Re: [PATCH 2/2] aio: fix the confliction of read events and migrating ring page

[Gu Zheng]

Hi Ben,
On 03/21/2014 12:30 AM, Benjamin LaHaise wrote:

> On Thu, Mar 20, 2014 at 10:32:07AM -0400, Dave Jones wrote:
>> On Thu, Mar 20, 2014 at 01:46:25PM +0800, Gu Zheng wrote:
>>
>>  > diff --git a/fs/aio.c b/fs/aio.c
>>  > index 88ad40c..e353085 100644
>>  > --- a/fs/aio.c
>>  > +++ b/fs/aio.c
>>  > @@ -319,6 +319,9 @@ static int aio_migratepage(struct address_space *mapping, struct page *new,
>>  >  	ctx->ring_pages[old->index] = new;
>>  >  	spin_unlock_irqrestore(&ctx->completion_lock, flags);
>>  >  
>>  > +	/* Ensure read event is completed before putting old page */
>>  > +	mutex_lock(&ctx->ring_lock);
>>  > +	mutex_unlock(&ctx->ring_lock);
>>  >  	put_page(old);
>>  >  
>>  >  	return rc;
>>
>> This looks a bit weird. Would using a completion work here ?
> 
> Nope.  This is actually the most elegant fix I've seen for this approach, 
> as everything else has relied on adding additional spin locks (which only 
> end up being needed in the migration case) around access to the ring_pages 
> on the reader side.  That said, this patch is not a complete solution to 
> the problem, as the update of the ring's head pointer could still get lost 
> with this patch.  I think the right thing is just taking the ring_lock 
> mutex over the entire page migration operation.  That should be safe, as 
> nowhere else is the ring_lock mutex nested with any other locks.

This one is based on linux-next which has merged the following patch:
commit 692c9b8c5ee8d263bb8348171f0bebd3d84eb2c1
Author: Tang Chen <tangchen@xxxxxxxxxxxxxx>
Date:   Mon Mar 10 16:15:33 2014 +0800
aio, memory-hotplug: Fix confliction when migrating and accessing ring pages.

With this patch, the update of the ring's head pointer is safe because it is protected
by completion_lock, so we do not need to enlarge the ring_lock protection region.
And on the other side, if we take the ring_lock over the entire page migration
operation, reading events will be affected if the page migration is going.

Thanks,
Gu

> 
> 		-ben
> 
>> 	Dave
> 


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