Re: [PATCH 7/8] block: refactor the bounce buffering code

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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
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