linux-kernel - Re: [HMM v15 01/16] mm/free_hot_cold_page: catch ZONE

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Date:   Mon, 9 Jan 2017 12:58:56 -0500
From:   Jerome Glisse <jglisse@...hat.com>
To:     Dave Hansen <dave.hansen@...el.com>
Cc:     Balbir Singh <bsingharora@...il.com>, akpm@...ux-foundation.org,
        linux-kernel@...r.kernel.org, linux-mm@...ck.org,
        John Hubbard <jhubbard@...dia.com>,
        Dan Williams <dan.j.williams@...el.com>,
        Ross Zwisler <ross.zwisler@...ux.intel.com>
Subject: Re: [HMM v15 01/16] mm/free_hot_cold_page: catch ZONE_DEVICE pages

On Mon, Jan 09, 2017 at 09:00:34AM -0800, Dave Hansen wrote:
> On 01/09/2017 08:57 AM, Jerome Glisse wrote:
> > On Mon, Jan 09, 2017 at 08:21:25AM -0800, Dave Hansen wrote:
> >> On 01/09/2017 01:19 AM, Balbir Singh wrote:
> >>>> +	/*
> >>>> +	 * This should never happen ! Page from ZONE_DEVICE always must have an
> >>>> +	 * active refcount. Complain about it and try to restore the refcount.
> >>>> +	 */
> >>>> +	if (is_zone_device_page(page)) {
> >>>> +		VM_BUG_ON_PAGE(is_zone_device_page(page), page);
> >>> This can be VM_BUG_ON_PAGE(1, page), hopefully the compiler does the right thing
> >>> here. I suspect this should be a BUG_ON, independent of CONFIG_DEBUG_VM
> >> BUG_ON() means "kill the machine dead".  Do we really want a guaranteed
> >> dead machine if someone screws up their refcounting?
> > VM_BUG_ON_PAGE ok with you ? It is just a safety net, i can simply drop that
> > patch if people have too much feeling about it.
> 
> Enough distros turn on DEBUG_VM that there's basically no difference
> between VM_BUG_ON() and BUG_ON().
> 
> I also think it would be much nicer if you buried the check in the
> allocator in a slow path somewhere instead of sticking it in one of the
> hottest paths in the whole kernel.

Well i will just drop that patch then. The point was to catch error
early on before anything happen. This is just a safety net so not
fundamental.

Cheers,
Jérôme