Re: [PATCH] thp, mm: remove comments on serializion of THP split vs. gup_fast

[Peter Zijlstra <peterz@infradead.org>]

On Thu, Feb 25, 2016 at 10:50:14PM -0800, Hugh Dickins wrote:

> For example, see the fallback tlb_remove_table_one() in mm/memory.c:
> that one uses smp_call_function() sending IPI to all CPUs concerned,
> without waiting an RCU grace period at all.

The better comment is with mmu_table_batch.

Its been too long for me to fully remember, nor have I really paid much
attention to this code in the past few years, so any memory I might have
had might be totally wrong.

But relying on rcu_read_lock_sched() and friends would mean replacing
that smp_call_function() with synchronize_sched().

A real quick look at the current code seems to suggest that _might_ just
work, but note that that will be slower, RT and HPC people will like you
for it though.

So it depends on how hard we hit that special, totally out of memory,
case, and if we care about some performance if we do.

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