linux-kernel - Re: [PATCH 5/5] dm: implement REQ

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Date:	Thu, 19 Aug 2010 19:32:04 +0900
From:	Kiyoshi Ueda <k-ueda@...jp.nec.com>
To:	Tejun Heo <tj@...nel.org>, Mike Snitzer <snitzer@...hat.com>
CC:	jaxboe@...ionio.com, linux-fsdevel@...r.kernel.org,
	linux-scsi@...r.kernel.org, linux-ide@...r.kernel.org,
	linux-kernel@...r.kernel.org, linux-raid@...r.kernel.org,
	hch@....de, James.Bottomley@...e.de, tytso@....edu,
	chris.mason@...cle.com, swhiteho@...hat.com,
	konishi.ryusuke@....ntt.co.jp, dm-devel@...hat.com, vst@...b.net,
	jack@...e.cz, rwheeler@...hat.com, hare@...e.de, neilb@...e.de,
	rusty@...tcorp.com.au, mst@...hat.com,
	Mikulas Patocka <mpatocka@...hat.com>,
	"Jun'ichi Nomura" <j-nomura@...jp.nec.com>
Subject: Re: [PATCH 5/5] dm: implement REQ_FLUSH/FUA support

Hi Tejun, Mike,

On 08/18/2010 01:51 AM +0900, Tejun Heo wrote:
> On 08/17/2010 04:07 PM, Mike Snitzer wrote:
>> NOTE: NEC has already given some preliminary feedback to hch in the
>> "[PATCH, RFC 2/2] dm: support REQ_FLUSH directly" thread:
>> https://www.redhat.com/archives/dm-devel/2010-August/msg00026.html
>> https://www.redhat.com/archives/dm-devel/2010-August/msg00033.html
> 
> Hmmm... I think both issues don't exist in this incarnation of
> conversion although I'm fairly sure there will be other issues.  :-)

The same issue is still there for request-based dm.  See below.


>>>     A related question: Is dm_wait_for_completion() used in
>>>     process_flush() safe against starvation under continuous influx of
>>>     other commands?
>> As for your specific dm_wait_for_completion() concern -- I'll defer to
>> Mikulas.  But I'll add: we haven't had any reported starvation issues
>> with DM's existing barrier support.  DM uses a mempool for its clones,
>> so it should naturally throttle (without starvation) when memory gets
>> low.
> 
> I see but single pending flush and steady write streams w/o saturating
> the mempool would be able to stall dm_wait_for_completeion(), no?  Eh
> well, it's a separate issue, I guess.

Your understanding is correct, dm_wait_for_completion() for flush
will stall in such cases for request-based dm.
That's why I mentioned below in
https://www.redhat.com/archives/dm-devel/2010-August/msg00026.html.

    In other words, current request-based device-mapper can't handle
    other requests while a flush request is in progress.

In flush request handling, request-based dm uses dm_wait_for_completion()
to wait for the completion of cloned flush requests, depending on
the fact that there should be only flush requests in flight owning
to the block layer sequencing.

It's not a separate issue and we need to resolve it at least.
I'm still considering how I can fix the request-based dm.

Thanks,
Kiyoshi Ueda
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