[V2 PATCH 2/2] aio, mem-hotplug: Add memory barrier to aio ring page migration.

[Tang Chen <tangchen@cn.fujitsu.com>]

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 and rmb to synchronize them.

v2:
   change smp_rmb() to smp_read_barrier_depends(). Thanks Miao.

Signed-off-by: Yasuaki Ishimatsu <isimatu.yasuaki@jp.fujitsu.com>
Signed-off-by: Tang Chen <tangchen@cn.fujitsu.com>
---
  fs/aio.c |   14 ++++++++++++++
  1 files changed, 14 insertions(+), 0 deletions(-)

diff --git a/fs/aio.c b/fs/aio.c
index dc70246..4133ba9 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 */
@@ -1069,6 +1077,12 @@ static long aio_read_events_ring(struct kioctx *ctx,
  		page = ctx->ring_pages[pos / AIO_EVENTS_PER_PAGE];
  		pos %= AIO_EVENTS_PER_PAGE;

+		/*
+		 * Ensure that the page's data was copied from old one by
+		 * aio_migratepage().
+		 */
+		smp_read_barrier_depends();
+
  		ev = kmap(page);
  		copy_ret = copy_to_user(event + ret, ev + pos,
  					sizeof(*ev) * avail);
-- 
1.7.7