[PATCH V3 07/13] powerpc/vas: Take reference to PID and mm for userspace windows

[Haren Myneni <haren@linux.ibm.com>]

Process close windows after its requests are completed. In multi-thread
applications, child can open a window but release FD will not be called
upon its exit. Parent thread will be closing it later upon its exit.

The parent can also send NX requests with this window and NX can
generate page faults. After kernel handles the page fault, send
signal to process by using PID if CSB address is invalid. Parent
thread will not receive signal since its PID is different from the one
saved in vas_window. So use tgid in case if the task for the pid saved
in window is not running and send signal to its parent.

To prevent reusing the pid until the window closed, take reference to
pid and task mm.

Signed-off-by: Haren Myneni <haren@us.ibm.com>
---
 arch/powerpc/platforms/powernv/vas-debug.c  |  2 +-
 arch/powerpc/platforms/powernv/vas-window.c | 53 ++++++++++++++++++++++++++---
 arch/powerpc/platforms/powernv/vas.h        |  9 ++++-
 3 files changed, 57 insertions(+), 7 deletions(-)

diff --git a/arch/powerpc/platforms/powernv/vas-debug.c b/arch/powerpc/platforms/powernv/vas-debug.c
index 09e63df..ef9a717 100644
--- a/arch/powerpc/platforms/powernv/vas-debug.c
+++ b/arch/powerpc/platforms/powernv/vas-debug.c
@@ -38,7 +38,7 @@ static int info_show(struct seq_file *s, void *private)
 
 	seq_printf(s, "Type: %s, %s\n", cop_to_str(window->cop),
 					window->tx_win ? "Send" : "Receive");
-	seq_printf(s, "Pid : %d\n", window->pid);
+	seq_printf(s, "Pid : %d\n", vas_window_pid(window));
 
 unlock:
 	mutex_unlock(&vas_mutex);
diff --git a/arch/powerpc/platforms/powernv/vas-window.c b/arch/powerpc/platforms/powernv/vas-window.c
index e36c5d2..0f27ac5 100644
--- a/arch/powerpc/platforms/powernv/vas-window.c
+++ b/arch/powerpc/platforms/powernv/vas-window.c
@@ -12,6 +12,8 @@
 #include <linux/log2.h>
 #include <linux/rcupdate.h>
 #include <linux/cred.h>
+#include <linux/sched/mm.h>
+#include <linux/mmu_context.h>
 #include <asm/switch_to.h>
 #include <asm/ppc-opcode.h>
 #include "vas.h"
@@ -877,8 +879,6 @@ struct vas_window *vas_rx_win_open(int vasid, enum vas_cop_type cop,
 	rxwin->user_win = rxattr->user_win;
 	rxwin->cop = cop;
 	rxwin->wcreds_max = rxattr->wcreds_max ?: VAS_WCREDS_DEFAULT;
-	if (rxattr->user_win)
-		rxwin->pid = task_pid_vnr(current);
 
 	init_winctx_for_rxwin(rxwin, rxattr, &winctx);
 	init_winctx_regs(rxwin, &winctx);
@@ -1028,7 +1028,6 @@ struct vas_window *vas_tx_win_open(int vasid, enum vas_cop_type cop,
 	txwin->tx_win = 1;
 	txwin->rxwin = rxwin;
 	txwin->nx_win = txwin->rxwin->nx_win;
-	txwin->pid = attr->pid;
 	txwin->user_win = attr->user_win;
 	txwin->wcreds_max = attr->wcreds_max ?: VAS_WCREDS_DEFAULT;
 
@@ -1069,8 +1068,43 @@ struct vas_window *vas_tx_win_open(int vasid, enum vas_cop_type cop,
 			goto free_window;
 	}
 
-	set_vinst_win(vinst, txwin);
+	if (txwin->user_win) {
+		/*
+		 * Window opened by child thread may not be closed when
+		 * it exits. So take reference to its pid and release it
+		 * when the window is free by parent thread.
+		 * Acquire a reference to the task's pid to make sure
+		 * pid will not be re-used - needed only for multithread
+		 * applications.
+		 */
+		txwin->pid = get_task_pid(current, PIDTYPE_PID);
+		/*
+		 * Acquire a reference to the task's mm.
+		 */
+		txwin->mm = get_task_mm(current);
 
+		if (!txwin->mm) {
+			put_pid(txwin->pid);
+			pr_err("VAS: pid(%d): mm_struct is not found\n",
+					current->pid);
+			rc = -EPERM;
+			goto free_window;
+		}
+
+		mmgrab(txwin->mm);
+		mmput(txwin->mm);
+		mm_context_add_copro(txwin->mm);
+		/*
+		 * Process closes window during exit. In the case of
+		 * multithread application, child can open window and
+		 * can exit without closing it. Expects parent thread
+		 * to use and close the window. So do not need to take
+		 * pid reference for parent thread.
+		 */
+		txwin->tgid = find_get_pid(task_tgid_vnr(current));
+	}
+
+	set_vinst_win(vinst, txwin);
 	return txwin;
 
 free_window:
@@ -1267,8 +1301,17 @@ int vas_win_close(struct vas_window *window)
 	poll_window_castout(window);
 
 	/* if send window, drop reference to matching receive window */
-	if (window->tx_win)
+	if (window->tx_win) {
+		if (window->user_win) {
+			/* Drop references to pid and mm */
+			put_pid(window->pid);
+			if (window->mm) {
+				mmdrop(window->mm);
+				mm_context_remove_copro(window->mm);
+			}
+		}
 		put_rx_win(window->rxwin);
+	}
 
 	vas_window_free(window);
 
diff --git a/arch/powerpc/platforms/powernv/vas.h b/arch/powerpc/platforms/powernv/vas.h
index 2621df1..af03aa0 100644
--- a/arch/powerpc/platforms/powernv/vas.h
+++ b/arch/powerpc/platforms/powernv/vas.h
@@ -340,7 +340,9 @@ struct vas_window {
 	bool user_win;		/* True if user space window */
 	void *hvwc_map;		/* HV window context */
 	void *uwc_map;		/* OS/User window context */
-	pid_t pid;		/* Linux process id of owner */
+	struct pid *pid;	/* Linux process id of owner */
+	struct pid *tgid;	/* Thread group ID of owner */
+	struct mm_struct *mm;	/* Linux process mm_struct */
 	int wcreds_max;		/* Window credits */
 
 	char *dbgname;
@@ -418,6 +420,11 @@ struct vas_winctx {
 extern struct vas_window *vas_pswid_to_window(struct vas_instance *vinst,
 						uint32_t pswid);
 
+static inline int vas_window_pid(struct vas_window *window)
+{
+	return pid_vnr(window->pid);
+}
+
 static inline void vas_log_write(struct vas_window *win, char *name,
 			void *regptr, u64 val)
 {
-- 
1.8.3.1