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

From: Shaohua Li <shli@fb.com>
To: Kent Overstreet <kent.overstreet@gmail.com>
Cc: Jeff Moyer <jmoyer@redhat.com>, Ming Lei <ming.lei@canonical.com>,
	Jens Axboe <axboe@fb.com>, <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>
Subject: Re: [PATCH] block: make sure big bio is splitted into at most 256 bvecs
Date: Tue, 5 Apr 2016 17:59:51 -0700	[thread overview]
Message-ID: <20160406005951.GA3129698@devbig084.prn1.facebook.com> (raw)
In-Reply-To: <20160406004555.GA32042@kmo-pixel>

On Tue, Apr 05, 2016 at 04:45:55PM -0800, Kent Overstreet wrote:
> 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.

We don't limit the max segments in blk_queue_max_segments(), but we can
add. On the other hand, limit max segments to 256 could be a problem,
because bvec page isn't always 4k, this might make some bio smaller.