mm/rmap: implement make_device_exclusive() using folio_walk instead of rmap walk

We require a writable PTE and only support anonymous folio: we can only
have exactly one PTE pointing at that page, which we can just lookup using
a folio walk, avoiding the rmap walk and the anon VMA lock.

So let's stop doing an rmap walk and perform a folio walk instead, so we
can easily just modify a single PTE and avoid relying on rmap/mapcounts.

We now effectively work on a single PTE instead of multiple PTEs of a
large folio, allowing for conversion of individual PTEs from non-exclusive
to device-exclusive -- note that the opposite direction always works on
single PTEs: restore_exclusive_pte().

With this change, device-exclusive handling is fully compatible with THPs
/ large folios.  We still require PMD-sized THPs to get PTE-mapped, and
supporting PMD-mapped THP (without the PTE-remapping) is a different
endeavour that might not be worth it at this point: it might even have
negative side-effects [1].

This gets rid of the "folio_mapcount()" usage and let's us fix ordinary
rmap walks (migration/swapout) next.  Spell out that messing with the
mapcount is wrong and must be fixed.

[1] https://lkml.kernel.org/r/Z5tI-cOSyzdLjoe_@phenom.ffwll.local

Link: https://lkml.kernel.org/r/20250210193801.781278-5-david@redhat.com
Signed-off-by: David Hildenbrand <david@redhat.com>
Tested-by: Alistair Popple <apopple@nvidia.com>
Cc: Alex Shi <alexs@kernel.org>
Cc: Danilo Krummrich <dakr@kernel.org>
Cc: Dave Airlie <airlied@gmail.com>
Cc: Jann Horn <jannh@google.com>
Cc: Jason Gunthorpe <jgg@nvidia.com>
Cc: Jerome Glisse <jglisse@redhat.com>
Cc: John Hubbard <jhubbard@nvidia.com>
Cc: Jonathan Corbet <corbet@lwn.net>
Cc: Karol Herbst <kherbst@redhat.com>
Cc: Liam Howlett <liam.howlett@oracle.com>
Cc: Lorenzo Stoakes <lorenzo.stoakes@oracle.com>
Cc: Lyude <lyude@redhat.com>
Cc: "Masami Hiramatsu (Google)" <mhiramat@kernel.org>
Cc: Oleg Nesterov <oleg@redhat.com>
Cc: Pasha Tatashin <pasha.tatashin@soleen.com>
Cc: Peter Xu <peterx@redhat.com>
Cc: Peter Zijlstra (Intel) <peterz@infradead.org>
Cc: SeongJae Park <sj@kernel.org>
Cc: Simona Vetter <simona.vetter@ffwll.ch>
Cc: Vlastimil Babka <vbabka@suse.cz>
Cc: Yanteng Si <si.yanteng@linux.dev>
Cc: Barry Song <v-songbaohua@oppo.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>

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