linux-kernel - Re: [PATCH] xen/blkfront: improve protection against issuing unsupported REQ

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Message-ID: <87vbmw9pkp.fsf@vitty.brq.redhat.com>
Date:	Mon, 03 Nov 2014 18:11:34 +0100
From:	Vitaly Kuznetsov <vkuznets@...hat.com>
To:	Boris Ostrovsky <boris.ostrovsky@...cle.com>
Cc:	Laszlo Ersek <lersek@...hat.com>,
	Konrad Rzeszutek Wilk <konrad.wilk@...cle.com>,
	David Vrabel <david.vrabel@...rix.com>,
	xen-devel@...ts.xenproject.org, linux-kernel@...r.kernel.org,
	Jeff Moyer <jmoyer@...hat.com>,
	Andrew Jones <drjones@...hat.com>
Subject: Re: [PATCH] xen/blkfront: improve protection against issuing unsupported REQ_FUA

Boris Ostrovsky <boris.ostrovsky@...cle.com> writes:

> On 11/03/2014 07:22 AM, Laszlo Ersek wrote:
>> On 10/27/14 14:44, Vitaly Kuznetsov wrote:
>>> Guard against issuing unsupported REQ_FUA and REQ_FLUSH was introduced
>>> in d11e61583 and was factored out into blkif_request_flush_valid() in
>>> 0f1ca65ee. However:
>>> 1) This check in incomplete. In case we negotiated to feature_flush = REQ_FLUSH
>>>     and flush_op = BLKIF_OP_FLUSH_DISKCACHE (so FUA is unsupported) FUA request
>>>     will still pass the check.
>>> 2) blkif_request_flush_valid() is misnamed. It is bool but returns true when
>>>     the request is invalid.
>>> 3) When blkif_request_flush_valid() fails -EIO is being returned. It seems that
>>>     -EOPNOTSUPP is more appropriate here.
>>> Fix all of the above issues.
>>>
>>> This patch is based on the original patch by Laszlo Ersek and a comment by
>>> Jeff Moyer.
>>>
>>> Signed-off-by: Vitaly Kuznetsov <vkuznets@...hat.com>
>>> ---
>>>   drivers/block/xen-blkfront.c | 14 ++++++++------
>>>   1 file changed, 8 insertions(+), 6 deletions(-)
>>>
>>> diff --git a/drivers/block/xen-blkfront.c b/drivers/block/xen-blkfront.c
>>> index 5ac312f..2e6c103 100644
>>> --- a/drivers/block/xen-blkfront.c
>>> +++ b/drivers/block/xen-blkfront.c
>>> @@ -582,12 +582,14 @@ static inline void flush_requests(struct blkfront_info *info)
>>>   		notify_remote_via_irq(info->irq);
>>>   }
>>>   -static inline bool blkif_request_flush_valid(struct request
>>> *req,
>>> -					     struct blkfront_info *info)
>>> +static inline bool blkif_request_flush_invalid(struct request *req,
>>> +					       struct blkfront_info *info)
>>>   {
>>>   	return ((req->cmd_type != REQ_TYPE_FS) ||
>>> -		((req->cmd_flags & (REQ_FLUSH | REQ_FUA)) &&
>>> -		!info->flush_op));
>>> +		((req->cmd_flags & REQ_FLUSH) &&
>>> +		 !(info->feature_flush & REQ_FLUSH)) ||
>>> +		((req->cmd_flags & REQ_FUA) &&
>>> +		 !(info->feature_flush & REQ_FUA)));
>
> Somewhat unrelated to the patch, but I am wondering whether we
> actually need flush_op field at all as it seems that it is
> unambiguously defined by REQ_FLUSH/REQ_FUA.

I was under an impression it was added for readability sake but we
definitely can remove it. If noone objects I'll send separate cleanup
patch (don't want to mix these two).

>
> -boris
>
>>>   }
>>>     /*
>>> @@ -612,8 +614,8 @@ static void do_blkif_request(struct request_queue *rq)
>>>     		blk_start_request(req);
>>>   -		if (blkif_request_flush_valid(req, info)) {
>>> -			__blk_end_request_all(req, -EIO);
>>> +		if (blkif_request_flush_invalid(req, info)) {
>>> +			__blk_end_request_all(req, -EOPNOTSUPP);
>>>   			continue;
>>>   		}
>>>   
>>>
>> Not sure if there has been some feedback yet (I can't see anything
>> threaded with this message in my inbox).
>>
>> FWIW I consulted "Documentation/block/writeback_cache_control.txt" for
>> this review. Apparently, REQ_FLUSH forces out "previously completed
>> write requests", whereas REQ_FUA delays the IO completion signal for
>> *this* request until "the data has been committed to non-volatile
>> storage". So, indeed, support for REQ_FLUSH only does not guarantee that
>> REQ_FUA can be served.
>>
>> Reviewed-by: Laszlo Ersek <lersek@...hat.com>
>>
>> Thanks
>> Laszlo

-- 
  Vitaly
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