[PATCH v3] mm, page_alloc: Fix has_unmovable_pages for HugePages

[Oscar Salvador <osalvador@suse.de>]

v3 -> v2: Get rid of the round_up()
v2 -> v1: Adjust skip pages logic per Michal

From 8c057ff497a078f28e293af8c0bd089893a57753 Mon Sep 17 00:00:00 2001
From: Oscar Salvador <osalvador@suse.de>
Date: Wed, 19 Dec 2018 00:04:18 +0000
Subject: [PATCH] mm, page_alloc: fix has_unmovable_pages for HugePages

While playing with gigantic hugepages and memory_hotplug, I triggered the
following #PF when "cat memoryX/removable":

<---
kernel: BUG: unable to handle kernel NULL pointer dereference at 0000000000000008
kernel: #PF error: [normal kernel read fault]
kernel: PGD 0 P4D 0
kernel: Oops: 0000 [#1] SMP PTI
kernel: CPU: 1 PID: 1481 Comm: cat Tainted: G            E     4.20.0-rc6-mm1-1-default+ #18
kernel: Hardware name: QEMU Standard PC (i440FX + PIIX, 1996), BIOS 1.0.0-prebuilt.qemu-project.org 04/01/2014
kernel: RIP: 0010:has_unmovable_pages+0x154/0x210
kernel: Code: 1b ff ff ff eb 32 48 8b 45 00 bf 00 10 00 00 a9 00 00 01 00 74 07 0f b6 4d 51 48 d3 e7 e8 c4 81 05 00 48 85 c0 49 89 c1 75 7e <41> 8b 41 08 83 f8 09 74 41 83 f8 1b 74 3c 4d 2b 64 24 58 49 81 ec
kernel: RSP: 0018:ffffc90000a1fd30 EFLAGS: 00010246
kernel: RAX: 0000000000000000 RBX: 0000000000000000 RCX: 0000000000000009
kernel: RDX: ffffffff82aed4f0 RSI: 0000000000001000 RDI: 0000000000001000
kernel: RBP: ffffea0001800000 R08: 0000000000200000 R09: 0000000000000000
kernel: R10: 0000000000001000 R11: 0000000000000003 R12: ffff88813ffd45c0
kernel: R13: 0000000000060000 R14: 0000000000000001 R15: ffffea0000000000
kernel: FS:  00007fd71d9b3500(0000) GS:ffff88813bb00000(0000) knlGS:0000000000000000
kernel: CS:  0010 DS: 0000 ES: 0000 CR0: 0000000080050033
kernel: CR2: 0000000000000008 CR3: 00000001371c2002 CR4: 00000000003606e0
kernel: DR0: 0000000000000000 DR1: 0000000000000000 DR2: 0000000000000000
kernel: DR3: 0000000000000000 DR6: 00000000fffe0ff0 DR7: 0000000000000400
kernel: Call Trace:
kernel:  is_mem_section_removable+0x7d/0x100
kernel:  removable_show+0x90/0xb0
kernel:  dev_attr_show+0x1c/0x50
kernel:  sysfs_kf_seq_show+0xca/0x1b0
kernel:  seq_read+0x133/0x380
kernel:  __vfs_read+0x26/0x180
kernel:  vfs_read+0x89/0x140
kernel:  ksys_read+0x42/0x90
kernel:  do_syscall_64+0x5b/0x180
kernel:  entry_SYSCALL_64_after_hwframe+0x44/0xa9
kernel: RIP: 0033:0x7fd71d4c8b41
kernel: Code: fe ff ff 48 8d 3d 27 9e 09 00 48 83 ec 08 e8 96 02 02 00 66 0f 1f 44 00 00 8b 05 ea fc 2c 00 48 63 ff 85 c0 75 13 31 c0 0f 05 <48> 3d 00 f0 ff ff 77 57 f3 c3 0f 1f 44 00 00 55 53 48 89 d5 48 89
kernel: RSP: 002b:00007ffeab5f6448 EFLAGS: 00000246 ORIG_RAX: 0000000000000000
kernel: RAX: ffffffffffffffda RBX: 0000000000020000 RCX: 00007fd71d4c8b41
kernel: RDX: 0000000000020000 RSI: 00007fd71d809000 RDI: 0000000000000003
kernel: RBP: 0000000000020000 R08: ffffffffffffffff R09: 0000000000000000
kernel: R10: 000000000000038b R11: 0000000000000246 R12: 00007fd71d809000
kernel: R13: 0000000000000003 R14: 00007fd71d80900f R15: 0000000000020000
kernel: Modules linked in: af_packet(E) xt_tcpudp(E) ipt_REJECT(E) xt_conntrack(E) nf_conntrack(E) nf_defrag_ipv4(E) ip_set(E) nfnetlink(E) ebtable_nat(E) ebtable_broute(E) bridge(E) stp(E) llc(E) iptable_mangle(E) iptable_raw(E) iptable_security(E) ebtable_filter(E) ebtables(E) iptable_filter(E) ip_tables(E) x_tables(E) kvm_intel(E) kvm(E) irqbypass(E) crct10dif_pclmul(E) crc32_pclmul(E) ghash_clmulni_intel(E) bochs_drm(E) ttm(E) drm_kms_helper(E) drm(E) aesni_intel(E) virtio_net(E) syscopyarea(E) net_failover(E) sysfillrect(E) failover(E) aes_x86_64(E) crypto_simd(E) sysimgblt(E) cryptd(E) pcspkr(E) glue_helper(E) parport_pc(E) fb_sys_fops(E) i2c_piix4(E) parport(E) button(E) btrfs(E) libcrc32c(E) xor(E) zstd_decompress(E) zstd_compress(E) raid6_pq(E) sd_mod(E) ata_generic(E) ata_piix(E) ahci(E) libahci(E) serio_raw(E) crc32c_intel(E) virtio_pci(E) virtio_ring(E) virtio(E) libata(E) sg(E) scsi_mod(E) autofs4(E)
kernel: CR2: 0000000000000008
kernel: ---[ end trace 49cade81474e40e7 ]---
kernel: RIP: 0010:has_unmovable_pages+0x154/0x210
kernel: Code: 1b ff ff ff eb 32 48 8b 45 00 bf 00 10 00 00 a9 00 00 01 00 74 07 0f b6 4d 51 48 d3 e7 e8 c4 81 05 00 48 85 c0 49 89 c1 75 7e <41> 8b 41 08 83 f8 09 74 41 83 f8 1b 74 3c 4d 2b 64 24 58 49 81 ec
kernel: RSP: 0018:ffffc90000a1fd30 EFLAGS: 00010246
kernel: RAX: 0000000000000000 RBX: 0000000000000000 RCX: 0000000000000009
kernel: RDX: ffffffff82aed4f0 RSI: 0000000000001000 RDI: 0000000000001000
kernel: RBP: ffffea0001800000 R08: 0000000000200000 R09: 0000000000000000
kernel: R10: 0000000000001000 R11: 0000000000000003 R12: ffff88813ffd45c0
kernel: R13: 0000000000060000 R14: 0000000000000001 R15: ffffea0000000000
kernel: FS:  00007fd71d9b3500(0000) GS:ffff88813bb00000(0000) knlGS:0000000000000000
kernel: CS:  0010 DS: 0000 ES: 0000 CR0: 0000000080050033
kernel: CR2: 0000000000000008 CR3: 00000001371c2002 CR4: 00000000003606e0
kernel: DR0: 0000000000000000 DR1: 0000000000000000 DR2: 0000000000000000
kernel: DR3: 0000000000000000 DR6: 00000000fffe0ff0 DR7: 0000000000000400
--->

