exec: make exec path waiting for mmap_sem killable

setup_arg_pages requires mmap_sem for write.  If the waiting task gets
killed by the oom killer it would block oom_reaper from asynchronous
address space reclaim and reduce the chances of timely OOM resolving.
Wait for the lock in the killable mode and return with EINTR if the task
got killed while waiting.  All the callers are already handling error
path and the fatal signal doesn't need any additional treatment.

The same applies to __bprm_mm_init.
Signed-off-by: Michal Hocko <mhocko@suse.com>
Acked-by: Oleg Nesterov <oleg@redhat.com>
Acked-by: Vlastimil Babka <vbabka@suse.cz>
Cc: Alexander Viro <viro@zeniv.linux.org.uk>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

fs/exec.c

@@ -263,7 +263,10 @@ static int __bprm_mm_init(struct linux_binprm *bprm)
 	if (!vma)
 		return -ENOMEM;
 
-	down_write(&mm->mmap_sem);
+	if (down_write_killable(&mm->mmap_sem)) {
+		err = -EINTR;
+		goto err_free;
+	}
 	vma->vm_mm = mm;
 
 	/*
@@ -290,6 +293,7 @@ static int __bprm_mm_init(struct linux_binprm *bprm)
 	return 0;
 err:
 	up_write(&mm->mmap_sem);
+err_free:
 	bprm->vma = NULL;
 	kmem_cache_free(vm_area_cachep, vma);
 	return err;
@@ -696,7 +700,9 @@ int setup_arg_pages(struct linux_binprm *bprm,
 		bprm->loader -= stack_shift;
 	bprm->exec -= stack_shift;
 
-	down_write(&mm->mmap_sem);
+	if (down_write_killable(&mm->mmap_sem))
+		return -EINTR;
+
 	vm_flags = VM_STACK_FLAGS;
 
 	/*