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 <[email protected]> --- 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
