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Subject: Re: [PATCH v1] xen/balloon: Set pages PageOffline() in
 balloon_add_region()
To:     David Hildenbrand <david@redhat.com>, linux-kernel@vger.kernel.org
Cc:     linux-mm@kvack.org, xen-devel@lists.xenproject.org,
        =?UTF-8?Q?Marek_Marczykowski-G=c3=b3recki?= 
        <marmarek@invisiblethingslab.com>, stable@vger.kernel.org,
        Juergen Gross <jgross@suse.com>,
        Stefano Stabellini <sstabellini@kernel.org>
References: <20190927154628.8480-1-david@redhat.com>
From:   Boris Ostrovsky <boris.ostrovsky@oracle.com>
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Message-ID: <4d6a1f9b-15ff-6594-44e0-bc262e9347f7@oracle.com>
Date:   Fri, 27 Sep 2019 14:08:58 -0400
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On 9/27/19 11:46 AM, David Hildenbrand wrote:
> We are missing a __SetPageOffline(), which is why we can get
> !PageOffline() pages onto the balloon list, where
> alloc_xenballooned_pages() will complain:
>
> page:ffffea0003e7ffc0 refcount:1 mapcount:0 mapping:0000000000000000 index:0x0
> flags: 0xffffe00001000(reserved)
> raw: 000ffffe00001000 dead000000000100 dead000000000200 0000000000000000
> raw: 0000000000000000 0000000000000000 00000001ffffffff 0000000000000000
> page dumped because: VM_BUG_ON_PAGE(!PageOffline(page))
> ------------[ cut here ]------------
> kernel BUG at include/linux/page-flags.h:744!
> invalid opcode: 0000 [#1] SMP NOPTI
>
> Reported-by: Marek Marczykowski-Górecki <marmarek@invisiblethingslab.com>
> Tested-by: Marek Marczykowski-Górecki <marmarek@invisiblethingslab.com>
> Fixes: 77c4adf6a6df ("xen/balloon: mark inflated pages PG_offline")
> Cc: stable@vger.kernel.org # v5.1+
> Cc: Boris Ostrovsky <boris.ostrovsky@oracle.com>
> Cc: Juergen Gross <jgross@suse.com>
> Cc: Stefano Stabellini <sstabellini@kernel.org>
> Signed-off-by: David Hildenbrand <david@redhat.com>
> ---
>  drivers/xen/balloon.c | 1 +
>  1 file changed, 1 insertion(+)
>
> diff --git a/drivers/xen/balloon.c b/drivers/xen/balloon.c
> index 05b1f7e948ef..29f6256363db 100644
> --- a/drivers/xen/balloon.c
> +++ b/drivers/xen/balloon.c
> @@ -687,6 +687,7 @@ static void __init balloon_add_region(unsigned long start_pfn,
>  		/* totalram_pages and totalhigh_pages do not
>  		   include the boot-time balloon extension, so
>  		   don't subtract from it. */
> +		__SetPageOffline(page);
>  		__balloon_append(page);


Given that when a page is appended to balloon list need to be marked
offline and, conversely, when a page is retrieved the bit should be
cleared I wonder whether it's better to move Set/ClearPageOffline to
balloon_append/retrieve().

-boris