[PATCH v2 16/17] mm/rmap: keep mapcount untouched for device-exclusive entries

[David Hildenbrand <david@redhat.com>]

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.

Fixes: b756a3b5e7ea ("mm: device exclusive memory access")
Signed-off-by: David Hildenbrand <david@redhat.com>
---
 mm/memory.c | 17 +----------------
 mm/rmap.c   |  7 -------
 2 files changed, 1 insertion(+), 23 deletions(-)

diff --git a/mm/memory.c b/mm/memory.c
index ba33ba3b7ea17..e9f54065b117f 100644
--- a/mm/memory.c
+++ b/mm/memory.c
@@ -741,20 +741,6 @@ static void restore_exclusive_pte(struct vm_area_struct *vma,
 
 	VM_BUG_ON_FOLIO(pte_write(pte) && (!folio_test_anon(folio) &&
 					   PageAnonExclusive(page)), folio);
-
-	/*
-	 * No need to take a page reference as one was already
-	 * created when the swap entry was made.
-	 */
-	if (folio_test_anon(folio))
-		folio_add_anon_rmap_pte(folio, page, vma, address, RMAP_NONE);
-	else
-		/*
-		 * Currently device exclusive access only supports anonymous
-		 * memory so the entry shouldn't point to a filebacked page.
-		 */
-		WARN_ON_ONCE(1);
-
 	set_pte_at(vma->vm_mm, address, ptep, pte);
 
 	/*
@@ -1626,8 +1612,7 @@ static inline int zap_nonpresent_ptes(struct mmu_gather *tlb,
 		 */
 		WARN_ON_ONCE(!vma_is_anonymous(vma));
 		rss[mm_counter(folio)]--;
-		if (is_device_private_entry(entry))
-			folio_remove_rmap_pte(folio, page, vma);
+		folio_remove_rmap_pte(folio, page, vma);
 		folio_put(folio);
 	} else if (!non_swap_entry(entry)) {
 		/* Genuine swap entries, hence a private anon pages */
diff --git a/mm/rmap.c b/mm/rmap.c
index 7b737f0f68fb5..e2a543f639ce3 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;
-- 
2.48.1