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>

drivers/misc/cxl/context.c

@@ -17,6 +17,7 @@
 #include <linux/debugfs.h>
 #include <linux/slab.h>
 #include <linux/idr.h>
+#include <linux/sched/mm.h>
 #include <asm/cputable.h>
 #include <asm/current.h>
 #include <asm/copro.h>
@@ -41,7 +42,7 @@ int cxl_context_init(struct cxl_context *ctx, struct cxl_afu *afu, bool master)
 	spin_lock_init(&ctx->sste_lock);
 	ctx->afu = afu;
 	ctx->master = master;
-	ctx->pid = ctx->glpid = NULL; /* Set in start work ioctl */
+	ctx->pid = NULL; /* Set in start work ioctl */
 	mutex_init(&ctx->mapping_lock);
 	ctx->mapping = NULL;
 
@@ -242,12 +243,16 @@ int __detach_context(struct cxl_context *ctx)
 
 	/* release the reference to the group leader and mm handling pid */
 	put_pid(ctx->pid);
-	put_pid(ctx->glpid);
 
 	cxl_ctx_put();
 
 	/* Decrease the attached context count on the adapter */
 	cxl_adapter_context_put(ctx->afu->adapter);
+
+	/* Decrease the mm count on the context */
+	cxl_context_mm_count_put(ctx);
+	ctx->mm = NULL;
+
 	return 0;
 }
 
@@ -325,3 +330,15 @@ void cxl_context_free(struct cxl_context *ctx)
 	mutex_unlock(&ctx->afu->contexts_lock);
 	call_rcu(&ctx->rcu, reclaim_ctx);
 }
+
+void cxl_context_mm_count_get(struct cxl_context *ctx)
+{
+	if (ctx->mm)
+		atomic_inc(&ctx->mm->mm_count);
+}
+
+void cxl_context_mm_count_put(struct cxl_context *ctx)
+{
+	if (ctx->mm)
+		mmdrop(ctx->mm);
+}