mm, virt: merge AS_UNMOVABLE and AS_INACCESSIBLE

The flags AS_UNMOVABLE and AS_INACCESSIBLE were both added just for guest_memfd;
AS_UNMOVABLE is already in existing versions of Linux, while AS_INACCESSIBLE was
acked for inclusion in 6.11.

But really, they are the same thing: only guest_memfd uses them, at least for
now, and guest_memfd pages are unmovable because they should not be
accessed by the CPU.

So merge them into one; use the AS_INACCESSIBLE name which is more comprehensive.
At the same time, this fixes an embarrassing bug where AS_INACCESSIBLE was used
as a bit mask, despite it being just a bit index.

The bug was mostly benign, because AS_INACCESSIBLE's bit representation (1010)
corresponded to setting AS_UNEVICTABLE (which is already set) and AS_ENOSPC
(except no async writes can happen on the guest_memfd).  So the AS_INACCESSIBLE
flag simply had no effect.

Fixes: 1d23040c ("KVM: guest_memfd: Use AS_INACCESSIBLE when creating guest_memfd inode")
Fixes: c72ceafb ("mm: Introduce AS_INACCESSIBLE for encrypted/confidential memory")
Cc: linux-mm@kvack.org
Acked-by: Vlastimil Babka <vbabka@suse.cz>
Acked-by: David Hildenbrand <david@redhat.com>
Tested-by: Michael Roth <michael.roth@amd.com>
Reviewed-by: Michael Roth <michael.roth@amd.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>

include/linux/pagemap.h

@@ -208,8 +208,8 @@ enum mapping_flags {
 	AS_RELEASE_ALWAYS,	/* Call ->release_folio(), even if no private data */
 	AS_STABLE_WRITES,	/* must wait for writeback before modifying
 				   folio contents */
-	AS_UNMOVABLE,		/* The mapping cannot be moved, ever */
-	AS_INACCESSIBLE,	/* Do not attempt direct R/W access to the mapping */
+	AS_INACCESSIBLE,	/* Do not attempt direct R/W access to the mapping,
+				   including to move the mapping */
 };
 
 /**
@@ -310,20 +310,20 @@ static inline void mapping_clear_stable_writes(struct address_space *mapping)
 	clear_bit(AS_STABLE_WRITES, &mapping->flags);
 }
 
-static inline void mapping_set_unmovable(struct address_space *mapping)
+static inline void mapping_set_inaccessible(struct address_space *mapping)
 {
 	/*
-	 * It's expected unmovable mappings are also unevictable. Compaction
+	 * It's expected inaccessible mappings are also unevictable. Compaction
 	 * migrate scanner (isolate_migratepages_block()) relies on this to
 	 * reduce page locking.
 	 */
 	set_bit(AS_UNEVICTABLE, &mapping->flags);
-	set_bit(AS_UNMOVABLE, &mapping->flags);
+	set_bit(AS_INACCESSIBLE, &mapping->flags);
 }
 
-static inline bool mapping_unmovable(struct address_space *mapping)
+static inline bool mapping_inaccessible(struct address_space *mapping)
 {
-	return test_bit(AS_UNMOVABLE, &mapping->flags);
+	return test_bit(AS_INACCESSIBLE, &mapping->flags);
 }
 
 static inline gfp_t mapping_gfp_mask(struct address_space * mapping)

mm/compaction.c

@@ -1172,22 +1172,22 @@ isolate_migratepages_block(struct compact_control *cc, unsigned long low_pfn,
 		if (((mode & ISOLATE_ASYNC_MIGRATE) && is_dirty) ||
 		    (mapping && is_unevictable)) {
 			bool migrate_dirty = true;
-			bool is_unmovable;
+			bool is_inaccessible;
 
 			/*
 			 * Only folios without mappings or that have
 			 * a ->migrate_folio callback are possible to migrate
 			 * without blocking.
 			 *
-			 * Folios from unmovable mappings are not migratable.
+			 * Folios from inaccessible mappings are not migratable.
 			 *
 			 * However, we can be racing with truncation, which can
 			 * free the mapping that we need to check. Truncation
 			 * holds the folio lock until after the folio is removed
 			 * from the page so holding it ourselves is sufficient.
 			 *
-			 * To avoid locking the folio just to check unmovable,
-			 * assume every unmovable folio is also unevictable,
+			 * To avoid locking the folio just to check inaccessible,
+			 * assume every inaccessible folio is also unevictable,
 			 * which is a cheaper test.  If our assumption goes
 			 * wrong, it's not a correctness bug, just potentially
 			 * wasted cycles.
@@ -1200,9 +1200,9 @@ isolate_migratepages_block(struct compact_control *cc, unsigned long low_pfn,
 				migrate_dirty = !mapping ||
 						mapping->a_ops->migrate_folio;
 			}
-			is_unmovable = mapping && mapping_unmovable(mapping);
+			is_inaccessible = mapping && mapping_inaccessible(mapping);
 			folio_unlock(folio);
-			if (!migrate_dirty || is_unmovable)
+			if (!migrate_dirty || is_inaccessible)
 				goto isolate_fail_put;
 		}
 

mm/migrate.c

@@ -965,7 +965,7 @@ static int move_to_new_folio(struct folio *dst, struct folio *src,
 
 		if (!mapping)
 			rc = migrate_folio(mapping, dst, src, mode);
-		else if (mapping_unmovable(mapping))
+		else if (mapping_inaccessible(mapping))
 			rc = -EOPNOTSUPP;
 		else if (mapping->a_ops->migrate_folio)
 			/*

mm/truncate.c

@@ -233,7 +233,7 @@ bool truncate_inode_partial_folio(struct folio *folio, loff_t start, loff_t end)
 	 * doing a complex calculation here, and then doing the zeroing
 	 * anyway if the page split fails.
 	 */
-	if (!(folio->mapping->flags & AS_INACCESSIBLE))
+	if (!mapping_inaccessible(folio->mapping))
 		folio_zero_range(folio, offset, length);
 
 	if (folio_has_private(folio))

virt/kvm/guest_memfd.c

@@ -416,11 +416,10 @@ static int __kvm_gmem_create(struct kvm *kvm, loff_t size, u64 flags)
 	inode->i_private = (void *)(unsigned long)flags;
 	inode->i_op = &kvm_gmem_iops;
 	inode->i_mapping->a_ops = &kvm_gmem_aops;
-	inode->i_mapping->flags |= AS_INACCESSIBLE;
 	inode->i_mode |= S_IFREG;
 	inode->i_size = size;
 	mapping_set_gfp_mask(inode->i_mapping, GFP_HIGHUSER);
-	mapping_set_unmovable(inode->i_mapping);
+	mapping_set_inaccessible(inode->i_mapping);
 	/* Unmovable mappings are supposed to be marked unevictable as well. */
 	WARN_ON_ONCE(!mapping_unevictable(inode->i_mapping));