[PATCH RFC 1/3] mm: split AS_UNMOVABLE back out of AS_INACCESSIBLE
From: Shivank Garg
Date: Thu Jun 11 2026 - 09:10:34 EST
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 31a848485ad9d9850d37185418349b89e6efe420..17f5abfa6e7be97c0dcb634346f21ce076798495 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 3648ce22c80728b894cffce502d8caa3e4532406..8262f08c01ff407eff8732ffe1d0eb4de469eaf2 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 8a64291ab5b44c401e1e0356bf39588e7b5d7b0d..c81b3900b5afd150681d973484e71982a8936221 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 69c9d6d546b287b4f75ef69868259c082ca50933..806a42f0e031a1c7729f53c786316d2502532553 100644
--- a/virt/kvm/guest_memfd.c
+++ b/virt/kvm/guest_memfd.c
@@ -592,6 +592,7 @@ 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