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.
v2:
Use spin_lock_irq rather than spin_lock_irqsave as Jeff suggested.
Reported-by: Yasuaki Ishimatsu <isimatu.yasuaki@xxxxxxxxxxxxxx>
Reviewed-by: Jeff Moyer <jmoyer@xxxxxxxxxx>
Signed-off-by: Tang Chen <tangchen@xxxxxxxxxxxxxx>
---
fs/aio.c | 28 ++++++++++++++++++++++++++++
1 files changed, 28 insertions(+), 0 deletions(-)
diff --git a/fs/aio.c b/fs/aio.c
index 062a5f6..dc70246 100644
--- a/fs/aio.c
+++ b/fs/aio.c
@@ -437,6 +437,14 @@ 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_irq(&ctx->completion_lock);
+
ring = kmap_atomic(ctx->ring_pages[0]);
ring->nr = nr_events; /* user copy */
ring->id = ~0U;
@@ -448,6 +456,8 @@ static int aio_setup_ring(struct kioctx *ctx)
kunmap_atomic(ring);
flush_dcache_page(ctx->ring_pages[0]);
+ spin_unlock_irq(&ctx->completion_lock);
+
return 0;
}
@@ -556,9 +566,17 @@ 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_irq(&ctx->completion_lock);
ring = kmap_atomic(ctx->ring_pages[0]);
ring->id = ctx->id;
kunmap_atomic(ring);
+ spin_unlock_irq(&ctx->completion_lock);
return 0;
}
@@ -1066,11 +1084,21 @@ 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_irq(&ctx->completion_lock);
+
ring = kmap_atomic(ctx->ring_pages[0]);
ring->head = head;
kunmap_atomic(ring);
flush_dcache_page(ctx->ring_pages[0]);
+ spin_unlock_irq(&ctx->completion_lock);
+
pr_debug("%li h%u t%u\n", ret, head, tail);
put_reqs_available(ctx, ret);
--
1.7.7
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