Re: [PATCH 1/5] Swap Migration V5: LRU operations

[Andrew Morton <akpm@osdl.org>]

Christoph Lameter <clameter@engr.sgi.com> wrote:
>
> On Mon, 14 Nov 2005, Andrew Morton wrote:
> 
> > >  +int isolate_lru_page(struct page *page)
> > >  +{
> > >  +	int rc = 0;
> > >  +	struct zone *zone = page_zone(page);
> > >  +
> > >  +redo:
> > >  +	spin_lock_irq(&zone->lru_lock);
> > >  +	rc = __isolate_lru_page(zone, page);
> > >  +	spin_unlock_irq(&zone->lru_lock);
> > >  +	if (rc == 0) {
> > >  +		/*
> > >  +		 * Maybe this page is still waiting for a cpu to drain it
> > >  +		 * from one of the lru lists?
> > >  +		 */
> > >  +		smp_call_function(&lru_add_drain_per_cpu, NULL, 0 , 1);
> > 
> > lru_add_drain() ends up doing spin_unlock_irq(), so we'll enable interrupts
> > within the smp_call_function() handler.  Is that legal on all
> > architectures?
> 
> isolate_lru_pages() is only called within a process context in the swap 
> migration patches. The hotplug folks may have to address this if they want 
> to isolate pages from interrupts etc.

But lru_add_drain_per_cpu() will be called from interrupt context: the IPI
handler.

I'm asking whether it is safe for the IPI handler to reenable interupts on
all architectures.  It might be so, but I don't recall ever having seen it
discussed, nor have I seen code which does it.
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