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From: Benjamin Block <bblock@linux.ibm.com>
To: Christoph Hellwig <hch@lst.de>
Cc: Matthew Wilcox <willy@infradead.org>,
	Jens Axboe <axboe@kernel.dk>, Khalid Aziz <khalid@gonehiking.org>,
	"Martin K. Petersen" <martin.petersen@oracle.com>,
	Hannes Reinecke <hare@suse.com>,
	Ondrej Zary <linux@rainbow-software.org>,
	linux-block@vger.kernel.org, linux-scsi@vger.kernel.org
Subject: Re: [PATCH 7/8] block: refactor the bounce buffering code
Date: Wed, 24 Mar 2021 19:07:40 +0100	[thread overview]
Message-ID: <YFt/7BdzecUmdySU@t480-pf1aa2c2.linux.ibm.com> (raw)
In-Reply-To: <20210324174458.GA13589@lst.de>

On Wed, Mar 24, 2021 at 06:44:58PM +0100, Christoph Hellwig wrote:
> On Wed, Mar 24, 2021 at 06:40:32PM +0100, Benjamin Block wrote:
> > Is blk_queue_bounce() called again when mpath submits the request to the
> > lower device? I thought when I looked at this code some time ago
> > bouncing would only be checked the first time a request is created
> > (dm-mpath), and then not again, so when we don't check for whether
> > bouncing is necessary in mpath, we still might screw the LLD - hence why
> > we might inherit this via the limits.
> 
> Every call to blk_mq_submit_bio also calls blk_queue_bounce, 

But map_request() -> multipath_clone_and_map() -> dm_dispatch_clone_request() 
doesn't call blk_mq_submit_bio() for requests that have been queued in a
request based mpath device. The requests gets cloned and then dispatched
on the lower queue. Or am I missing something?

> and
> blk_queue_bounce then checks if it needs to do anything based on the
> bounce limit and max_pfn, and if needed proceeds to check every bvec.
> 
> So for extremely unlikely case thay someone is running multipath over one
> of the few remaining drivers that need block layering bounce buffering
> this inheritance just leads to (harmless) extra work.

Yeah fair enough, I don't know whether anyone would care for those old
drivers; it just crossed my mind.

-- 
Best Regards, Benjamin Block  / Linux on IBM Z Kernel Development / IBM Systems
IBM Deutschland Research & Development GmbH    /    https://www.ibm.com/privacy
Vorsitz. AufsR.: Gregor Pillen         /        Geschäftsführung: Dirk Wittkopp
Sitz der Gesellschaft: Böblingen / Registergericht: AmtsG Stuttgart, HRB 243294

  reply	other threads:[~2021-03-24 18:09 UTC|newest]

Thread overview: 21+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2021-03-18  6:39 start removing block bounce buffering support Christoph Hellwig
2021-03-18  6:39 ` [PATCH 1/8] aha1542: use a local bounce buffer Christoph Hellwig
2021-03-18  6:39 ` [PATCH 2/8] Buslogic: remove ISA support Christoph Hellwig
2021-03-18  6:39 ` [PATCH 3/8] BusLogic: reject broken old firmware that requires ISA-style bounce buffering Christoph Hellwig
2021-03-18  6:39 ` [PATCH 4/8] advansys: remove ISA support Christoph Hellwig
2021-03-18 11:20   ` Matthew Wilcox
2021-03-18 12:54     ` Christoph Hellwig
2021-03-18 14:34       ` Matthew Wilcox
2021-03-18  6:39 ` [PATCH 5/8] scsi: remove the unchecked_isa_dma flag Christoph Hellwig
2021-03-18  6:39 ` [PATCH 6/8] block: remove BLK_BOUNCE_ISA support Christoph Hellwig
2021-03-18  6:39 ` [PATCH 7/8] block: refactor the bounce buffering code Christoph Hellwig
2021-03-18 11:29   ` Matthew Wilcox
2021-03-18 12:53     ` Christoph Hellwig
2021-03-24 17:40       ` Benjamin Block
2021-03-24 17:44         ` Christoph Hellwig
2021-03-24 18:07           ` Benjamin Block [this message]
2021-03-24 18:13             ` Christoph Hellwig
2021-03-18  6:39 ` [PATCH 8/8] block: stop calling blk_queue_bounce for passthrough requests Christoph Hellwig
2021-03-26  5:58 start removing block bounce buffering support v2 Christoph Hellwig
2021-03-26  5:58 ` [PATCH 7/8] block: refactor the bounce buffering code Christoph Hellwig
2021-03-29  6:34   ` Hannes Reinecke
2021-03-31  7:29 start removing block bounce buffering support v3 Christoph Hellwig
2021-03-31  7:30 ` [PATCH 7/8] block: refactor the bounce buffering code Christoph Hellwig

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