mm: check pfn_valid first in zero_resv_unavail

With latest kernel I get below bug while testing kdump:

[    0.000000] BUG: unable to handle kernel paging request at ffffea00034b1040
[    0.000000] IP: zero_resv_unavail+0xbd/0x126
[    0.000000] PGD 37b98067 P4D 37b98067 PUD 37b97067 PMD 0
[    0.000000] Oops: 0002 [#1] SMP
[    0.000000] Modules linked in:
[    0.000000] CPU: 0 PID: 0 Comm: swapper Not tainted 4.15.0-rc1+ torvalds#316
[    0.000000] Hardware name: LENOVO 20ARS1BJ02/20ARS1BJ02, BIOS GJET92WW (2.42 ) 03/03/2017
[    0.000000] task: ffffffff81a0e4c0 task.stack: ffffffff81a00000
[    0.000000] RIP: 0010:zero_resv_unavail+0xbd/0x126
[    0.000000] RSP: 0000:ffffffff81a03d88 EFLAGS: 00010006
[    0.000000] RAX: 0000000000000000 RBX: ffffea00034b1040 RCX: 0000000000000010
[    0.000000] RDX: 0000000000000000 RSI: 0000000000000092 RDI: ffffea00034b1040
[    0.000000] RBP: 00000000000d2c41 R08: 00000000000000c0 R09: 0000000000000a0d
[    0.000000] R10: 0000000000000002 R11: 0000000000007f01 R12: ffffffff81a03d90
[    0.000000] R13: ffffea0000000000 R14: 0000000000000063 R15: 0000000000000062
[    0.000000] FS:  0000000000000000(0000) GS:ffffffff81c73000(0000) knlGS:0000000000000000
[    0.000000] CS:  0010 DS: 0000 ES: 0000 CR0: 0000000080050033
[    0.000000] CR2: ffffea00034b1040 CR3: 0000000037609000 CR4: 00000000000606b0
[    0.000000] Call Trace:
[    0.000000]  ? free_area_init_nodes+0x640/0x664
[    0.000000]  ? zone_sizes_init+0x58/0x72
[    0.000000]  ? setup_arch+0xb50/0xc6c
[    0.000000]  ? start_kernel+0x64/0x43d
[    0.000000]  ? secondary_startup_64+0xa5/0xb0
[    0.000000] Code: c1 e8 0c 48 39 d8 76 27 48 89 de 48 c1 e3 06 48 c7 c7 7a 87 79 81 e8 b0 c0 3e ff 4c 01 eb b9 10 00 00 00 31 c0 48 89 df 49 ff c6 <f3> ab eb bc 6a 00 49
c7 c0 f0 93 d1 81 31 d2 83 ce ff 41 54 49
[    0.000000] RIP: zero_resv_unavail+0xbd/0x126 RSP: ffffffff81a03d88
[    0.000000] CR2: ffffea00034b1040
[    0.000000] ---[ end trace f5ba9e8f73c7ee26 ]---

This is introduced by a4a3ede ("mm: zero reserved and unavailable
struct pages").

The reason is some efi reserved boot ranges is not reported in E820 ram.
In my case it is a bgrt buffer:
efi: mem00: [Boot Data          |RUN|  |  |  |  |  |  |   |WB|WT|WC|UC] range=[0x00000000d2c41000-0x00000000d2c85fff] (0MB)

Use "add_efi_memmap" can workaround the problem with another fix:
http://lkml.kernel.org/r/[email protected]

In zero_resv_unavail it would be better to check pfn_valid first before zero
the page struct. This fixes the problem and potential other similar problems.
Also as Pavel Tatashin suggested checks pfn_valid at the beginning of the
section.

Link: http://lkml.kernel.org/r/[email protected]
Fixes: a4a3ede ("mm: zero reserved and unavailable struct pages")
Signed-off-by: Dave Young <[email protected]>
Cc: Michal Hocko <[email protected]>
Cc: Pavel Tatashin <[email protected]>
Signed-off-by: Andrew Morton <[email protected]>

Author: daveyoung

mm/page_alloc.c

@@ -6260,6 +6260,8 @@ void __paginginit zero_resv_unavail(void)
 	pgcnt = 0;
 	for_each_resv_unavail_range(i, &start, &end) {
 		for (pfn = PFN_DOWN(start); pfn < PFN_UP(end); pfn++) {
+			if (!pfn_valid(ALIGN_DOWN(pfn, pageblock_nr_pages)))
+				continue;
 			mm_zero_struct_page(pfn_to_page(pfn));
 			pgcnt++;
 		}