Re: [PATCH] hv_balloon: avoid touching uninitialized struct page during tail onlining
From: Vitaly Kuznetsov
Date: Mon Jan 07 2019 - 08:44:45 EST
David Hildenbrand <david@xxxxxxxxxx> writes:
> 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)
I'm worried a bit about racing with mm code here as we're not doing
mem_hotplug_begin()/done() so I'd slightly prefer keeping
online_section_nr() (pfn_to_online_page() also uses it but then it gets
to the particular struct page). Moreover, with pfn_to_online_page() we
will be looking at some other pfn - because the start_pfn is definitelly
offline (pre-patch we were looking at start_pfn-1). Just looking at the
whole section seems cleaner.
P.S. I still think about bringing mem_hotplug_begin()/done() to
hv_balloon but that's going to be a separate discussion, here I want to
have a small fix backportable to stable.