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Date: Thu, 27 Feb 2014 05:03:27 -0500
From: Benjamin LaHaise <bcrl@...ck.org>
To: Tang Chen <tangchen@...fujitsu.com>
Cc: viro@...iv.linux.org.uk, jmoyer@...hat.com,
kosaki.motohiro@...il.com, kosaki.motohiro@...fujitsu.com,
isimatu.yasuaki@...fujitsu.com, guz.fnst@...fujitsu.com,
linux-fsdevel@...r.kernel.org, linux-aio@...ck.org,
linux-kernel@...r.kernel.org
Subject: Re: [PATCH 1/1] aio, memory-hotplug: Fix confliction when migrating and accessing ring pages.
On Thu, Feb 27, 2014 at 08:26:16AM +0800, Tang Chen wrote:
> Forgot to mention that the above patch was merged when Linux 3.12 was
> released.
> So I think this problem exists in 3.12 stable tree.
>
> If the following solution is acceptable, we need to merge it to 3.12
> stable tree, too.
>
> Please reply ASAP.
I'm travelling right now and won't be testing this patch until I get back
home in about a week, so, for now, I'll apply the patch to my aio-next tree
so that it gets some exposure to the various trinty runs and other tools
people run against the -next tree. I'll then push it out to Linus once I've
run my own sanity tests next week. Regards,
-ben
> Thanks.
>
> >
> >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@...fujitsu.com>
> >Signed-off-by: Tang Chen<tangchen@...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);
--
"Thought is the essence of where you are now."
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