Minchan Kim asked the following question -- what locks protects
address_space destroying when race happens between inode trauncation and
__isolate_lru_page? Jan Kara clarified by describing the race as follows
CPU1 CPU2
truncate(inode) __isolate_lru_page()
...
truncate_inode_page(mapping, page);
delete_from_page_cache(page)
spin_lock_irqsave(&mapping->tree_lock, flags);
__delete_from_page_cache(page, NULL)
page_cache_tree_delete(..)
... mapping = page_mapping(page);
page->mapping = NULL;
...
spin_unlock_irqrestore(&mapping->tree_lock, flags);
page_cache_free_page(mapping, page)
put_page(page)
if (put_page_testzero(page)) -> false
- inode now has no pages and can be freed including embedded address_space
if (mapping && !mapping->a_ops->migratepage)
- we've dereferenced mapping which is potentially already free.
The race is theoritically possible but unlikely. Before the
delete_from_page_cache, truncate_cleanup_page is called so the page is
likely to be !PageDirty or PageWriteback which gets skipped by the only
caller that checks the mappping in __isolate_lru_page. Even if the race
occurs, a substantial amount of work has to happen during a tiny window
with no preemption but it could potentially be done using a virtual machine
to artifically slow one CPU or halt it during the critical window.
This patch should eliminate the race with truncation by try-locking the page
before derefencing mapping and aborting if the lock was not acquired. There
was a suggestion from Huang Ying to use RCU as a side-effect to prevent
mapping being freed. However, I do not like the solution as it's an
unconventional means of preserving a mapping and it's not a context where
rcu_read_lock is obviously protecting rcu data.
Fixes: c82449352854 ("mm: compaction: make isolate_lru_page() filter-aware again")
Signed-off-by: Mel Gorman <mgorman@xxxxxxxxxxxxxxxxxxx>
---
mm/vmscan.c | 14 ++++++++++++--
1 file changed, 12 insertions(+), 2 deletions(-)
diff --git a/mm/vmscan.c b/mm/vmscan.c
index c02c850ea349..61bf0bc60d96 100644
--- a/mm/vmscan.c
+++ b/mm/vmscan.c
@@ -1433,14 +1433,24 @@ int __isolate_lru_page(struct page *page, isolate_mode_t mode)
if (PageDirty(page)) {
struct address_space *mapping;
+ bool migrate_dirty;
/*
* Only pages without mappings or that have a
* ->migratepage callback are possible to migrate
- * without blocking
+ * without blocking. However, we can be racing with
+ * truncation so it's necessary to lock the page
+ * to stabilise the mapping as truncation holds
+ * the page lock until after the page is removed
+ * from the page cache.
*/
+ if (!trylock_page(page))
+ return ret;
+
mapping = page_mapping(page);
- if (mapping && !mapping->a_ops->migratepage)
+ migrate_dirty = mapping && mapping->a_ops->migratepage;
+ unlock_page(page);
+ if (!migrate_dirty)
return ret;
}
}
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