Commit 0881ace2 ("mm/mremap: use pmd/pud_poplulate to update page
table entries") introduced a regression when running as Xen PV guest.

Today pmd_populate() for Xen PV assumes that the PFN inserted is
referencing a not yet used page table. In case of move_normal_pmd()
this is not true, resulting in WARN splats like:

[34321.304270] ------------[ cut here ]------------
[34321.304277] WARNING: CPU: 0 PID: 23628 at arch/x86/xen/multicalls.c:102 xen_mc_flush+0x176/0x1a0
[34321.304288] Modules linked in:
[34321.304291] CPU: 0 PID: 23628 Comm: apt-get Not tainted 5.14.1-20210906-doflr-mac80211debug+ #1
[34321.304294] Hardware name: MSI MS-7640/890FXA-GD70 (MS-7640)  , BIOS V1.8B1 09/13/2010
[34321.304296] RIP: e030:xen_mc_flush+0x176/0x1a0
[34321.304300] Code: 89 45 18 48 c1 e9 3f 48 89 ce e9 20 ff ff ff e8 60 03 00 00 66 90 5b 5d 41 5c 41 5d c3 48 c7 45 18 ea ff ff ff be 01 00 00 00 <0f> 0b 8b 55 00 48 c7 c7 10 97 aa 82 31 db 49 c7 c5 38 97 aa 82 65
[34321.304303] RSP: e02b:ffffc90000a97c90 EFLAGS: 00010002
[34321.304305] RAX: ffff88807d416398 RBX: ffff88807d416350 RCX: ffff88807d416398
[34321.304306] RDX: 0000000000000001 RSI: 0000000000000001 RDI: deadbeefdeadf00d
[34321.304308] RBP: ffff88807d416300 R08: aaaaaaaaaaaaaaaa R09: ffff888006160cc0
[34321.304309] R10: deadbeefdeadf00d R11: ffffea000026a600 R12: 0000000000000000
[34321.304310] R13: ffff888012f6b000 R14: 0000000012f6b000 R15: 0000000000000001
[34321.304320] FS:  00007f5071177800(0000) GS:ffff88807d400000(0000) knlGS:0000000000000000
[34321.304322] CS:  10000e030 DS: 0000 ES: 0000 CR0: 0000000080050033
[34321.304323] CR2: 00007f506f542000 CR3: 00000000160cc000 CR4: 0000000000000660
[34321.304326] Call Trace:
[34321.304331]  xen_alloc_pte+0x294/0x320
[34321.304334]  move_pgt_entry+0x165/0x4b0
[34321.304339]  move_page_tables+0x6fa/0x8d0
[34321.304342]  move_vma.isra.44+0x138/0x500
[34321.304345]  __x64_sys_mremap+0x296/0x410
[34321.304348]  do_syscall_64+0x3a/0x80
[34321.304352]  entry_SYSCALL_64_after_hwframe+0x44/0xae
[34321.304355] RIP: 0033:0x7f507196301a
[34321.304358] Code: 73 01 c3 48 8b 0d 76 0e 0c 00 f7 d8 64 89 01 48 83 c8 ff c3 66 2e 0f 1f 84 00 00 00 00 00 66 90 49 89 ca b8 19 00 00 00 0f 05 <48> 3d 01 f0 ff ff 73 01 c3 48 8b 0d 46 0e 0c 00 f7 d8 64 89 01 48
[34321.304360] RSP: 002b:00007ffda1eecd38 EFLAGS: 00000246 ORIG_RAX: 0000000000000019
[34321.304362] RAX: ffffffffffffffda RBX: 000056205f950f30 RCX: 00007f507196301a
[34321.304363] RDX: 0000000001a00000 RSI: 0000000001900000 RDI: 00007f506dc56000
[34321.304364] RBP: 0000000001a00000 R08: 0000000000000010 R09: 0000000000000004
[34321.304365] R10: 0000000000000001 R11: 0000000000000246 R12: 00007f506dc56060
[34321.304367] R13: 00007f506dc56000 R14: 00007f506dc56060 R15: 000056205f950f30
[34321.304368] ---[ end trace a19885b78fe8f33e ]---
[34321.304370] 1 of 2 multicall(s) failed: cpu 0
[34321.304371]   call  2: op=12297829382473034410 arg=[aaaaaaaaaaaaaaaa] result=-22

Fix that by modifying xen_alloc_ptpage() to only pin the page table in
case it wasn't pinned already.

Fixes: 0881ace2 ("mm/mremap: use pmd/pud_poplulate to update page table entries")
Cc: <stable@vger.kernel.org>
Reported-by: Sander Eikelenboom <linux@eikelenboom.it>
Tested-by: Sander Eikelenboom <linux@eikelenboom.it>
Signed-off-by: Juergen Gross <jgross@suse.com>
Link: https://lore.kernel.org/r/20210908073640.11299-1-jgross@suse.comSigned-off-by: Juergen Gross <jgross@suse.com>