diff options
| -rw-r--r-- | include/linux/mm.h | 55 | ||||
| -rw-r--r-- | mm/migrate.c | 22 |
2 files changed, 59 insertions, 18 deletions
diff --git a/include/linux/mm.h b/include/linux/mm.h index deeb535f920c..41f5c88bdf3b 100644 --- a/include/linux/mm.h +++ b/include/linux/mm.h @@ -2200,6 +2200,61 @@ static inline bool folio_likely_mapped_shared(struct folio *folio) return atomic_read(&folio->_mapcount) > 0; } +/** + * folio_expected_ref_count - calculate the expected folio refcount + * @folio: the folio + * + * Calculate the expected folio refcount, taking references from the pagecache, + * swapcache, PG_private and page table mappings into account. Useful in + * combination with folio_ref_count() to detect unexpected references (e.g., + * GUP or other temporary references). + * + * Does currently not consider references from the LRU cache. If the folio + * was isolated from the LRU (which is the case during migration or split), + * the LRU cache does not apply. + * + * Calling this function on an unmapped folio -- !folio_mapped() -- that is + * locked will return a stable result. + * + * Calling this function on a mapped folio will not result in a stable result, + * because nothing stops additional page table mappings from coming (e.g., + * fork()) or going (e.g., munmap()). + * + * Calling this function without the folio lock will also not result in a + * stable result: for example, the folio might get dropped from the swapcache + * concurrently. + * + * However, even when called without the folio lock or on a mapped folio, + * this function can be used to detect unexpected references early (for example, + * if it makes sense to even lock the folio and unmap it). + * + * The caller must add any reference (e.g., from folio_try_get()) it might be + * holding itself to the result. + * + * Returns the expected folio refcount. + */ +static inline int folio_expected_ref_count(const struct folio *folio) +{ + const int order = folio_order(folio); + int ref_count = 0; + + if (WARN_ON_ONCE(folio_test_slab(folio))) + return 0; + + if (folio_test_anon(folio)) { + /* One reference per page from the swapcache. */ + ref_count += folio_test_swapcache(folio) << order; + } else if (!((unsigned long)folio->mapping & PAGE_MAPPING_FLAGS)) { + /* One reference per page from the pagecache. */ + ref_count += !!folio->mapping << order; + /* One reference from PG_private. */ + ref_count += folio_test_private(folio); + } + + /* One reference per page table mapping. */ + return ref_count + folio_mapcount(folio); +} + #ifndef HAVE_ARCH_MAKE_FOLIO_ACCESSIBLE static inline int arch_make_folio_accessible(struct folio *folio) { diff --git a/mm/migrate.c b/mm/migrate.c index 25e7438af968..8619aa884eaa 100644 --- a/mm/migrate.c +++ b/mm/migrate.c @@ -453,20 +453,6 @@ unlock: } #endif -static int folio_expected_refs(struct address_space *mapping, - struct folio *folio) -{ - int refs = 1; - if (!mapping) - return refs; - - refs += folio_nr_pages(folio); - if (folio_test_private(folio)) - refs++; - - return refs; -} - /* * Replace the folio in the mapping. * @@ -609,7 +595,7 @@ static int __folio_migrate_mapping(struct address_space *mapping, int folio_migrate_mapping(struct address_space *mapping, struct folio *newfolio, struct folio *folio, int extra_count) { - int expected_count = folio_expected_refs(mapping, folio) + extra_count; + int expected_count = folio_expected_ref_count(folio) + extra_count + 1; if (folio_ref_count(folio) != expected_count) return -EAGAIN; @@ -626,7 +612,7 @@ int migrate_huge_page_move_mapping(struct address_space *mapping, struct folio *dst, struct folio *src) { XA_STATE(xas, &mapping->i_pages, folio_index(src)); - int rc, expected_count = folio_expected_refs(mapping, src); + int rc, expected_count = folio_expected_ref_count(src) + 1; if (folio_ref_count(src) != expected_count) return -EAGAIN; @@ -756,7 +742,7 @@ static int __migrate_folio(struct address_space *mapping, struct folio *dst, struct folio *src, void *src_private, enum migrate_mode mode) { - int rc, expected_count = folio_expected_refs(mapping, src); + int rc, expected_count = folio_expected_ref_count(src) + 1; /* Check whether src does not have extra refs before we do more work */ if (folio_ref_count(src) != expected_count) @@ -844,7 +830,7 @@ static int __buffer_migrate_folio(struct address_space *mapping, return migrate_folio(mapping, dst, src, mode); /* Check whether page does not have extra refs before we do more work */ - expected_count = folio_expected_refs(mapping, src); + expected_count = folio_expected_ref_count(src) + 1; if (folio_ref_count(src) != expected_count) return -EAGAIN; |