev = tmp;
···
 			goto out;
 	}
 	retval = 0;
-
 out:
+	up_write(&mm->mmap_sem);
 	flush_tlb_mm(oldmm);
 	up_write(&oldmm->mmap_sem);
 	return retval;