Re: [PATCH v13 14/35] mm: Add AS_UNMOVABLE to mark mapping as completely unmovable

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On 10/27/23 20:21, Sean Christopherson wrote:
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>

I think this could even be "From: Vlastimil", but no biggie.

Paolo

---
  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 2053b54556ca..ed874e43ecd7 100644
--- a/mm/migrate.c
+++ b/mm/migrate.c
@@ -956,6 +956,8 @@ 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




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