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

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Message-ID: <de3ede24-c883-aa6c-c7de-e76f1d0931bc@intel.com>
Date:   Mon, 9 Jan 2017 09:00:34 -0800
From:   Dave Hansen <dave.hansen@...el.com>
To:     Jerome Glisse <jglisse@...hat.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 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.