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Subject[PATCH 4.9 073/329] mm: pin address_space before dereferencing it while isolating an LRU page
Date
4.9-stable review patch.  If anyone has any objections, please let me know.

------------------

From: Mel Gorman <mgorman@techsingularity.net>

[ Upstream commit 69d763fc6d3aee787a3e8c8c35092b4f4960fa5d ]

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 theoretically 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.

Link: http://lkml.kernel.org/r/20180104102512.2qos3h5vqzeisrek@techsingularity.net
Fixes: c82449352854 ("mm: compaction: make isolate_lru_page() filter-aware again")
Signed-off-by: Mel Gorman <mgorman@techsingularity.net>
Acked-by: Minchan Kim <minchan@kernel.org>
Cc: "Huang, Ying" <ying.huang@intel.com>
Cc: Jan Kara <jack@suse.cz>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>
Signed-off-by: Sasha Levin <alexander.levin@microsoft.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
---
mm/vmscan.c | 14 ++++++++++++--
1 file changed, 12 insertions(+), 2 deletions(-)

--- a/mm/vmscan.c
+++ b/mm/vmscan.c
@@ -1374,6 +1374,7 @@ int __isolate_lru_page(struct page *page

if (PageDirty(page)) {
struct address_space *mapping;
+ bool migrate_dirty;

/* ISOLATE_CLEAN means only clean pages */
if (mode & ISOLATE_CLEAN)
@@ -1382,10 +1383,19 @@ int __isolate_lru_page(struct page *page
/*
* 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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