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

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Date:   Mon, 9 Jan 2017 14:49:52 +0530
From:   Balbir Singh <bsingharora@...il.com>
To:     Jérôme Glisse <jglisse@...hat.com>
Cc:     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 Fri, Jan 06, 2017 at 11:46:28AM -0500, Jérôme Glisse wrote:
> Catch page from ZONE_DEVICE in free_hot_cold_page(). This should never
> happen as ZONE_DEVICE page must always have an elevated refcount.
> 
> This is safety-net to catch any refcounting issues in a sane way for any
> ZONE_DEVICE pages.
> 
> Signed-off-by: Jérôme Glisse <jglisse@...hat.com>
> Cc: Dan Williams <dan.j.williams@...el.com>
> Cc: Ross Zwisler <ross.zwisler@...ux.intel.com>
> ---
>  mm/page_alloc.c | 10 ++++++++++
>  1 file changed, 10 insertions(+)
> 
> diff --git a/mm/page_alloc.c b/mm/page_alloc.c
> index 1c24112..355beb4 100644
> --- a/mm/page_alloc.c
> +++ b/mm/page_alloc.c
> @@ -2445,6 +2445,16 @@ void free_hot_cold_page(struct page *page, bool cold)
>  	unsigned long pfn = page_to_pfn(page);
>  	int migratetype;
>  
> +	/*
> +	 * 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

> +		page_ref_inc(page);
> +		return;
> +	}
> +

Balbir Singh.