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SubjectRe: [PATCH 1/2] aio, memory-hotplug: Fix confliction when migrating and accessing ring pages.
Date
Tang Chen <tangchen@cn.fujitsu.com> writes:

> 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.

Thanks for the good explanation and for adding comments to the code.
This looks right to me. I guess the only issue I have with it is that
the code paths changed don't run in interrupt context, so I think you
could just use spin_lock_irq. That's not a big deal, though.

Reviewed-by: Jeff Moyer <jmoyer@redhat.com>


> 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);


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