The reason is we do not pass the Head to page_hstate(), and so, the call
to compound_order() in page_hstate() returns 0, so we end up checking all
hstates's size to match PAGE_SIZE.

Obviously, we do not find any hstate matching that size, and we return
NULL.  Then, we dereference that NULL pointer in
hugepage_migration_supported() and we got the #PF from above.

Fix that by getting the head page before calling page_hstate().

Also, since gigantic pages span several pageblocks, re-adjust the logic
for skipping pages.
While are it, we can also get rid of the round_up().

Signed-off-by: Oscar Salvador <osalvador@suse.de>
Acked-by: Michal Hocko <mhocko@suse.com>
---
 mm/page_alloc.c | 7 +++++--
 1 file changed, 5 insertions(+), 2 deletions(-)

diff --git a/mm/page_alloc.c b/mm/page_alloc.c
index 2ec9cc407216..995d1079f958 100644
--- a/mm/page_alloc.c
+++ b/mm/page_alloc.c
@@ -7802,11 +7802,14 @@ bool has_unmovable_pages(struct zone *zone, struct page *page, int count,
 		 * handle each tail page individually in migration.
 		 */
 		if (PageHuge(page)) {
+			struct page *head = compound_head(page);
+			unsigned int skip_pages;
 
-			if (!hugepage_migration_supported(page_hstate(page)))
+			if (!hugepage_migration_supported(page_hstate(head)))
 				goto unmovable;
 
-			iter = round_up(iter + 1, 1<<compound_order(page)) - 1;
+			skip_pages = (1 << compound_order(head)) - (page - head);
+			iter += skip_pages - 1;
 			continue;
 		}
 
-- 
2.13.7