Re: [PATCH] block: make sure big bio is splitted into at most 256 bvecs

From: Ming Lei <ming.lei@canonical.com>
To: Kent Overstreet <kent.overstreet@gmail.com>
Cc: Jens Axboe <axboe@fb.com>,
	Linux Kernel Mailing List <linux-kernel@vger.kernel.org>,
	linux-block@vger.kernel.org,
	Christoph Hellwig <hch@infradead.org>,
	Eric Wheeler <bcache@lists.ewheeler.net>,
	Sebastian Roesner <sroesner-kernelorg@roesner-online.de>,
	"4.2+" <stable@vger.kernel.org>, Shaohua Li <shli@fb.com>
Subject: Re: [PATCH] block: make sure big bio is splitted into at most 256 bvecs
Date: Wed, 6 Apr 2016 10:37:05 +0800	[thread overview]
Message-ID: <CACVXFVPD=fb3iFwY9xO7s_daZ-gePiXKQs7XEO48vCGx+WSOzg@mail.gmail.com> (raw)
In-Reply-To: <20160406023411.GA7793@kmo-pixel>

On Wed, Apr 6, 2016 at 10:34 AM, Kent Overstreet
<kent.overstreet@gmail.com> wrote:
> On Wed, Apr 06, 2016 at 10:30:22AM +0800, Ming Lei wrote:
>> Now limit.max_segments is for limiting segments from hw view, one this
>> segment may include lots of pages/bvecs.
>>
>> The current bio_clone() issue is that we can't clone from one bio which
>> includes more than 256 bvecs, maybe all these 256 bvecs belong to
>> one same hw segment.
>
> So the distinction is purely a post multipage bvec thing?

Even after multipage bvec is applied, the limit for max bvecs is still needed
for some cases like bio bounce, in which bio_clone() need to
clone single page bvec.

Thanks,

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