Re: [RFC] CMA page migration failure due to buffers on bh_lru

[Andrew Morton]

On Mon, 30 Jun 2014 19:02:45 -0700 Laura Abbott <lauraa@xxxxxxxxxxxxxx> wrote:

> On 6/30/2014 6:07 PM, Gioh Kim wrote:
> > Hi,Laura.
> > 
> > I have a question.
> > 
> > Does the __evict_bh_lru() not need bh_lru_lock()?
> > The get_cpu_var() has already preenpt_disable() and can prevent other thread.
> > But get_cpu_var cannot prevent IRQ context such like page-fault.
> > I think if a page-fault occured and a file is read in IRQ context it can change cpu-lru.
> > 
> > Is my concern correct?
> > 
> > 
> 
> __evict_bh_lru is called via on_each_cpu_cond which I believe will disable irqs.
> I based the code on the existing invalidate_bh_lru which did not take the bh_lru_lock
> either. It's possible I missed something though.

I fear that running on_each_cpu() within try_to_free_buffers() is going
to be horridly expensive in some cases.

Maybe CMA can use invalidate_bh_lrus() to shoot down everything before
trying the allocation attempt.  That should increase the success rate
greatly and doesn't burden page reclaim.  The bh LRU isn't terribly
important from a performance point of view, so emptying it occasionally
won't hurt.

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