linux-kernel - Re: [PATCH] xen/blkfront: fix ring info addressing

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Date:   Thu, 5 Mar 2020 12:04:27 +0100
From:   Jürgen Groß <jgross@...e.com>
To:     Roger Pau Monné <roger.pau@...rix.com>
Cc:     xen-devel@...ts.xenproject.org, linux-block@...r.kernel.org,
        linux-kernel@...r.kernel.org,
        Konrad Rzeszutek Wilk <konrad.wilk@...cle.com>,
        Boris Ostrovsky <boris.ostrovsky@...cle.com>,
        Stefano Stabellini <sstabellini@...nel.org>,
        Jens Axboe <axboe@...nel.dk>
Subject: Re: [PATCH] xen/blkfront: fix ring info addressing

On 05.03.20 11:49, Roger Pau Monné wrote:
> On Thu, Mar 05, 2020 at 11:03:31AM +0100, Juergen Gross wrote:
>> Commit 0265d6e8ddb890 ("xen/blkfront: limit allocated memory size to
>> actual use case") made struct blkfront_ring_info size dynamic. This is
>> fine when running with only one queue, but with multiple queues the
>> addressing of the single queues has to be adapted as the structs are
>> allocated in an array.
> 
> Thanks, and sorry for not catching this during review.
> 
>>
>> Fixes: 0265d6e8ddb890 ("xen/blkfront: limit allocated memory size to actual use case")
>> Signed-off-by: Juergen Gross <jgross@...e.com>
>> ---
>>   drivers/block/xen-blkfront.c | 82 ++++++++++++++++++++++++--------------------
>>   1 file changed, 45 insertions(+), 37 deletions(-)
>>
>> diff --git a/drivers/block/xen-blkfront.c b/drivers/block/xen-blkfront.c
>> index e2ad6bba2281..a8d4a3838e5d 100644
>> --- a/drivers/block/xen-blkfront.c
>> +++ b/drivers/block/xen-blkfront.c
>> @@ -213,6 +213,7 @@ struct blkfront_info
>>   	struct blk_mq_tag_set tag_set;
>>   	struct blkfront_ring_info *rinfo;
>>   	unsigned int nr_rings;
>> +	unsigned int rinfo_size;
>>   	/* Save uncomplete reqs and bios for migration. */
>>   	struct list_head requests;
>>   	struct bio_list bio_list;
>> @@ -259,6 +260,21 @@ static int blkfront_setup_indirect(struct blkfront_ring_info *rinfo);
>>   static void blkfront_gather_backend_features(struct blkfront_info *info);
>>   static int negotiate_mq(struct blkfront_info *info);
>>   
>> +#define rinfo_ptr(rinfo, off) \
>> +	(struct blkfront_ring_info *)((unsigned long)(rinfo) + (off))
>                                        ^ void * would seem more natural IMO.
> 
> Also if you use void * you don't need the extra (struct
> blkfront_ring_info *) cast I think?

Yes, can change that.

> I however think this macro is kind of weird, since it's just doing an
> addition. I would rather have that calculation in get_rinfo and code
> for_each_rinfo on top of that.

I wanted to avoid the multiplication in the rather common
for_each_rinfo() usage.

> 
> I agree this might be a question of taste, so I'm not going to insist
> but that would reduce the number of helpers from 3 to 2.
> 
>> +
>> +#define for_each_rinfo(info, rinfo, idx)				\
>> +	for (rinfo = info->rinfo, idx = 0;				\
>> +	     idx < info->nr_rings;					\
>> +	     idx++, rinfo = rinfo_ptr(rinfo, info->rinfo_size))
> 
> I think the above is missing proper parentheses around macro
> parameters.

rinfo and idx are simple variables, so I don't think they need
parentheses. info maybe. But just seeing it now: naming the
parameter "rinfo" and trying to access info->rinfo isn't a good
idea. It is working only as I always use "rinfo" as the pointer.

> 
>> +
>> +static struct blkfront_ring_info *get_rinfo(struct blkfront_info *info,
>> +					    unsigned int i)
> 
> inline attribute might be appropriate here.

See "the inline disease" in the kernel's coding style.


Juergen