lkml.org 
[lkml]   [2020]   [Nov]   [5]   [last100]   RSS Feed
Views: [wrap][no wrap]   [headers]  [forward] 
 
Messages in this thread
Patch in this message
/
From
Subject[PATCH v21 06/19] mm/rmap: stop store reordering issue on page->mapping
Date
Hugh Dickins and Minchan Kim observed a long time issue which
discussed here, but actully the mentioned fix missed.
https://lore.kernel.org/lkml/20150504031722.GA2768@blaptop/
The store reordering may cause problem in the scenario:

CPU 0 CPU1
do_anonymous_page
page_add_new_anon_rmap()
page->mapping = anon_vma + PAGE_MAPPING_ANON
lru_cache_add_inactive_or_unevictable()
spin_lock(lruvec->lock)
SetPageLRU()
spin_unlock(lruvec->lock)
/* idletacking judged it as LRU
* page so pass the page in
* page_idle_clear_pte_refs
*/
page_idle_clear_pte_refs
rmap_walk
if PageAnon(page)

Johannes give detailed examples how the store reordering could cause
a trouble:
The concern is the SetPageLRU may get reorder before 'page->mapping'
setting, That would make CPU 1 will observe at page->mapping after
observing PageLRU set on the page.

1. anon_vma + PAGE_MAPPING_ANON

That's the in-order scenario and is fine.

2. NULL

That's possible if the page->mapping store gets reordered to occur
after SetPageLRU. That's fine too because we check for it.

3. anon_vma without the PAGE_MAPPING_ANON bit

That would be a problem and could lead to all kinds of undesirable
behavior including crashes and data corruption.

Is it possible? AFAICT the compiler is allowed to tear the store to
page->mapping and I don't see anything that would prevent it.

That said, I also don't see how the reader testing PageLRU under the
lru_lock would prevent that in the first place. AFAICT we need that
WRITE_ONCE() around the page->mapping assignment.

Signed-off-by: Alex Shi <alex.shi@linux.alibaba.com>
Cc: Johannes Weiner <hannes@cmpxchg.org>
Cc: Andrew Morton <akpm@linux-foundation.org>
Cc: Hugh Dickins <hughd@google.com>
Cc: Matthew Wilcox <willy@infradead.org>
Cc: Minchan Kim <minchan@kernel.org>
Cc: Vladimir Davydov <vdavydov.dev@gmail.com>
Cc: linux-kernel@vger.kernel.org
Cc: linux-mm@kvack.org
---
mm/rmap.c | 7 ++++++-
1 file changed, 6 insertions(+), 1 deletion(-)

diff --git a/mm/rmap.c b/mm/rmap.c
index 1b84945d655c..078d54da59d4 100644
--- a/mm/rmap.c
+++ b/mm/rmap.c
@@ -1054,8 +1054,13 @@ static void __page_set_anon_rmap(struct page *page,
if (!exclusive)
anon_vma = anon_vma->root;

+ /*
+ * Prevent page->mapping from pointing to an anon_vma without
+ * the PAGE_MAPPING_ANON bit set. This could happen if the
+ * compiler stores anon_vma and then adds PAGE_MAPPING_ANON to it.
+ */
anon_vma = (void *) anon_vma + PAGE_MAPPING_ANON;
- page->mapping = (struct address_space *) anon_vma;
+ WRITE_ONCE(page->mapping, (struct address_space *) anon_vma);
page->index = linear_page_index(vma, address);
}

--
1.8.3.1
\
 
 \ /
  Last update: 2020-11-05 09:57    [W:0.383 / U:0.016 seconds]
©2003-2020 Jasper Spaans|hosted at Digital Ocean and TransIP|Read the blog|Advertise on this site