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 08:59:31 +0800	[thread overview]
Message-ID: <CACVXFVOP62+2ahfiVa9hZX5fX4C2owQDS1xSbsRdMQ3Jr-hNJg@mail.gmail.com> (raw)
In-Reply-To: <20160406003025.GC31161@kmo-pixel>

On Wed, Apr 6, 2016 at 8:30 AM, Kent Overstreet
<kent.overstreet@gmail.com> wrote:
> On Wed, Apr 06, 2016 at 01:44:06AM +0800, Ming Lei wrote:
>> After arbitrary bio size is supported, the incoming bio may
>> be very big. We have to split the bio into small bios so that
>> each holds at most BIO_MAX_PAGES bvecs for safety reason, such
>> as bio_clone().
>>
>> This patch fixes the following kernel crash:
>
> Ming, let's not do it this way; drivers that don't clone biovecs are the norm -
> instead, md has its own queue limits that it ought to be setting up correctly.

Except for md, there are also several usages of bio_clone:

         - drbd
         - osdblk
         - pktcdvd
         - xen-blkfront
         - verify code of bcache

I don't like bio_clone() too, which can cause trouble to multipage bvecs.

How about fixing the issue by this simple patch first? Then once we limits
all above queues by max sectors, the global limit can be removed as
mentioned by the comment.

thanks,
Ming

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