linux-kernel - Re: [PATCH 1/2] xen/grant-table: Use put_page instead of free

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Date:   Thu, 11 Jan 2018 15:48:36 +0000
From:   Ross Lagerwall <ross.lagerwall@...rix.com>
To:     <xen-devel@...ts.xenproject.org>
CC:     Boris Ostrovsky <boris.ostrovsky@...cle.com>,
        Juergen Gross <jgross@...e.com>,
        <linux-kernel@...r.kernel.org>, <netdev@...r.kernel.org>
Subject: Re: [PATCH 1/2] xen/grant-table: Use put_page instead of free_page

+CC netdev

On 01/11/2018 09:36 AM, Ross Lagerwall wrote:
> The page given to gnttab_end_foreign_access() to free could be a
> compound page so use put_page() instead of free_page() since it can
> handle both compound and single pages correctly.
> 
> This bug was discovered when migrating a Xen VM with several VIFs and
> CONFIG_DEBUG_VM enabled. It hits a BUG usually after fewer than 10
> iterations. All netfront devices disconnect from the backend during a
> suspend/resume and this will call gnttab_end_foreign_access() if a
> netfront queue has an outstanding skb. The mismatch between calling
> get_page() and free_page() on a compound page causes a reference
> counting error which is detected when DEBUG_VM is enabled.
> 
> Signed-off-by: Ross Lagerwall <ross.lagerwall@...rix.com>
> ---
>   drivers/xen/grant-table.c | 4 ++--
>   1 file changed, 2 insertions(+), 2 deletions(-)
> 
> diff --git a/drivers/xen/grant-table.c b/drivers/xen/grant-table.c
> index f45114f..27be107 100644
> --- a/drivers/xen/grant-table.c
> +++ b/drivers/xen/grant-table.c
> @@ -382,7 +382,7 @@ static void gnttab_handle_deferred(struct timer_list *unused)
>   			if (entry->page) {
>   				pr_debug("freeing g.e. %#x (pfn %#lx)\n",
>   					 entry->ref, page_to_pfn(entry->page));
> -				__free_page(entry->page);
> +				put_page(entry->page);
>   			} else
>   				pr_info("freeing g.e. %#x\n", entry->ref);
>   			kfree(entry);
> @@ -438,7 +438,7 @@ void gnttab_end_foreign_access(grant_ref_t ref, int readonly,
>   	if (gnttab_end_foreign_access_ref(ref, readonly)) {
>   		put_free_entry(ref);
>   		if (page != 0)
> -			free_page(page);
> +			put_page(virt_to_page(page));
>   	} else
>   		gnttab_add_deferred(ref, readonly,
>   				    page ? virt_to_page(page) : NULL);
>