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

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Date:	Tue, 5 Apr 2016 16:45:55 -0800
From:	Kent Overstreet <kent.overstreet@...il.com>
To:	Shaohua Li <shli@...com>
Cc:	Jeff Moyer <jmoyer@...hat.com>, Ming Lei <ming.lei@...onical.com>,
	Jens Axboe <axboe@...com>, linux-kernel@...r.kernel.org,
	linux-block@...r.kernel.org, Christoph Hellwig <hch@...radead.org>,
	Eric Wheeler <bcache@...ts.ewheeler.net>,
	Sebastian Roesner <sroesner-kernelorg@...sner-online.de>,
	"4.2+" <stable@...r.kernel.org>
Subject: Re: [PATCH] block: make sure big bio is splitted into at most 256
 bvecs

On Tue, Apr 05, 2016 at 05:41:47PM -0700, Shaohua Li wrote:
> On Tue, Apr 05, 2016 at 04:36:04PM -0800, Kent Overstreet wrote:
> > On Tue, Apr 05, 2016 at 05:30:07PM -0700, Shaohua Li wrote:
> > > this one:
> > > http://marc.info/?l=linux-kernel&m=145926976808760&w=2
> > 
> > Ah. that patch won't actually fix the bug, since md isn't using
> > blk_default_limits, it's using blk_set_stacking_limits().
> 
> Not really, the limit is set by under layer disk not md, otherwise it
> should be BLK_SAFE_MAX_SECTORS, but the reported bio has 2560 sectors.
> blk_set_stacking_limits() will use it.

What? Well, that could should just be deleted, there's no reason anymore for md
to care about the queue limits of the devices underneath it.

Regardless, using BLK_DEF_MAX_SECTORS to limit # of pages in the biovec is
_crazy_. Why would you even do that? We have a separate field in queue limits
for # max segments, use it.