Re: [PATCH v2 0/3] staging: zcache: xcfmalloc support

From: Dave Hansen
Date: Mon Oct 03 2011 - 14:24:35 EST

On Mon, 2011-10-03 at 13:54 -0400, Nitin Gupta wrote:
> I think disabling preemption on the local CPU is the cheapest we can get
> to protect PCPU buffers. We may experiment with, say, multiple buffers
> per CPU, so we end up disabling preemption only in highly improbable
> case of getting preempted just too many times exactly within critical
> section.

I guess the problem is two-fold: preempt_disable() and

> static int zcache_put_page(int cli_id, int pool_id, struct tmem_oid *oidp,
> uint32_t index, struct page *page)
> {
> struct tmem_pool *pool;
> int ret = -1;
> BUG_ON(!irqs_disabled());

That tells me "zcache" doesn't work with interrupts on. It seems like
awfully high-level code to have interrupts disabled. The core page
allocator has some irq-disabling spinlock calls, but that's only really
because it has to be able to service page allocations from interrupts.
What's the high-level reason for zcache?

I'll save the discussion about preempt for when Seth posts his patch.

-- Dave

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