Add an "unmovable" flag for mappings that cannot be migrated under any
circumstance. KVM will use the flag for its upcoming GUEST_MEMFD support,
which will not support compaction/migration, at least not in the
foreseeable future.
Test AS_UNMOVABLE under folio lock as already done for the async
compaction/dirty folio case, as the mapping can be removed by truncation
while compaction is running. To avoid having to lock every folio with a
mapping, assume/require that unmovable mappings are also unevictable, and
have mapping_set_unmovable() also set AS_UNEVICTABLE.
Cc: Matthew Wilcox <willy@xxxxxxxxxxxxx>
Co-developed-by: Vlastimil Babka <vbabka@xxxxxxx>
Signed-off-by: Vlastimil Babka <vbabka@xxxxxxx>
Signed-off-by: Sean Christopherson <seanjc@xxxxxxxxxx>
---
include/linux/pagemap.h | 19 +++++++++++++++++-
mm/compaction.c | 43 +++++++++++++++++++++++++++++------------
mm/migrate.c | 2 ++
3 files changed, 51 insertions(+), 13 deletions(-)
diff --git a/include/linux/pagemap.h b/include/linux/pagemap.h
index 351c3b7f93a1..82c9bf506b79 100644
--- a/include/linux/pagemap.h
+++ b/include/linux/pagemap.h
@@ -203,7 +203,8 @@ enum mapping_flags {
/* writeback related tags are not used */
AS_NO_WRITEBACK_TAGS = 5,
AS_LARGE_FOLIO_SUPPORT = 6,
- AS_RELEASE_ALWAYS, /* Call ->release_folio(), even if no private data */
+ AS_RELEASE_ALWAYS = 7, /* Call ->release_folio(), even if no private data */
+ AS_UNMOVABLE = 8, /* The mapping cannot be moved, ever */
};
/**
@@ -289,6 +290,22 @@ static inline void mapping_clear_release_always(struct address_space *mapping)
clear_bit(AS_RELEASE_ALWAYS, &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(struct address_space *mapping)
+{
+ return test_bit(AS_UNMOVABLE, &mapping->flags);
+}
+
static inline gfp_t mapping_gfp_mask(struct address_space * mapping)
{
return mapping->gfp_mask;
diff --git a/mm/compaction.c b/mm/compaction.c
index 38c8d216c6a3..12b828aed7c8 100644
--- a/mm/compaction.c
+++ b/mm/compaction.c
@@ -883,6 +883,7 @@ isolate_migratepages_block(struct compact_control *cc, unsigned long low_pfn,
/* Time to isolate some pages for migration */
for (; low_pfn < end_pfn; low_pfn++) {
+ bool is_dirty, is_unevictable;
if (skip_on_failure && low_pfn >= next_skip_pfn) {
/*
@@ -1080,8 +1081,10 @@ isolate_migratepages_block(struct compact_control *cc, unsigned long low_pfn,
if (!folio_test_lru(folio))
goto isolate_fail_put;
+ is_unevictable = folio_test_unevictable(folio);
+
/* Compaction might skip unevictable pages but CMA takes them */
- if (!(mode & ISOLATE_UNEVICTABLE) && folio_test_unevictable(folio))
+ if (!(mode & ISOLATE_UNEVICTABLE) && is_unevictable)
goto isolate_fail_put;
/*
@@ -1093,26 +1096,42 @@ isolate_migratepages_block(struct compact_control *cc, unsigned long low_pfn,
if ((mode & ISOLATE_ASYNC_MIGRATE) && folio_test_writeback(folio))
goto isolate_fail_put;
- if ((mode & ISOLATE_ASYNC_MIGRATE) && folio_test_dirty(folio)) {
- bool migrate_dirty;
+ is_dirty = folio_test_dirty(folio);
+
+ if (((mode & ISOLATE_ASYNC_MIGRATE) && is_dirty) ||
+ (mapping && is_unevictable)) {
+ bool migrate_dirty = true;
+ bool is_unmovable;
/*
* Only folios without mappings or that have
- * a ->migrate_folio callback are possible to
- * migrate without blocking. However, we may
- * be racing with truncation, which can free
- * the mapping. Truncation holds the folio lock
- * until after the folio is removed from the page
- * cache so holding it ourselves is sufficient.
+ * a ->migrate_folio callback are possible to migrate
+ * without blocking.
+ *
+ * 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 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.
*/
if (!folio_trylock(folio))
goto isolate_fail_put;
mapping = folio_mapping(folio);
- migrate_dirty = !mapping ||
- mapping->a_ops->migrate_folio;
+ if ((mode & ISOLATE_ASYNC_MIGRATE) && is_dirty) {
+ migrate_dirty = !mapping ||
+ mapping->a_ops->migrate_folio;
+ }
+ is_unmovable = mapping && mapping_unmovable(mapping);
folio_unlock(folio);
- if (!migrate_dirty)
+ if (!migrate_dirty || is_unmovable)
goto isolate_fail_put;
}
diff --git a/mm/migrate.c b/mm/migrate.c
index b7fa020003f3..3d25c145098d 100644
--- a/mm/migrate.c
+++ b/mm/migrate.c
@@ -953,6 +953,8 @@ 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))
+ rc = -EOPNOTSUPP;
else if (mapping->a_ops->migrate_folio)
/*
* Most folios have a mapping and most filesystems
--
2.42.0.283.g2d96d420d3-goog
