mm/rmap: keep mapcount untouched for device-exclusive entries

Now that conversion to device-exclusive does no longer perform an rmap
walk and all page_vma_mapped_walk() users were taught to properly handle
device-exclusive entries, let's treat device-exclusive entries just as if
they would be present, similar to how we handle device-private entries
already.

This fixes swapout/migration/split/hwpoison of folios with
device-exclusive entries.

We only had to take care of page_vma_mapped_walk() users, because these
traditionally assume pte_present().  Other page table walkers already have
to handle !pte_present(), and some of them might simply skip them (e.g.,
MADV_PAGEOUT) if they are not specialized on them.  This change doesn't
modify the latter.

Note that while folios with device-exclusive PTEs can now get migrated,
khugepaged will not collapse a THP if there is device-exclusive PTE. 
Doing so might also not be desired if the device frequently performs
atomics to the same page.  Similarly, KSM will never merge order-0 folios
that are device-exclusive.

Link: https://lkml.kernel.org/r/20250210193801.781278-17-david@redhat.com
Fixes: b756a3b5e7ea ("mm: device exclusive memory access")
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>

1 files changed, 0 insertions, 7 deletions

diff --git a/mm/rmap.c b/mm/rmap.c
index 7b737f0f68fb..e2a543f639ce 100644
--- a/mm/rmap.c
+++ b/mm/rmap.c
@@ -2511,13 +2511,6 @@ struct page *make_device_exclusive(struct mm_struct *mm, unsigned long addr,
 	/* The pte is writable, uffd-wp does not apply. */
 	set_pte_at(mm, addr, fw.ptep, swp_pte);
 
-	/*
-	 * TODO: The device-exclusive PFN swap PTE holds a folio reference but
-	 * does not count as a mapping (mapcount), which is wrong and must be
-	 * fixed, otherwise RMAP walks don't behave as expected.
-	 */
-	folio_remove_rmap_pte(folio, page, vma);
-
 	folio_walk_end(&fw, vma);
 	mmu_notifier_invalidate_range_end(&range);
 	*foliop = folio;