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Subject[PATCH 4.19 050/103] hv_balloon: avoid touching uninitialized struct page during tail onlining
Date
4.19-stable review patch.  If anyone has any objections, please let me know.

------------------

From: Vitaly Kuznetsov <vkuznets@redhat.com>

commit da8ced360ca8ad72d8f41f5c8fcd5b0e63e1555f upstream.

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@redhat.com>
Cc: stable@kernel.org
Signed-off-by: Sasha Levin <sashal@kernel.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

---
drivers/hv/hv_balloon.c | 10 ++++++----
1 file changed, 6 insertions(+), 4 deletions(-)

--- a/drivers/hv/hv_balloon.c
+++ b/drivers/hv/hv_balloon.c
@@ -888,12 +888,14 @@ static unsigned long handle_pg_range(uns
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);

}

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