Re: [PATCH] mm: disable preemption in apply_to_pte_range
From: Jeremy Fitzhardinge
Date: Thu Feb 12 2009 - 20:39:19 EST
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
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