diff options
Diffstat (limited to '')
| -rw-r--r-- | mm/ksm.c | 94 |
1 files changed, 73 insertions, 21 deletions
@@ -499,6 +499,7 @@ static void remove_node_from_stable_tree(struct stable_node *stable_node) * In which case we can trust the content of the page, and it * returns the gotten page; but if the page has now been zapped, * remove the stale node from the stable tree and return NULL. + * But beware, the stable node's page might be being migrated. * * You would expect the stable_node to hold a reference to the ksm page. * But if it increments the page's count, swapping out has to wait for @@ -509,44 +510,77 @@ static void remove_node_from_stable_tree(struct stable_node *stable_node) * pointing back to this stable node. This relies on freeing a PageAnon * page to reset its page->mapping to NULL, and relies on no other use of * a page to put something that might look like our key in page->mapping. - * - * include/linux/pagemap.h page_cache_get_speculative() is a good reference, - * but this is different - made simpler by ksm_thread_mutex being held, but - * interesting for assuming that no other use of the struct page could ever - * put our expected_mapping into page->mapping (or a field of the union which - * coincides with page->mapping). - * - * Note: it is possible that get_ksm_page() will return NULL one moment, - * then page the next, if the page is in between page_freeze_refs() and - * page_unfreeze_refs(): this shouldn't be a problem anywhere, the page * is on its way to being freed; but it is an anomaly to bear in mind. */ static struct page *get_ksm_page(struct stable_node *stable_node, bool locked) { struct page *page; void *expected_mapping; + unsigned long kpfn; - page = pfn_to_page(stable_node->kpfn); expected_mapping = (void *)stable_node + (PAGE_MAPPING_ANON | PAGE_MAPPING_KSM); - if (page->mapping != expected_mapping) - goto stale; - if (!get_page_unless_zero(page)) +again: + kpfn = ACCESS_ONCE(stable_node->kpfn); + page = pfn_to_page(kpfn); + + /* + * page is computed from kpfn, so on most architectures reading + * page->mapping is naturally ordered after reading node->kpfn, + * but on Alpha we need to be more careful. + */ + smp_read_barrier_depends(); + if (ACCESS_ONCE(page->mapping) != expected_mapping) goto stale; - if (page->mapping != expected_mapping) { + + /* + * We cannot do anything with the page while its refcount is 0. + * Usually 0 means free, or tail of a higher-order page: in which + * case this node is no longer referenced, and should be freed; + * however, it might mean that the page is under page_freeze_refs(). + * The __remove_mapping() case is easy, again the node is now stale; + * but if page is swapcache in migrate_page_move_mapping(), it might + * still be our page, in which case it's essential to keep the node. + */ + while (!get_page_unless_zero(page)) { + /* + * Another check for page->mapping != expected_mapping would + * work here too. We have chosen the !PageSwapCache test to + * optimize the common case, when the page is or is about to + * be freed: PageSwapCache is cleared (under spin_lock_irq) + * in the freeze_refs section of __remove_mapping(); but Anon + * page->mapping reset to NULL later, in free_pages_prepare(). + */ + if (!PageSwapCache(page)) + goto stale; + cpu_relax(); + } + + if (ACCESS_ONCE(page->mapping) != expected_mapping) { put_page(page); goto stale; } + if (locked) { lock_page(page); - if (page->mapping != expected_mapping) { + if (ACCESS_ONCE(page->mapping) != expected_mapping) { unlock_page(page); put_page(page); goto stale; } } return page; + stale: + /* + * We come here from above when page->mapping or !PageSwapCache + * suggests that the node is stale; but it might be under migration. + * We need smp_rmb(), matching the smp_wmb() in ksm_migrate_page(), + * before checking whether node->kpfn has been changed. + */ + smp_rmb(); + if (ACCESS_ONCE(stable_node->kpfn) != kpfn) + goto again; remove_node_from_stable_tree(stable_node); return NULL; } @@ -1103,15 +1137,25 @@ static struct page *stable_tree_search(struct page *page) return NULL; ret = memcmp_pages(page, tree_page); + put_page(tree_page); - if (ret < 0) { - put_page(tree_page); + if (ret < 0) node = node->rb_left; - } else if (ret > 0) { - put_page(tree_page); + else if (ret > 0) node = node->rb_right; - } else + else { + /* + * Lock and unlock the stable_node's page (which + * might already have been migrated) so that page + * migration is sure to notice its raised count. + * It would be more elegant to return stable_node + * than kpage, but that involves more changes. + */ + tree_page = get_ksm_page(stable_node, true); + if (tree_page) + unlock_page(tree_page); return tree_page; + } } return NULL; @@ -1903,6 +1947,14 @@ void ksm_migrate_page(struct page *newpage, struct page *oldpage) if (stable_node) { VM_BUG_ON(stable_node->kpfn != page_to_pfn(oldpage)); stable_node->kpfn = page_to_pfn(newpage); + /* + * newpage->mapping was set in advance; now we need smp_wmb() + * to make sure that the new stable_node->kpfn is visible + * to get_ksm_page() before it can see that oldpage->mapping + * has gone stale (or that PageSwapCache has been cleared). + */ + smp_wmb(); + set_page_stable_node(oldpage, NULL); } } #endif /* CONFIG_MIGRATION */ |