cxl: Keep track of mm struct associated with a context

The mm_struct corresponding to the current task is acquired each time
an interrupt is raised. So to simplify the code, we only get the
mm_struct when attaching an AFU context to the process.
The mm_count reference is increased to ensure that the mm_struct can't
be freed. The mm_struct will be released when the context is detached.
A reference on mm_users is not kept to avoid a circular dependency if
the process mmaps its cxl mmio and forget to unmap before exiting.
The field glpid (pid of the group leader associated with the pid), of
the structure cxl_context, is removed because it's no longer useful.

Signed-off-by: Christophe Lombard <clombard@linux.vnet.ibm.com>
Reviewed-by: Andrew Donnellan <andrew.donnellan@au1.ibm.com>
Acked-by: Frederic Barrat <fbarrat@linux.vnet.ibm.com>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>

1 files changed, 14 insertions, 3 deletions

diff --git a/drivers/misc/cxl/api.c b/drivers/misc/cxl/api.c
index bcc030eacab7..1a138c83f877 100644
--- a/drivers/misc/cxl/api.c
+++ b/drivers/misc/cxl/api.c
@@ -14,6 +14,7 @@
 #include <linux/msi.h>
 #include <linux/module.h>
 #include <linux/mount.h>
+#include <linux/sched/mm.h>
 
 #include "cxl.h"
 
@@ -321,19 +322,29 @@ int cxl_start_context(struct cxl_context *ctx, u64 wed,
 
 	if (task) {
 		ctx->pid = get_task_pid(task, PIDTYPE_PID);
-		ctx->glpid = get_task_pid(task->group_leader, PIDTYPE_PID);
 		kernel = false;
 		ctx->real_mode = false;
+
+		/* acquire a reference to the task's mm */
+		ctx->mm = get_task_mm(current);
+
+		/* ensure this mm_struct can't be freed */
+		cxl_context_mm_count_get(ctx);
+
+		/* decrement the use count */
+		if (ctx->mm)
+			mmput(ctx->mm);
 	}
 
 	cxl_ctx_get();
 
 	if ((rc = cxl_ops->attach_process(ctx, kernel, wed, 0))) {
-		put_pid(ctx->glpid);
 		put_pid(ctx->pid);
-		ctx->glpid = ctx->pid = NULL;
+		ctx->pid = NULL;
 		cxl_adapter_context_put(ctx->afu->adapter);
 		cxl_ctx_put();
+		if (task)
+			cxl_context_mm_count_put(ctx);
 		goto out;
 	}