[PATCH v1 1/3] mm: fix uninitialized memmaps on a partially populated last section

[David Hildenbrand <david@redhat.com>]

If max_pfn is not aligned to a section boundary, we can easily run into
BUGs. This can e.g., be triggered on x86-64 under QEMU by specifying a
memory size that is not a multiple of 128MB (e.g., 4097MB, but also
4160MB). I was told that on real HW, we can easily have this scenario
(esp., one of the main reasons sub-section hotadd of devmem was added).

The issue is, that we have a valid memmap (pfn_valid()) for the
whole section, and the whole section will be marked "online".
pfn_to_online_page() will succeed, but the memmap contains garbage.

E.g., doing a "cat /proc/kpageflags > /dev/null" results in

[  303.218313] BUG: unable to handle page fault for address: fffffffffffffffe
[  303.218899] #PF: supervisor read access in kernel mode
[  303.219344] #PF: error_code(0x0000) - not-present page
[  303.219787] PGD 12614067 P4D 12614067 PUD 12616067 PMD 0
[  303.220266] Oops: 0000 [#1] SMP NOPTI
[  303.220587] CPU: 0 PID: 424 Comm: cat Not tainted 5.4.0-next-20191128+ #17
[  303.221169] Hardware name: QEMU Standard PC (i440FX + PIIX, 1996), BIOS rel-1.12.0-59-gc9ba5276e321-prebuilt.qemu4
[  303.222140] RIP: 0010:stable_page_flags+0x4d/0x410
[  303.222554] Code: f3 ff 41 89 c0 48 b8 00 00 00 00 01 00 00 00 45 84 c0 0f 85 cd 02 00 00 48 8b 53 08 48 8b 2b 48f
[  303.224135] RSP: 0018:ffff9f5980187e58 EFLAGS: 00010202
[  303.224576] RAX: fffffffffffffffe RBX: ffffda1285004000 RCX: ffff9f5980187dd4
[  303.225178] RDX: 0000000000000001 RSI: ffffffff92662420 RDI: 0000000000000246
[  303.225789] RBP: ffffffffffffffff R08: 0000000000000000 R09: 0000000000000000
[  303.226405] R10: 0000000000000000 R11: 0000000000000000 R12: 00007f31d070e000
[  303.227012] R13: 0000000000140100 R14: 00007f31d070e800 R15: ffffda1285004000
[  303.227629] FS:  00007f31d08f6580(0000) GS:ffff90a6bba00000(0000) knlGS:0000000000000000
[  303.228329] CS:  0010 DS: 0000 ES: 0000 CR0: 0000000080050033
[  303.228820] CR2: fffffffffffffffe CR3: 00000001332a2000 CR4: 00000000000006f0
[  303.229438] Call Trace:
[  303.229654]  kpageflags_read.cold+0x57/0xf0
[  303.230016]  proc_reg_read+0x3c/0x60
[  303.230332]  vfs_read+0xc2/0x170
[  303.230614]  ksys_read+0x65/0xe0
[  303.230898]  do_syscall_64+0x5c/0xa0
[  303.231216]  entry_SYSCALL_64_after_hwframe+0x49/0xbe

This patch fixes that by at least zero-ing out that memmap (so e.g.,
page_to_pfn() will not crash). Commit 907ec5fca3dc ("mm: zero remaining
unavailable struct pages") tried to fix a similar issue, but forgot to
consider this special case.

After this patch, there are still problems to solve. E.g., not all of these
pages falling into a memory hole will actually get initialized later
and set PageReserved - they are only zeroed out - but at least the
immediate crashes are gone. A follow-up patch will take care of this.

Fixes: f7f99100d8d9 ("mm: stop zeroing memory during allocation in vmemmap")
Cc: <stable@vger.kernel.org> # v4.15+
Cc: Naoya Horiguchi <n-horiguchi@ah.jp.nec.com>
Cc: Pavel Tatashin <pasha.tatashin@oracle.com>
Cc: Andrew Morton <akpm@linux-foundation.org>
Cc: Steven Sistare <steven.sistare@oracle.com>
Cc: Michal Hocko <mhocko@suse.com>
Cc: Daniel Jordan <daniel.m.jordan@oracle.com>
Cc: Bob Picco <bob.picco@oracle.com>
Cc: Oscar Salvador <osalvador@suse.de>
Signed-off-by: David Hildenbrand <david@redhat.com>
---
 mm/page_alloc.c | 14 ++++++++++++--
 1 file changed, 12 insertions(+), 2 deletions(-)

diff --git a/mm/page_alloc.c b/mm/page_alloc.c
index 62dcd6b76c80..1eb2ce7c79e4 100644
--- a/mm/page_alloc.c
+++ b/mm/page_alloc.c
@@ -6932,7 +6932,8 @@ static u64 zero_pfn_range(unsigned long spfn, unsigned long epfn)
  * This function also addresses a similar issue where struct pages are left
  * uninitialized because the physical address range is not covered by
  * memblock.memory or memblock.reserved. That could happen when memblock
- * layout is manually configured via memmap=.
+ * layout is manually configured via memmap=, or when the highest physical
+ * address (max_pfn) does not end on a section boundary.
  */
 void __init zero_resv_unavail(void)
 {
@@ -6950,7 +6951,16 @@ void __init zero_resv_unavail(void)
 			pgcnt += zero_pfn_range(PFN_DOWN(next), PFN_UP(start));
 		next = end;
 	}
-	pgcnt += zero_pfn_range(PFN_DOWN(next), max_pfn);
+
+	/*
+	 * Early sections always have a fully populated memmap for the whole
+	 * section - see pfn_valid(). If the last section has holes at the
+	 * end and that section is marked "online", the memmap will be
+	 * considered initialized. Make sure that memmap has a well defined
+	 * state.
+	 */
+	pgcnt += zero_pfn_range(PFN_DOWN(next),
+				round_up(max_pfn, PAGES_PER_SECTION));
 
 	/*
 	 * Struct pages that do not have backing memory. This could be because
-- 
2.21.0