linux-kernel - Re: [PATCH v3 02/10] block: Introduce queue limits for copy-offload support

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Date:   Wed, 23 Feb 2022 10:29:18 +0900
From:   Damien Le Moal <damien.lemoal@...nsource.wdc.com>
To:     Luis Chamberlain <mcgrof@...nel.org>,
        Nitesh Shetty <nj.shetty@...sung.com>
Cc:     hch@....de, javier@...igon.com, chaitanyak@...dia.com,
        linux-block@...r.kernel.org, linux-scsi@...r.kernel.org,
        dm-devel@...hat.com, linux-nvme@...ts.infradead.org,
        linux-fsdevel@...r.kernel.org, axboe@...nel.dk,
        msnitzer@...hat.com, bvanassche@....org,
        martin.petersen@...cle.com, hare@...e.de, kbusch@...nel.org,
        Frederick.Knight@...app.com, osandov@...com,
        lsf-pc@...ts.linux-foundation.org, djwong@...nel.org,
        josef@...icpanda.com, clm@...com, dsterba@...e.com, tytso@....edu,
        jack@...e.com, joshi.k@...sung.com, arnav.dawn@...sung.com,
        nitheshshetty@...il.com, SelvaKumar S <selvakuma.s1@...sung.com>,
        Alasdair Kergon <agk@...hat.com>,
        Mike Snitzer <snitzer@...hat.com>,
        Sagi Grimberg <sagi@...mberg.me>,
        James Smart <james.smart@...adcom.com>,
        Chaitanya Kulkarni <kch@...dia.com>,
        Alexander Viro <viro@...iv.linux.org.uk>,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH v3 02/10] block: Introduce queue limits for copy-offload
 support

On 2/23/22 09:55, Luis Chamberlain wrote:
> On Thu, Feb 17, 2022 at 06:29:01PM +0530, Nitesh Shetty wrote:
>>  Thu, Feb 17, 2022 at 01:07:00AM -0800, Luis Chamberlain wrote:
>>> The subject says limits for copy-offload...
>>>
>>> On Mon, Feb 14, 2022 at 01:29:52PM +0530, Nitesh Shetty wrote:
>>>> Add device limits as sysfs entries,
>>>>         - copy_offload (RW)
>>>>         - copy_max_bytes (RW)
>>>>         - copy_max_hw_bytes (RO)
>>>>         - copy_max_range_bytes (RW)
>>>>         - copy_max_range_hw_bytes (RO)
>>>>         - copy_max_nr_ranges (RW)
>>>>         - copy_max_nr_ranges_hw (RO)
>>>
>>> Some of these seem like generic... and also I see a few more max_hw ones
>>> not listed above...
>>>
>> queue_limits and sysfs entries are differently named.
>> All sysfs entries start with copy_* prefix. Also it makes easy to lookup
>> all copy sysfs.
>> For queue limits naming, I tried to following existing queue limit
>> convention (like discard).
> 
> My point was that your subject seems to indicate the changes are just
> for copy-offload, but you seem to be adding generic queue limits as
> well. Is that correct? If so then perhaps the subject should be changed
> or the patch split up.
> 
>>>> +static ssize_t queue_copy_offload_store(struct request_queue *q,
>>>> +				       const char *page, size_t count)
>>>> +{
>>>> +	unsigned long copy_offload;
>>>> +	ssize_t ret = queue_var_store(&copy_offload, page, count);
>>>> +
>>>> +	if (ret < 0)
>>>> +		return ret;
>>>> +
>>>> +	if (copy_offload && !q->limits.max_hw_copy_sectors)
>>>> +		return -EINVAL;
>>>
>>>
>>> If the kernel schedules, copy_offload may still be true and
>>> max_hw_copy_sectors may be set to 0. Is that an issue?
>>>
>>
>> This check ensures that, we dont enable offload if device doesnt support
>> offload. I feel it shouldn't be an issue.
> 
> My point was this:
> 
> CPU1                                       CPU2
> Time
> 1) if (copy_offload 
> 2)    ---> preemption so it schedules      
> 3)    ---> some other high priority task  Sets q->limits.max_hw_copy_sectors to 0
> 4) && !q->limits.max_hw_copy_sectors)
> 
> Can something bad happen if we allow for this?

max_hw_copy_sectors describes the device capability to offload copy. So
this is read-only and "max_hw_copy_sectors != 0" means that the device
supports copy offload (this attribute should really be named
max_hw_copy_offload_sectors).

The actual loop to issue copy offload BIOs, however, must use the soft
version of the attribute: max_copy_sectors, which defaults to
max_hw_copy_sectors if copy offload is truned on and I guess to
max_sectors for the emulation case.

Now, with this in mind, I do not see how allowing max_copy_sectors to be
0 makes sense. I fail to see why that should be allowed since:
1) If copy_offload is true, we will rely on the device and chunk copy
offload BIOs up to max_copy_sectors
2) If copy_offload is false (or device does not support it), emulation
will be used by issuing read/write BIOs of up to max_copy_sectors.

Thus max_copy_sectors must always be at least equal to the device
minimum IO size, that is, the logical block size.


-- 
Damien Le Moal
Western Digital Research