linux-kernel - [PATCH 2/2] aio, mem-hotplug: Add memory barrier to aio ring page migration.

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Message-Id: <1393497616-16428-3-git-send-email-tangchen@cn.fujitsu.com>
Date:	Thu, 27 Feb 2014 18:40:16 +0800
From:	Tang Chen <tangchen@...fujitsu.com>
To:	viro@...iv.linux.org.uk, bcrl@...ck.org, jmoyer@...hat.com,
	kosaki.motohiro@...il.com, kosaki.motohiro@...fujitsu.com,
	isimatu.yasuaki@...fujitsu.com, guz.fnst@...fujitsu.com
Cc:	linux-fsdevel@...r.kernel.org, linux-aio@...ck.org,
	linux-kernel@...r.kernel.org
Subject: [PATCH 2/2] aio, mem-hotplug: Add memory barrier to aio ring page migration.

When doing aio ring page migration, we migrated the page, and update
ctx->ring_pages[]. Like the following:

aio_migratepage()
 |-> migrate_page_copy(new, old)
 |   ......				/* Need barrier here */
 |-> ctx->ring_pages[idx] = new

Actually, we need a memory barrier between these two operations.
Otherwise, if ctx->ring_pages[] is updated before memory copy due to
the compiler optimization, other processes may have an opportunity
to access to the not fully initialized new ring page.

So add a wmb to synchronize them.

Reported-by: Yasuaki Ishimatsu <isimatu.yasuaki@...fujitsu.com>
Signed-off-by: Tang Chen <tangchen@...fujitsu.com>
---
 fs/aio.c | 8 ++++++++
 1 file changed, 8 insertions(+)

diff --git a/fs/aio.c b/fs/aio.c
index 50c089c..f0ed838 100644
--- a/fs/aio.c
+++ b/fs/aio.c
@@ -327,6 +327,14 @@ static int aio_migratepage(struct address_space *mapping, struct page *new,
 		pgoff_t idx;
 		spin_lock_irqsave(&ctx->completion_lock, flags);
 		migrate_page_copy(new, old);
+
+		/*
+		 * Ensure memory copy is finished before updating
+		 * ctx->ring_pages[]. Otherwise other processes may access to
+		 * new ring pages which are not fully initialized.
+		 */
+		smp_wmb();
+
 		idx = old->index;
 		if (idx < (pgoff_t)ctx->nr_pages) {
 			/* And only do the move if things haven't changed */
-- 
1.8.3.1

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