mm/rmap: fix dereferencing invalid subpage pointer in try_to_migrate_one()

mm/rmap: fix dereferencing invalid subpage pointer in try_to_migrate_one()

The subpage we calculate is an invalid pointer for device private pages,
because device private pages are mapped via non-present device private
entries, not ordinary present PTEs.

Let's just not compute broken pointers and fixup later.  Move the proper
assignment of the correct subpage to the beginning of the function and
assert that we really only have a single page in our folio.

This currently results in a BUG when tying to compute anon_exclusive,
because:

[  528.727237] BUG: unable to handle page fault for address: ffffea1fffffffc0
[  528.739585] #PF: supervisor read access in kernel mode
[  528.745324] #PF: error_code(0x0000) - not-present page
[  528.751062] PGD 44eaf2067 P4D 44eaf2067 PUD 0
[  528.756026] Oops: 0000 [#1] PREEMPT SMP NOPTI
[  528.760890] CPU: 120 PID: 18275 Comm: hmm-tests Not tainted 5.19.0-rc3-kfd-alex #257
[  528.769542] Hardware name: AMD Corporation BardPeak/BardPeak, BIOS RTY1002BDS 09/17/2021
[  528.778579] RIP: 0010:try_to_migrate_one+0x21a/0x1000
[  528.784225] Code: f6 48 89 c8 48 2b 05 45 d1 6a 01 48 c1 f8 06 48 29
c3 48 8b 45 a8 48 c1 e3 06 48 01 cb f6 41 18 01 48 89 85 50 ff ff ff 74
0b <4c> 8b 33 49 c1 ee 11 41 83 e6 01 48 8b bd 48 ff ff ff e8 3f 99 02
[  528.805194] RSP: 0000:ffffc90003cdfaa0 EFLAGS: 00010202
[  528.811027] RAX: 00007ffff7ff4000 RBX: ffffea1fffffffc0 RCX: ffffeaffffffffc0
[  528.818995] RDX: 0000000000000000 RSI: 0000000000000000 RDI: ffffc90003cdfaf8
[  528.826962] RBP: ffffc90003cdfb70 R08: 0000000000000000 R09: 0000000000000000
[  528.834930] R10: ffffc90003cdf910 R11: 0000000000000002 R12: ffff888194450540
[  528.842899] R13: ffff888160d057c0 R14: 0000000000000000 R15: 03ffffffffffffff
[  528.850865] FS:  00007ffff7fdb740(0000) GS:ffff8883b0600000(0000) knlGS:0000000000000000
[  528.859891] CS:  0010 DS: 0000 ES: 0000 CR0: 0000000080050033
[  528.866308] CR2: ffffea1fffffffc0 CR3: 00000001562b4003 CR4: 0000000000770ee0
[  528.874275] PKRU: 55555554
[  528.877286] Call Trace:
[  528.880016]  <TASK>
[  528.882356]  ? lock_is_held_type+0xdf/0x130
[  528.887033]  rmap_walk_anon+0x167/0x410
[  528.891316]  try_to_migrate+0x90/0xd0
[  528.895405]  ? try_to_unmap_one+0xe10/0xe10
[  528.900074]  ? anon_vma_ctor+0x50/0x50
[  528.904260]  ? put_anon_vma+0x10/0x10
[  528.908347]  ? invalid_mkclean_vma+0x20/0x20
[  528.913114]  migrate_vma_setup+0x5f4/0x750
[  528.917691]  dmirror_devmem_fault+0x8c/0x250 [test_hmm]
[  528.923532]  do_swap_page+0xac0/0xe50
[  528.927623]  ? __lock_acquire+0x4b2/0x1ac0
[  528.932199]  __handle_mm_fault+0x949/0x1440
[  528.936876]  handle_mm_fault+0x13f/0x3e0
[  528.941256]  do_user_addr_fault+0x215/0x740
[  528.945928]  exc_page_fault+0x75/0x280
[  528.950115]  asm_exc_page_fault+0x27/0x30
[  528.954593] RIP: 0033:0x40366b
...

Link: https://lkml.kernel.org/r/20220623205332.319257-1-david@redhat.com
Fixes: 6c287605fd56 ("mm: remember exclusively mapped anonymous pages with PG_anon_exclusive")
Signed-off-by: David Hildenbrand <david@redhat.com>
Reported-by: "Sierra Guiza, Alejandro (Alex)" <alex.sierra@amd.com>
Reviewed-by: Alistair Popple <apopple@nvidia.com>
Tested-by: Alistair Popple <apopple@nvidia.com>
Cc: Vlastimil Babka <vbabka@suse.cz>
Cc: Christoph Hellwig <hch@lst.de>
Cc: "Matthew Wilcox (Oracle)" <willy@infradead.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>