Re: [PATCH v2 1/5] powerpc/mm: Introduce temporary mm

Hi Chris,
  

This is on the nitpicky end, but I wonder if this should be named
temp_mm, or should be labelled something else to capture its broader
purpose as a context for code patching? I'm thinking that a store of
breakpoints is perhaps unusual in a memory-managment structure?

I don't have a better suggestion off the top of my head and I'm happy
for you to leave it, I just wanted to flag it as a possible way we could
be clearer.

+
+static inline void init_temp_mm(struct temp_mm *temp_mm, struct mm_struct *mm)
+{
+	temp_mm->temp = mm;
+	temp_mm->prev = NULL;
+	temp_mm->is_kernel_thread = false;
+	memset(&temp_mm->brk, 0, sizeof(temp_mm->brk));
+}
+
+static inline void use_temporary_mm(struct temp_mm *temp_mm)
+{
+	lockdep_assert_irqs_disabled();
+
+	temp_mm->is_kernel_thread = current->mm == NULL;
+	if (temp_mm->is_kernel_thread)
+		temp_mm->prev = current->active_mm;

You don't seem to restore active_mm below. I don't know what active_mm
does, so I don't know if this is a problem.

+	else
+		temp_mm->prev = current->mm;
+
+	/*
+	 * Hash requires a non-NULL current->mm to allocate a userspace address
+	 * when handling a page fault. Does not appear to hurt in Radix either.
+	 */
+	current->mm = temp_mm->temp;
+	switch_mm_irqs_off(NULL, temp_mm->temp, current);
+
+	if (ppc_breakpoint_available()) {

I wondered if this could be changed during a text-patching operation.
AIUI, it potentially can on a P9 via "dawr_enable_dangerous" in debugfs.

I don't know if that's a problem. My concern is that you could turn off
breakpoints, call 'use_temporary_mm', then turn them back on again
before 'unuse_temporary_mm' and get a breakpoint while that can access
the temporary mm. Is there something else that makes that safe?
disabling IRQs maybe?

+		struct arch_hw_breakpoint null_brk = {0};
+		int i = 0;
+
+		for (; i < nr_wp_slots(); ++i) {

super nitpicky, and I'm not sure if this is actually documented, but I'd
usually see this written as:

for (i = 0; i < nr_wp_slots(); i++) {

Not sure if there's any reason that it _shouldn't_ be written the way
you've written it (and I do like initialising the variable when it's
defined!), I'm just not used to it. (Likewise with the unuse function.)

+			__get_breakpoint(i, &temp_mm->brk[i]);
+			if (temp_mm->brk[i].type != 0)
+				__set_breakpoint(i, &null_brk);
+		}
+	}
+}
+

Kind regards,
Daniel