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

From: Vitaly Kuznetsov
Date: Fri Jan 04 2019 - 09:19:47 EST

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);