linux-kernel - [PATCH 3.16 183/410] mm: pin address_space before dereferencing it while isolating an LRU page

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Message-ID: <lsq.1528380321.441239617@decadent.org.uk>
Date:   Thu, 07 Jun 2018 15:05:21 +0100
From:   Ben Hutchings <ben@...adent.org.uk>
To:     linux-kernel@...r.kernel.org, stable@...r.kernel.org
CC:     akpm@...ux-foundation.org,
        "Linus Torvalds" <torvalds@...ux-foundation.org>,
        "Mel Gorman" <mgorman@...hsingularity.net>,
        "Jan Kara" <jack@...e.cz>, "Minchan Kim" <minchan@...nel.org>,
        "Huang, Ying" <ying.huang@...el.com>
Subject: [PATCH 3.16 183/410] mm: pin address_space before dereferencing
 it while isolating an LRU page

3.16.57-rc1 review patch.  If anyone has any objections, please let me know.

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

From: Mel Gorman <mgorman@...hsingularity.net>

commit 69d763fc6d3aee787a3e8c8c35092b4f4960fa5d upstream.

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@...hsingularity.net>
Acked-by: Minchan Kim <minchan@...nel.org>
Cc: "Huang, Ying" <ying.huang@...el.com>
Cc: Jan Kara <jack@...e.cz>
Signed-off-by: Andrew Morton <akpm@...ux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@...ux-foundation.org>
[bwh: Backported to 3.16: adjust context]
Signed-off-by: Ben Hutchings <ben@...adent.org.uk>
---
 mm/vmscan.c | 14 ++++++++++++--
 1 file changed, 12 insertions(+), 2 deletions(-)

--- a/mm/vmscan.c
+++ b/mm/vmscan.c
@@ -1206,6 +1206,7 @@ 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)
@@ -1214,10 +1215,19 @@ 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;
 		}
 	}