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Date: Tue, 1 Nov 2016 19:08:13 -0800
From: Kent Overstreet <kent.overstreet@gmail.com>
To: Christoph Hellwig <hch@infradead.org>
Cc: Ming Lei <tom.leiming@gmail.com>, Jens Axboe <axboe@fb.com>,
	linux-kernel@vger.kernel.org, linux-block@vger.kernel.org,
	linux-fsdevel@vger.kernel.org,
	"Kirill A . Shutemov" <kirill.shutemov@linux.intel.com>,
	Jens Axboe <axboe@kernel.dk>, Hannes Reinecke <hare@suse.de>,
	Mike Christie <mchristi@redhat.com>,
	Dan Williams <dan.j.williams@intel.com>,
	Toshi Kani <toshi.kani@hpe.com>
Subject: Re: [PATCH 28/60] block: introduce QUEUE_FLAG_SPLIT_MP
Message-ID: <20161102030813.spwwymvdrym3lrah@kmo-pixel>
References: <1477728600-12938-1-git-send-email-tom.leiming@gmail.com>
 <1477728600-12938-29-git-send-email-tom.leiming@gmail.com>
 <20161031153915.GL30919@infradead.org>
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On Mon, Oct 31, 2016 at 08:39:15AM -0700, Christoph Hellwig wrote:
> On Sat, Oct 29, 2016 at 04:08:27PM +0800, Ming Lei wrote:
> > Some drivers(such as dm) should be capable of dealing with multipage
> > bvec, but the incoming bio may be too big, such as, a new singlepage bvec
> > bio can't be cloned from the bio, or can't be allocated to singlepage
> > bvec with same size.
> > 
> > At least crypt dm, log writes and bcache have this kind of issue.
> 
> We already have the segment_size limitation for request based drivers.
> I'd rather extent it to bio drivers if really needed.
> 
> But then again we should look into not having this limitation.  E.g.
> for bcache I'd be really surprised if it's that limited, given that
> Kent came up with this whole multipage bvec scheme.

AFAIK the only issue is with drivers that may have to bounce bios - pages that
were contiguous in the original bio won't necessarily be contiguous in the
bounced bio, thus bouncing might require more than BIO_MAX_SEGMENTS bvecs.

I don't know what Ming's referring to by "singlepage bvec bios".

Anyways, bouncing comes up in multiple places so we probably need to come up
with a generic solution for that. Other than that, there shouldn't be any issues
or limitations - if you're not bouncing, there's no need to clone the bvecs.