Re: [Xen-devel] VM_BUG_ON_PAGE(!PageOffline(page), page) in alloc_xenballooned_pages

[Marek Marczykowski-Górecki <marmarek@xxxxxxxxxxxxxxxxxxxxxx>]

On Fri, Sep 27, 2019 at 09:44:35AM +0200, David Hildenbrand wrote:
> On 26.09.19 23:34, Marek Marczykowski-Górecki wrote:
> > Hi,
> > 
> > I've hit VM_BUG_ON_PAGE(!PageOffline(page), page) in
> > alloc_xenballooned_pages, when trying to use gnttab from userspace
> > application. It happens on Xen PV, but not on Xen PVH or HVM with the
> > same kernel. This happens at least with 5.1.6, but also 5.2.15
> > (as seen below). Based on this, it looks related to 0266def91377
> > (xen/balloon: Fix mapping PG_offline pages to user space) and probably
> > 77c4adf6a6df (xen/balloon: mark inflated pages PG_offline).
> > 
> > Any idea? Below is full message.
> 
> Now that's weird. Weird because half a year passed since 
> 0266def91377 (xen/balloon: Fix mapping PG_offline pages to user space).

Not sure about others, but in Qubes we use PVH/HVM VMs mostly.

> > page:ffffea0003e7ffc0 refcount:1 mapcount:0 mapping:0000000000000000 
> > index:0x0
> > flags: 0xffffe00001000(reserved)
> 
> So we have a PageReserved page that is not PageOffline. I assume this
> happens when we do a __ClearPageOffline() in alloc_xenballooned_pages().
> 
> That means, that we get such a page via balloon_retrieve(true). Which
> means that we have such a page sitting in the ballooned_pages list, which
> is weird.
> 
> Pages enter ballooned_pages via __balloon_append() only.
> 
> 1. Via xen_online_page(). We have a __SetPageOffline() right in front
>    of it.
> 2. Via balloon_add_region(). I don't see a __SetPageOffline().
> 3. Via decrease_reservation(). We seem to do a __SetPageOffline() on all
>    pages in the previous loop.
> 4. Via free_xenballooned_pages(). We have a __SetPageOffline() right
>    in front of it.
> 
> 
> So this smells like #2 (and matches your PV only observation). Also,
> it makes sense that the page is PageReserved that way.
> 
> 
> Wonder if it is as easy as:

Yes, besides missing semicolon it works. Thanks!

> From 0955beef5aa11da4a8398472ce3106a92599cbe6 Mon Sep 17 00:00:00 2001
> From: David Hildenbrand <david@xxxxxxxxxx>
> Date: Fri, 27 Sep 2019 09:39:31 +0200
> Subject: [PATCH v1] xen/balloon: Set pages PageOffline() in
>  balloon_add_region()
> MIME-Version: 1.0
> Content-Type: text/plain; charset=UTF-8
> Content-Transfer-Encoding: 8bit
> 
> 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@xxxxxxxxxxxxxxxxxxxxxx>
> Fixes: 77c4adf6a6df ("xen/balloon: mark inflated pages PG_offline")
> Cc: stable@xxxxxxxxxxxxxxx # v5.1+
> Signed-off-by: David Hildenbrand <david@xxxxxxxxxx>

Tested-by: Marek Marczykowski-Górecki <marmarek@xxxxxxxxxxxxxxxxxxxxxx>

> ---
>  drivers/xen/balloon.c | 1 +
>  1 file changed, 1 insertion(+)
> 
> diff --git a/drivers/xen/balloon.c b/drivers/xen/balloon.c
> index 05b1f7e948ef..d31149068448 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);
>       }
>  

-- 
Best Regards,
Marek Marczykowski-Górecki
Invisible Things Lab
A: Because it messes up the order in which people normally read text.
Q: Why is top-posting such a bad thing?

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