Re: [PATCH] hv_balloon: avoid touching uninitialized struct page during tail onlining
From: David Hildenbrand
Date: Mon Jan 07 2019 - 05:21:24 EST
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:
> 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)
David / dhildenb