[RFC PATCH 2/3] powerpc/lib: Initialize a temporary mm for code patching

[Steven Rostedt]

On Fri, 17 Apr 2020 10:57:10 +1000
Michael Ellerman <mpe at ellerman.id.au> wrote:

> >>> 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