[PATCH RFC v2 1/4] mm: split AS_UNMOVABLE back out of AS_INACCESSIBLE

[Shivank Garg]

Commit 27e6a24a4cf3 ("mm, virt: merge AS_UNMOVABLE and AS_INACCESSIBLE")
folded the two flags into one, on the grounds that guest_memfd was the
only user and always set both. But the two flags were added for
different reasons and guard different things:

  AS_UNMOVABLE (0003e2a41468) marks a mapping whose folios cannot be
  migrated.

  AS_INACCESSIBLE (c72ceafbd12c) marks a mapping whose contents must
  not be directly R/W accessed. Its only job is to stop
  truncate_inode_partial_folio() from zeroing the folio.

The merge assumed unmovable and inaccessible were the same thing.
This cannot express a mapping that is inaccessible yet still movable,
which is exactly what guest_memfd wants.

Reintroduce AS_UNMOVABLE and restore the original split: truncate keeps
checking AS_INACCESSIBLE, while migration and compaction go back to
checking AS_UNMOVABLE.

Currently guest_memfd sets both, so the resulting flags and behaviour
are unchanged. Preparatory change to support folio migration for
non-confidential guest_memfd VMs.

Signed-off-by: Shivank Garg <shivankg@xxxxxxx>
---
 include/linux/pagemap.h | 24 ++++++++++++++++++++----
 mm/compaction.c         | 12 ++++++------
 mm/migrate.c            |  2 +-
 virt/kvm/guest_memfd.c  |  1 +
 4 files changed, 28 insertions(+), 11 deletions(-)

diff --git a/include/linux/pagemap.h b/include/linux/pagemap.h
index 2c3718d592d6..a7dcaa66e4e3 100644
--- a/include/linux/pagemap.h
+++ b/include/linux/pagemap.h
@@ -210,6 +210,7 @@ enum mapping_flags {
 	AS_WRITEBACK_MAY_DEADLOCK_ON_RECLAIM = 9,
 	AS_KERNEL_FILE = 10,	/* mapping for a fake kernel file that shouldn't
 				   account usage to user cgroups */
+	AS_UNMOVABLE = 11,	/* The mapping cannot be moved, ever */
 	/* Bits 16-25 are used for FOLIO_ORDER */
 	AS_FOLIO_ORDER_BITS = 5,
 	AS_FOLIO_ORDER_MIN = 16,
@@ -322,11 +323,10 @@ static inline void mapping_clear_stable_writes(struct address_space *mapping)
 static inline void mapping_set_inaccessible(struct address_space *mapping)
 {
 	/*
-	 * It's expected inaccessible mappings are also unevictable. Compaction
-	 * migrate scanner (isolate_migratepages_block()) relies on this to
-	 * reduce page locking.
+	 * The mapping's contents must not be accessed by the CPU through
+	 * the kernel direct map or other internal paths (e.g. zeroing of
+	 * pages during truncation).
 	 */
-	set_bit(AS_UNEVICTABLE, &mapping->flags);
 	set_bit(AS_INACCESSIBLE, &mapping->flags);
 }
 
@@ -335,6 +335,22 @@ static inline bool mapping_inaccessible(const struct address_space *mapping)
 	return test_bit(AS_INACCESSIBLE, &mapping->flags);
 }
 
+static inline void mapping_set_unmovable(struct address_space *mapping)
+{
+	/*
+	 * It's expected unmovable 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);
+}
+
+static inline bool mapping_unmovable(const struct address_space *mapping)
+{
+	return test_bit(AS_UNMOVABLE, &mapping->flags);
+}
+
 static inline void mapping_set_writeback_may_deadlock_on_reclaim(struct address_space *mapping)
 {
 	set_bit(AS_WRITEBACK_MAY_DEADLOCK_ON_RECLAIM, &mapping->flags);
diff --git a/mm/compaction.c b/mm/compaction.c
index f08765ade014..e6b0fdfaf79d 100644
--- a/mm/compaction.c
+++ b/mm/compaction.c
@@ -1133,22 +1133,22 @@ 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_inaccessible;
+			bool is_unmovable;
 
 			/*
 			 * Only folios without mappings or that have
 			 * a ->migrate_folio callback are possible to migrate
 			 * without blocking.
 			 *
-			 * Folios from inaccessible mappings are not migratable.
+			 * Folios from unmovable 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 inaccessible,
-			 * assume every inaccessible folio is also unevictable,
+			 * To avoid locking the folio just to check unmovable,
+			 * assume every unmovable folio is also unevictable,
 			 * which is a cheaper test.  If our assumption goes
 			 * wrong, it's not a correctness bug, just potentially
 			 * wasted cycles.
@@ -1161,9 +1161,9 @@ isolate_migratepages_block(struct compact_control *cc, unsigned long low_pfn,
 				migrate_dirty = !mapping ||
 						mapping->a_ops->migrate_folio;
 			}
-			is_inaccessible = mapping && mapping_inaccessible(mapping);
+			is_unmovable = mapping && mapping_unmovable(mapping);
 			folio_unlock(folio);
-			if (!migrate_dirty || is_inaccessible)
+			if (!migrate_dirty || is_unmovable)
 				goto isolate_fail_put;
 		}
 
diff --git a/mm/migrate.c b/mm/migrate.c
index d9b23909d716..e48ccd5a9485 100644
--- a/mm/migrate.c
+++ b/mm/migrate.c
@@ -1100,7 +1100,7 @@ static int move_to_new_folio(struct folio *dst, struct folio *src,
 
 	if (!mapping)
 		rc = migrate_folio(mapping, dst, src, mode);
-	else if (mapping_inaccessible(mapping))
+	else if (mapping_unmovable(mapping))
 		rc = -EOPNOTSUPP;
 	else if (mapping->a_ops->migrate_folio)
 		/*
diff --git a/virt/kvm/guest_memfd.c b/virt/kvm/guest_memfd.c
index db57c5766ab6..d3b1186b03b3 100644
--- a/virt/kvm/guest_memfd.c
+++ b/virt/kvm/guest_memfd.c
@@ -593,6 +593,7 @@ static int __kvm_gmem_create(struct kvm *kvm, loff_t size, u64 flags)
 	inode->i_size = size;
 	mapping_set_gfp_mask(inode->i_mapping, GFP_HIGHUSER);
 	mapping_set_inaccessible(inode->i_mapping);
+	mapping_set_unmovable(inode->i_mapping);
 	/* Unmovable mappings are supposed to be marked unevictable as well. */
 	WARN_ON_ONCE(!mapping_unevictable(inode->i_mapping));
 

-- 
2.43.0