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Subject[Update 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 and rmb to synchronize them.

Signed-off-by: Tang Chen <tangchen@cn.fujitsu.com>
Signed-off-by: Yasuaki Ishimatsu <isimatu.yasuaki@jp.fujitsu.com>

---
fs/aio.c | 14 ++++++++++++++
1 file changed, 14 insertions(+)

diff --git a/fs/aio.c b/fs/aio.c
index 50c089c..8d9b82b 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 */
@@ -1074,6 +1082,12 @@ 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_rmb();
+
ev = kmap(page);
copy_ret = copy_to_user(event + ret, ev + pos,
sizeof(*ev) * avail);


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