On Fri, 17 Apr 2020 10:57:10 +1000
Michael Ellerman [off-list ref] wrote:

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Does it needs to be a BUG_ON() ? Can't we fail gracefully with just a
WARN_ON ?
 

I'm not sure what failing gracefully means here? The main reason this could
fail is if there is not enough memory to allocate the patching_mm. The
previous implementation had this justification for BUG_ON():  

But the system can continue running just fine after this failure.
Only the things that make use of code patching will fail (ftrace, kgdb, ...)  

That's probably true of ftrace, but we can't fail patching for jump
labels (static keys).

See:

void arch_jump_label_transform(struct jump_entry *entry,
			       enum jump_label_type type)
{
	u32 *addr = (u32 *)(unsigned long)entry->code;

	if (type == JUMP_LABEL_JMP)
		patch_branch(addr, entry->target, 0);
	else
		patch_instruction(addr, PPC_INST_NOP);
}

I would still error on a WARN_ON() as a lot of static keys should still
work if they don't get switched over.

If a user is concerned about something like this, they can always set
panic_on_warn.

-- Steve