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/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;
 	}
 

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);
+}

drivers/misc/cxl/cxl.h

@@ -482,8 +482,6 @@ struct cxl_context {
 	unsigned int sst_size, sst_lru;
 
 	wait_queue_head_t wq;
-	/* pid of the group leader associated with the pid */
-	struct pid *glpid;
 	/* use mm context associated with this pid for ds faults */
 	struct pid *pid;
 	spinlock_t lock; /* Protects pending_irq_mask, pending_fault and fault_addr */
@@ -551,6 +549,8 @@ struct cxl_context {
 	 * CX4 only:
 	 */
 	struct list_head extra_irq_contexts;
+
+	struct mm_struct *mm;
 };
 
 struct cxl_service_layer_ops {
@@ -1012,4 +1012,10 @@ int cxl_adapter_context_lock(struct cxl *adapter);
 /* Unlock the contexts-lock if taken. Warn and force unlock otherwise */
 void cxl_adapter_context_unlock(struct cxl *adapter);
 
+/* Increases the reference count to "struct mm_struct" */
+void cxl_context_mm_count_get(struct cxl_context *ctx);
+
+/* Decrements the reference count to "struct mm_struct" */
+void cxl_context_mm_count_put(struct cxl_context *ctx);
+
 #endif

drivers/misc/cxl/fault.c

@@ -170,81 +170,18 @@ static void cxl_handle_page_fault(struct cxl_context *ctx,
 }
 
 /*
- * Returns the mm_struct corresponding to the context ctx via ctx->pid
- * In case the task has exited we use the task group leader accessible
- * via ctx->glpid to find the next task in the thread group that has a
- * valid  mm_struct associated with it. If a task with valid mm_struct
- * is found the ctx->pid is updated to use the task struct for subsequent
- * translations. In case no valid mm_struct is found in the task group to
- * service the fault a NULL is returned.
+ * Returns the mm_struct corresponding to the context ctx.
+ * mm_users == 0, the context may be in the process of being closed.
  */
 static struct mm_struct *get_mem_context(struct cxl_context *ctx)
 {
-	struct task_struct *task = NULL;
-	struct mm_struct *mm = NULL;
-	struct pid *old_pid = ctx->pid;
-
-	if (old_pid == NULL) {
-		pr_warn("%s: Invalid context for pe=%d\n",
-			 __func__, ctx->pe);
+	if (ctx->mm == NULL)
 		return NULL;
-	}
-
-	task = get_pid_task(old_pid, PIDTYPE_PID);
-
-	/*
-	 * pid_alive may look racy but this saves us from costly
-	 * get_task_mm when the task is a zombie. In worst case
-	 * we may think a task is alive, which is about to die
-	 * but get_task_mm will return NULL.
-	 */
-	if (task != NULL && pid_alive(task))
-		mm = get_task_mm(task);
 
-	/* release the task struct that was taken earlier */
-	if (task)
-		put_task_struct(task);
-	else
-		pr_devel("%s: Context owning pid=%i for pe=%i dead\n",
-			__func__, pid_nr(old_pid), ctx->pe);
-
-	/*
-	 * If we couldn't find the mm context then use the group
-	 * leader to iterate over the task group and find a task
-	 * that gives us mm_struct.
-	 */
-	if (unlikely(mm == NULL && ctx->glpid != NULL)) {
-
-		rcu_read_lock();
-		task = pid_task(ctx->glpid, PIDTYPE_PID);
-		if (task)
-			do {
-				mm = get_task_mm(task);
-				if (mm) {
-					ctx->pid = get_task_pid(task,
-								PIDTYPE_PID);
-					break;
-				}
-				task = next_thread(task);
-			} while (task && !thread_group_leader(task));
-		rcu_read_unlock();
-
-		/* check if we switched pid */
-		if (ctx->pid != old_pid) {
-			if (mm)
-				pr_devel("%s:pe=%i switch pid %i->%i\n",
-					 __func__, ctx->pe, pid_nr(old_pid),
-					 pid_nr(ctx->pid));
-			else
-				pr_devel("%s:Cannot find mm for pid=%i\n",
-					 __func__, pid_nr(old_pid));
-
-			/* drop the reference to older pid */
-			put_pid(old_pid);
-		}
-	}
+	if (!atomic_inc_not_zero(&ctx->mm->mm_users))
+		return NULL;
 
-	return mm;
+	return ctx->mm;
 }
 
 
@@ -282,7 +219,6 @@ void cxl_handle_fault(struct work_struct *fault_work)
 	if (!ctx->kernel) {
 
 		mm = get_mem_context(ctx);
-		/* indicates all the thread in task group have exited */
 		if (mm == NULL) {
 			pr_devel("%s: unable to get mm for pe=%d pid=%i\n",
 				 __func__, ctx->pe, pid_nr(ctx->pid));

drivers/misc/cxl/file.c

@@ -18,6 +18,7 @@
 #include <linux/fs.h>
 #include <linux/mm.h>
 #include <linux/slab.h>
+#include <linux/sched/mm.h>
 #include <asm/cputable.h>
 #include <asm/current.h>
 #include <asm/copro.h>
@@ -216,18 +217,26 @@ static long afu_ioctl_start_work(struct cxl_context *ctx,
 	 * process is still accessible.
 	 */
 	ctx->pid = get_task_pid(current, PIDTYPE_PID);
-	ctx->glpid = get_task_pid(current->group_leader, PIDTYPE_PID);
 
+	/* 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);
 
 	trace_cxl_attach(ctx, work.work_element_descriptor, work.num_interrupts, amr);
 
 	if ((rc = cxl_ops->attach_process(ctx, false, work.work_element_descriptor,
 							amr))) {
 		afu_release_irqs(ctx, ctx);
 		cxl_adapter_context_put(ctx->afu->adapter);
-		put_pid(ctx->glpid);
 		put_pid(ctx->pid);
-		ctx->glpid = ctx->pid = NULL;
+		ctx->pid = NULL;
+		cxl_context_mm_count_put(ctx);
 		goto out;
 	}
 

drivers/misc/cxl/main.c

@@ -59,16 +59,10 @@ int cxl_afu_slbia(struct cxl_afu *afu)
 
 static inline void _cxl_slbia(struct cxl_context *ctx, struct mm_struct *mm)
 {
-	struct task_struct *task;
 	unsigned long flags;
-	if (!(task = get_pid_task(ctx->pid, PIDTYPE_PID))) {
-		pr_devel("%s unable to get task %i\n",
-			 __func__, pid_nr(ctx->pid));
-		return;
-	}
 
-	if (task->mm != mm)
-		goto out_put;
+	if (ctx->mm != mm)
+		return;
 
 	pr_devel("%s matched mm - card: %i afu: %i pe: %i\n", __func__,
 		 ctx->afu->adapter->adapter_num, ctx->afu->slice, ctx->pe);
@@ -79,8 +73,6 @@ static inline void _cxl_slbia(struct cxl_context *ctx, struct mm_struct *mm)
 	spin_unlock_irqrestore(&ctx->sste_lock, flags);
 	mb();
 	cxl_afu_slbia(ctx->afu);
-out_put:
-	put_task_struct(task);
 }
 
 static inline void cxl_slbia_core(struct mm_struct *mm)