Re: [RFC][PATCH v2 5/5] sched: User Mode Concurency Groups

Peter Zijlstra
2022-01-21T12:26:51Z

On Thu, Jan 20, 2022 at 04:55:22PM +0100, Peter Zijlstra wrote:
> +/*
> + * Pinning a page inhibits rmap based unmap for Anon pages. Doing a load
> + * through the user mapping ensures the user mapping exists.
> + */
> +#define umcg_pin_and_load(_self, _pagep, _member) \
> +({ \
> + __label__ __out; \
> + int __ret = -EFAULT; \
> + \
> + if (pin_user_pages_fast((unsigned long)(_self), 1, 0, &(_pagep)) != 1) \
> + goto __out; \
> + \
> + if (!PageAnon(_pagep) || \
> + get_user(_member, &(_self)->_member)) { \
> + unpin_user_page(_pagep); \
> + goto __out; \
> + } \
> + __ret = 0; \
> +__out: __ret; \
> +})

Per the thread with David, this wants changing like so.

---

--- a/kernel/sched/umcg.c
+++ b/kernel/sched/umcg.c
@@ -34,25 +34,26 @@ static struct task_struct *umcg_get_task
}

/*
- * Pinning a page inhibits rmap based unmap for Anon pages. Doing a load
- * through the user mapping ensures the user mapping exists.
+ * Pinning a page inhibits rmap based unmap for Anon pages. Doing a store
+ * through the user mapping ensures the user mapping exists and is writable.
*/
-#define umcg_pin_and_load(_self, _pagep, _member) \
-({ \
- __label__ __out; \
- int __ret = -EFAULT; \
- \
- if (pin_user_pages_fast((unsigned long)(_self), 1, 0, &(_pagep)) != 1) \
- goto __out; \
- \
- if (!PageAnon(_pagep) || \
- get_user(_member, &(_self)->_member)) { \
- unpin_user_page(_pagep); \
- goto __out; \
- } \
- __ret = 0; \
-__out: __ret; \
-})
+static int umcg_pin_page(struct umcg_task __user *self, struct page **pagep)
+{
+ int ret = -EFAULT;
+
+ if (pin_user_pages_fast((unsigned long)self, 1, FOLL_WRITE, pagep) != 1)
+ goto out;
+
+ if (!PageAnon(*pagep) ||
+ put_user(0ULL, &self->__zero[0])) {
+ unpin_user_page(*pagep);
+ goto out;
+ }
+
+ ret = 0;
+out:
+ return ret;
+}

/**
* umcg_pin_pages: pin pages containing struct umcg_task of
@@ -72,10 +73,13 @@ static int umcg_pin_pages(void)
tsk->umcg_server))
return -EBUSY;

- ret = umcg_pin_and_load(self, tsk->umcg_page, server_tid);
+ ret = umcg_pin_page(self, &tsk->umcg_page);
if (ret)
goto clear_self;

+ if (get_user(server_tid, &self->server_tid))
+ goto unpin_self;
+
ret = -ESRCH;
server = umcg_get_task(server_tid);
if (!server)
@@ -83,7 +87,7 @@ static int umcg_pin_pages(void)

/* must cache due to possible concurrent change */
tsk->umcg_server_task = READ_ONCE(server->umcg_task);
- ret = umcg_pin_and_load(tsk->umcg_server_task, tsk->umcg_server_page, server_tid);
+ ret = umcg_pin_page(tsk->umcg_server_task, &tsk->umcg_server_page);
if (ret)
goto clear_server;

@@ -414,7 +418,7 @@ static int umcg_wait(u64 timo)
break;
}

- ret = umcg_pin_and_load(self, page, state);
+ ret = umcg_pin_page(self, &page);
if (ret) {
page = NULL;
break;