mm: mremap: downgrade mmap_sem to read when shrinking
[muen/linux.git] / mm / mremap.c
index a9617e72e6b796c967a0fb858bc3897e964c3102..7f9f9180e4013084a02fd27390321240af972e3c 100644 (file)
@@ -521,6 +521,7 @@ SYSCALL_DEFINE5(mremap, unsigned long, addr, unsigned long, old_len,
        unsigned long ret = -EINVAL;
        unsigned long charged = 0;
        bool locked = false;
+       bool downgraded = false;
        struct vm_userfaultfd_ctx uf = NULL_VM_UFFD_CTX;
        LIST_HEAD(uf_unmap_early);
        LIST_HEAD(uf_unmap);
@@ -557,12 +558,20 @@ SYSCALL_DEFINE5(mremap, unsigned long, addr, unsigned long, old_len,
        /*
         * Always allow a shrinking remap: that just unmaps
         * the unnecessary pages..
-        * do_munmap does all the needed commit accounting
+        * __do_munmap does all the needed commit accounting, and
+        * downgrades mmap_sem to read if so directed.
         */
        if (old_len >= new_len) {
-               ret = do_munmap(mm, addr+new_len, old_len - new_len, &uf_unmap);
-               if (ret && old_len != new_len)
+               int retval;
+
+               retval = __do_munmap(mm, addr+new_len, old_len - new_len,
+                                 &uf_unmap, true);
+               if (retval < 0 && old_len != new_len) {
+                       ret = retval;
                        goto out;
+               /* Returning 1 indicates mmap_sem is downgraded to read. */
+               } else if (retval == 1)
+                       downgraded = true;
                ret = addr;
                goto out;
        }
@@ -627,7 +636,10 @@ out:
                vm_unacct_memory(charged);
                locked = 0;
        }
-       up_write(&current->mm->mmap_sem);
+       if (downgraded)
+               up_read(&current->mm->mmap_sem);
+       else
+               up_write(&current->mm->mmap_sem);
        if (locked && new_len > old_len)
                mm_populate(new_addr + old_len, new_len - old_len);
        userfaultfd_unmap_complete(mm, &uf_unmap_early);