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Subject[PATCH 1/2] aio, memory-hotplug: Fix confliction when migrating and accessing ring pages.
Date
AIO ring page migration has been implemented by the following patch:

https://git.kernel.org/cgit/linux/kernel/git/torvalds/linux.git/commit/fs/aio.c?id=36bc08cc01709b4a9bb563b35aa530241ddc63e3

In this patch, ctx->completion_lock is used to prevent other processes
from accessing the ring page being migrated.

But in aio_setup_ring(), ioctx_add_table() and aio_read_events_ring(),
when writing to the ring page, they didn't take ctx->completion_lock.

As a result, for example, we have the following problem:

thread 1 | thread 2
|
aio_migratepage() |
|-> take ctx->completion_lock |
|-> migrate_page_copy(new, old) |
| *NOW*, ctx->ring_pages[idx] == old |
|
| *NOW*, ctx->ring_pages[idx] == old
| aio_read_events_ring()
| |-> ring = kmap_atomic(ctx->ring_pages[0])
| |-> ring->head = head; *HERE, write to the old ring page*
| |-> kunmap_atomic(ring);
|
|-> ctx->ring_pages[idx] = new |
| *BUT NOW*, the content of |
| ring_pages[idx] is old. |
|-> release ctx->completion_lock |

As above, the new ring page will not be updated.

The solution is taking ctx->completion_lock in thread 2, which means,
in aio_setup_ring(), ioctx_add_table() and aio_read_events_ring() when
writing to ring pages.

Reported-by: Yasuaki Ishimatsu <isimatu.yasuaki@jp.fujitsu.com>
Signed-off-by: Tang Chen <tangchen@cn.fujitsu.com>
---
fs/aio.c | 33 +++++++++++++++++++++++++++++++++
1 file changed, 33 insertions(+)

diff --git a/fs/aio.c b/fs/aio.c
index 062a5f6..50c089c 100644
--- a/fs/aio.c
+++ b/fs/aio.c
@@ -366,6 +366,7 @@ static int aio_setup_ring(struct kioctx *ctx)
int nr_pages;
int i;
struct file *file;
+ unsigned long flags;

/* Compensate for the ring buffer's head/tail overlap entry */
nr_events += 2; /* 1 is required, 2 for good luck */
@@ -437,6 +438,14 @@ static int aio_setup_ring(struct kioctx *ctx)
ctx->user_id = ctx->mmap_base;
ctx->nr_events = nr_events; /* trusted copy */

+ /*
+ * The aio ring pages are user space pages, so they can be migrated.
+ * When writing to an aio ring page, we should ensure the page is not
+ * being migrated. Aio page migration procedure is protected by
+ * ctx->completion_lock, so we add this lock here.
+ */
+ spin_lock_irqsave(&ctx->completion_lock, flags);
+
ring = kmap_atomic(ctx->ring_pages[0]);
ring->nr = nr_events; /* user copy */
ring->id = ~0U;
@@ -448,6 +457,8 @@ static int aio_setup_ring(struct kioctx *ctx)
kunmap_atomic(ring);
flush_dcache_page(ctx->ring_pages[0]);

+ spin_unlock_irqrestore(&ctx->completion_lock, flags);
+
return 0;
}

@@ -542,6 +553,7 @@ static int ioctx_add_table(struct kioctx *ctx, struct mm_struct *mm)
unsigned i, new_nr;
struct kioctx_table *table, *old;
struct aio_ring *ring;
+ unsigned long flags;

spin_lock(&mm->ioctx_lock);
rcu_read_lock();
@@ -556,9 +568,19 @@ static int ioctx_add_table(struct kioctx *ctx, struct mm_struct *mm)
rcu_read_unlock();
spin_unlock(&mm->ioctx_lock);

+ /*
+ * Accessing ring pages must be done
+ * holding ctx->completion_lock to
+ * prevent aio ring page migration
+ * procedure from migrating ring pages.
+ */
+ spin_lock_irqsave(&ctx->completion_lock,
+ flags);
ring = kmap_atomic(ctx->ring_pages[0]);
ring->id = ctx->id;
kunmap_atomic(ring);
+ spin_unlock_irqrestore(
+ &ctx->completion_lock, flags);
return 0;
}

@@ -1021,6 +1043,7 @@ static long aio_read_events_ring(struct kioctx *ctx,
unsigned head, tail, pos;
long ret = 0;
int copy_ret;
+ unsigned long flags;

mutex_lock(&ctx->ring_lock);

@@ -1066,11 +1089,21 @@ static long aio_read_events_ring(struct kioctx *ctx,
head %= ctx->nr_events;
}

+ /*
+ * The aio ring pages are user space pages, so they can be migrated.
+ * When writing to an aio ring page, we should ensure the page is not
+ * being migrated. Aio page migration procedure is protected by
+ * ctx->completion_lock, so we add this lock here.
+ */
+ spin_lock_irqsave(&ctx->completion_lock, flags);
+
ring = kmap_atomic(ctx->ring_pages[0]);
ring->head = head;
kunmap_atomic(ring);
flush_dcache_page(ctx->ring_pages[0]);

+ spin_unlock_irqrestore(&ctx->completion_lock, flags);
+
pr_debug("%li h%u t%u\n", ret, head, tail);

put_reqs_available(ctx, ret);
--
1.8.3.1


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