Re: [PATCH] hv_balloon: avoid touching uninitialized struct page during tail onlining

[David Hildenbrand]

On 04.01.19 15:19, Vitaly Kuznetsov wrote:
> Hyper-V memory hotplug protocol has 2M granularity and in Linux x86 we use
> 128M. To deal with it we implement partial section onlining by registering
> custom page onlining callback (hv_online_page()). Later, when more memory
> arrives we try to online the 'tail' (see hv_bring_pgs_online()).
> 
> It was found that in some cases this 'tail' onlining causes issues:
> 
>  BUG: Bad page state in process kworker/0:2  pfn:109e3a
>  page:ffffe08344278e80 count:0 mapcount:1 mapping:0000000000000000 index:0x0
>  flags: 0xfffff80000000()
>  raw: 000fffff80000000 dead000000000100 dead000000000200 0000000000000000
>  raw: 0000000000000000 0000000000000000 0000000000000000 0000000000000000
>  page dumped because: nonzero mapcount
>  ...
>  Workqueue: events hot_add_req [hv_balloon]
>  Call Trace:
>   dump_stack+0x5c/0x80
>   bad_page.cold.112+0x7f/0xb2
>   free_pcppages_bulk+0x4b8/0x690
>   free_unref_page+0x54/0x70
>   hv_page_online_one+0x5c/0x80 [hv_balloon]
>   hot_add_req.cold.24+0x182/0x835 [hv_balloon]
>   ...
> 
> Turns out that we now have deferred struct page initialization for memory
> hotplug so e.g. memory_block_action() in drivers/base/memory.c does
> pages_correctly_probed() check and in that check it avoids inspecting
> struct pages and checks sections instead. But in Hyper-V balloon driver we
> do PageReserved(pfn_to_page()) check and this is now wrong.
> 
> Switch to checking online_section_nr() instead.
> 
> Signed-off-by: Vitaly Kuznetsov <vkuznets@xxxxxxxxxx>
> ---
>  drivers/hv/hv_balloon.c | 10 ++++++----
>  1 file changed, 6 insertions(+), 4 deletions(-)
> 
> diff --git a/drivers/hv/hv_balloon.c b/drivers/hv/hv_balloon.c
> index 5301fef16c31..7c6349a50ef1 100644
> --- a/drivers/hv/hv_balloon.c
> +++ b/drivers/hv/hv_balloon.c
> @@ -888,12 +888,14 @@ static unsigned long handle_pg_range(unsigned long pg_start,
>  			pfn_cnt -= pgs_ol;
>  			/*
>  			 * Check if the corresponding memory block is already
> -			 * online by checking its last previously backed page.
> -			 * In case it is we need to bring rest (which was not
> -			 * backed previously) online too.
> +			 * online. It is possible to observe struct pages still
> +			 * being uninitialized here so check section instead.
> +			 * In case the section is online we need to bring the
> +			 * rest of pfns (which were not backed previously)
> +			 * online too.
>  			 */
>  			if (start_pfn > has->start_pfn &&
> -			    !PageReserved(pfn_to_page(start_pfn - 1)))
> +			    online_section_nr(pfn_to_section_nr(start_pfn)))
>  				hv_bring_pgs_online(has, start_pfn, pgs_ol);
>  
>  		}
> 

I wonder if you should use pfn_to_online_page() and check for PageOffline().

(I guess online_section_nr() should also do the trick)

-- 

Thanks,

David / dhildenb