mm: Band aid for 5fd4ca2

Prevents
[   76.175502] page:ffffea00098e0000 count:0 mapcount:0 mapping:0000000000000000 index:0x1
[   76.175525] flags: 0x8000000000000000()
[   76.175533] raw: 8000000000000000 ffffea0004a7e988 ffffea000445c3c8 0000000000000000
[   76.175538] raw: 0000000000000001 0000000000000000 00000000ffffffff 0000000000000000
[   76.175543] page dumped because: VM_BUG_ON_PAGE(entry != page)
[   76.175560] ------------[ cut here ]------------
[   76.175564] kernel BUG at mm/swap_state.c:170!
[   76.175574] invalid opcode: 0000 [rib#1] PREEMPT SMP
[   76.175581] CPU: 0 PID: 131 Comm: kswapd0 Tainted: G     U            5.1.0+ torvalds#247
[   76.175586] Hardware name:  /NUC6CAYB, BIOS AYAPLCEL.86A.0029.2016.1124.1625 11/24/2016
[   76.175598] RIP: 0010:__delete_from_swap_cache+0x22e/0x340
[   76.175604] Code: e8 b7 3e fd ff 48 01 1d a8 7e 04 01 48 83 c4 30 5b 5d 41 5c 41 5d 41 5e 41 5f c3 48 c7 c6 03 7e bf 81 48 89 c7 e8 92 f8 fd ff <0f> 0b 48 c7 c6 c8 7c bf 81 48 89 df e8 81 f8 fd ff 0f 0b 48 c7 c6
[   76.175613] RSP: 0000:ffffc900008dba88 EFLAGS: 00010046
[   76.175619] RAX: 0000000000000032 RBX: ffffea00098e0040 RCX: 0000000000000006
[   76.175624] RDX: 0000000000000007 RSI: 0000000000000000 RDI: ffffffff81bf6d4c
[   76.175629] RBP: ffff888265ed8640 R08: 00000000000002c2 R09: 0000000000000000
[   76.175634] R10: 0000000273a4626d R11: 0000000000000000 R12: 0000000000000001
[   76.175639] R13: 0000000000000040 R14: 0000000000000000 R15: ffffea00098e0000
[   76.175645] FS:  0000000000000000(0000) GS:ffff888277a00000(0000) knlGS:0000000000000000
[   76.175651] CS:  0010 DS: 0000 ES: 0000 CR0: 0000000080050033
[   76.175656] CR2: 00007f24e4399000 CR3: 0000000002c09000 CR4: 00000000001406f0
[   76.175661] Call Trace:
[   76.175671]  __remove_mapping+0x1c2/0x380
[   76.175678]  shrink_page_list+0x11db/0x1d10
[   76.175684]  shrink_inactive_list+0x14b/0x420
[   76.175690]  shrink_node_memcg+0x20e/0x740
[   76.175696]  shrink_node+0xba/0x420
[   76.175702]  balance_pgdat+0x27d/0x4d0
[   76.175709]  kswapd+0x216/0x300
[   76.175715]  ? wait_woken+0x80/0x80
[   76.175721]  ? balance_pgdat+0x4d0/0x4d0
[   76.175726]  kthread+0x106/0x120
[   76.175732]  ? kthread_create_on_node+0x40/0x40
[   76.175739]  ret_from_fork+0x1f/0x30
[   76.175745] Modules linked in: i915 intel_gtt drm_kms_helper
[   76.175754] ---[ end trace 8faf2ec849d50724 ]---
[   76.206689] RIP: 0010:__delete_from_swap_cache+0x22e/0x340
[   76.206708] Code: e8 b7 3e fd ff 48 01 1d a8 7e 04 01 48 83 c4 30 5b 5d 41 5c 41 5d 41 5e 41 5f c3 48 c7 c6 03 7e bf 81 48 89 c7 e8 92 f8 fd ff <0f> 0b 48 c7 c6 c8 7c bf 81 48 89 df e8 81 f8 fd ff 0f 0b 48 c7 c6
[   76.206718] RSP: 0000:ffffc900008dba88 EFLAGS: 00010046
[   76.206723] RAX: 0000000000000032 RBX: ffffea00098e0040 RCX: 0000000000000006
[   76.206729] RDX: 0000000000000007 RSI: 0000000000000000 RDI: ffffffff81bf6d4c
[   76.206734] RBP: ffff888265ed8640 R08: 00000000000002c2 R09: 0000000000000000
[   76.206740] R10: 0000000273a4626d R11: 0000000000000000 R12: 0000000000000001
[   76.206745] R13: 0000000000000040 R14: 0000000000000000 R15: ffffea00098e0000
[   76.206750] FS:  0000000000000000(0000) GS:ffff888277a00000(0000) knlGS:0000000000000000
[   76.206757] CS:  0010 DS: 0000 ES: 0000 CR0: 0000000080050033

Bugzilla: https://bugs.freedesktop.org/show_bug.cgi?id=110789
Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>

mm/huge_memory.c

@@ -2501,6 +2501,11 @@ static void __split_huge_page(struct page *page, struct list_head *list,
 		} else if (!PageAnon(page)) {
 			__xa_store(&head->mapping->i_pages, head[i].index,
 					head + i, 0);
+		} else if (PageSwapCache(page)) {
+			swp_entry_t entry = { .val = page_private(head + i) };
+			__xa_store(&swap_address_space(entry)->i_pages,
+				   swp_offset(entry),
+				   head + i, 0);
 		}
 	}