mm: don't defer struct page initialization for Xen pv guests

Commit f7f9910 ("mm: stop zeroing
memory during allocation in vmemmap") broke Xen pv domains in some
configurations, as the "Pinned" information in struct page of early
page tables could get lost. This will lead to the kernel trying to
write directly into the page tables instead of asking the hypervisor
to do so. The result is a crash like the following:

[    0.004000] BUG: unable to handle kernel paging request at ffff8801ead19008
[    0.004000] IP: xen_set_pud+0x4e/0xd0
[    0.004000] PGD 1c0a067 P4D 1c0a067 PUD 23a0067 PMD 1e9de0067 PTE 80100001ead19065
[    0.004000] Oops: 0003 [#1] PREEMPT SMP
[    0.004000] Modules linked in:
[    0.004000] CPU: 0 PID: 0 Comm: swapper/0 Not tainted 4.14.0-default+ torvalds#271
[    0.004000] Hardware name: Dell Inc. Latitude E6440/0159N7, BIOS A07 06/26/2014
[    0.004000] task: ffffffff81c10480 task.stack: ffffffff81c00000
[    0.004000] RIP: e030:xen_set_pud+0x4e/0xd0
[    0.004000] RSP: e02b:ffffffff81c03cd8 EFLAGS: 00010246
[    0.004000] RAX: 002ffff800000800 RBX: ffff88020fd31000 RCX: 0000000000000000
[    0.004000] RDX: ffffea0000000000 RSI: 00000001b8308067 RDI: ffff8801ead19008
[    0.004000] RBP: ffff8801ead19008 R08: aaaaaaaaaaaaaaaa R09: 00000000063f4c80
[    0.004000] R10: aaaaaaaaaaaaaaaa R11: 0720072007200720 R12: 00000001b8308067
[    0.004000] R13: ffffffff81c8a9cc R14: ffff88018fd31000 R15: 000077ff80000000
[    0.004000] FS:  0000000000000000(0000) GS:ffff88020f600000(0000) knlGS:0000000000000000
[    0.004000] CS:  e033 DS: 0000 ES: 0000 CR0: 0000000080050033
[    0.004000] CR2: ffff8801ead19008 CR3: 0000000001c09000 CR4: 0000000000042660
[    0.004000] Call Trace:
[    0.004000]  __pmd_alloc+0x128/0x140
[    0.004000]  ? acpi_os_map_iomem+0x175/0x1b0
[    0.004000]  ioremap_page_range+0x3f4/0x410
[    0.004000]  ? acpi_os_map_iomem+0x175/0x1b0
[    0.004000]  __ioremap_caller+0x1c3/0x2e0
[    0.004000]  acpi_os_map_iomem+0x175/0x1b0
[    0.004000]  acpi_tb_acquire_table+0x39/0x66
[    0.004000]  acpi_tb_validate_table+0x44/0x7c
[    0.004000]  acpi_tb_verify_temp_table+0x45/0x304
[    0.004000]  ? acpi_ut_acquire_mutex+0x12a/0x1c2
[    0.004000]  acpi_reallocate_root_table+0x12d/0x141
[    0.004000]  acpi_early_init+0x4d/0x10a
[    0.004000]  start_kernel+0x3eb/0x4a1
[    0.004000]  ? set_init_arg+0x55/0x55
[    0.004000]  xen_start_kernel+0x528/0x532
[    0.004000] Code: 48 01 e8 48 0f 42 15 a2 fd be 00 48 01 d0 48 ba 00 00 00 00 00 ea ff ff 48 c1 e8 0c 48 c1 e0 06 48 01 d0 48 8b 00 f6 c4 02 75 5d <4c> 89 65 00 5b 5d 41 5c c3 65 8b 05 52 9f fe 7e 89 c0 48 0f a3
[    0.004000] RIP: xen_set_pud+0x4e/0xd0 RSP: ffffffff81c03cd8
[    0.004000] CR2: ffff8801ead19008
[    0.004000] ---[ end trace 38eca2e56f1b642e ]---

Avoid this problem by not deferring struct page initialization when
running as Xen pv guest.

Cc: <stable@vger.kernel.org> #4.15
Signed-off-by: Juergen Gross <jgross@suse.com>

mm/page_alloc.c

@@ -348,6 +348,9 @@ static inline bool update_defer_init(pg_data_t *pgdat,
 	/* Always populate low zones for address-constrained allocations */
 	if (zone_end < pgdat_end_pfn(pgdat))
 		return true;
+	/* Xen PV domains need page structures early */
+	if (xen_pv_domain())
+		return true;
 	(*nr_initialised)++;
 	if ((*nr_initialised > pgdat->static_init_pgcnt) &&
 	    (pfn & (PAGES_PER_SECTION - 1)) == 0) {