Re: [PATCH] mm: disable preemption in apply_to_pte_range

[Jeremy Fitzhardinge <jeremy@goop.org>]

Andrew Morton wrote:
> This weakens the apply_to_page_range() utility by newly requiring that
> the callback function be callable under preempt_disable() if the target
> mm is init_mm.  I guess we can live with that.
>
> It's OK for the two present in-tree callers.  There might of course be
> out-of-tree callers which break, but it is unlikely.
>
> The patch should include a comment explaining why there is a random
> preempt_disable() in this function.
>   

I cuddled them up to their corresponding arch_X_lazy_mmu_mode calls to 
get this across, but I guess some prose would be helpful here.

> Why is apply_to_page_range() exported to modules, btw?  I can find no
> modules which need it.  Unexporting that function would make the
> proposed weakening even less serious.
>   

I have some yet-to-be upstreamed code that can use it from modules.

> The patch assumes that
> arch_enter_lazy_mmu_mode()/arch_leave_lazy_mmu_mode() must have
> preemption disabled for all architectures.  Is this a sensible
> assumption?
>   

In general the model for lazy updates is that you're batching the 
updates in some queue somewhere, which is almost certainly a piece of 
percpu state being maintained by someone.  Its therefore broken and/or 
meaningless to have the code making the updates wandering between cpus 
for the duration of the lazy updates.

> If so, should we do the preempt_disable/enable within those functions? 
> Probably not worth the cost, I guess.

The specific rules are that 
arch_enter_lazy_mmu_mode()/arch_leave_lazy_mmu_mode() require you to be 
holding the appropriate pte locks for the ptes you're updating, so 
preemption is naturally disabled in that case.

This all goes a bit strange with init_mm's non-requirement for taking 
pte locks.  The caller has to arrange for some kind of serialization on 
updating the range in question, and that could be a mutex.  Explicitly 
disabling preemption in enter_lazy_mmu_mode would make sense for this 
case, but it would be redundant for the common case of batched updates 
to usermode ptes.

    J