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From: Matthew Wilcox <willy@infradead.org>
To: Christoph Hellwig <hch@lst.de>
Cc: Ming Lei <ming.lei@redhat.com>, Jens Axboe <axboe@kernel.dk>,
	linux-block@vger.kernel.org,
	Vitaly Kuznetsov <vkuznets@redhat.com>,
	Dave Chinner <dchinner@redhat.com>,
	Linux FS Devel <linux-fsdevel@vger.kernel.org>,
	"Darrick J . Wong" <darrick.wong@oracle.com>,
	xfs@vger.kernel.org, Bart Van Assche <bvanassche@acm.org>
Subject: Re: [PATCH 0/5] block: introduce helpers for allocating io buffer from slab
Date: Thu, 18 Oct 2018 08:06:05 -0700	[thread overview]
Message-ID: <20181018150605.GC32429@bombadil.infradead.org> (raw)
In-Reply-To: <20181018140551.GA26500@lst.de>

On Thu, Oct 18, 2018 at 04:05:51PM +0200, Christoph Hellwig wrote:
> On Thu, Oct 18, 2018 at 07:03:42AM -0700, Matthew Wilcox wrote:
> > Before we go down this road, could we have a discussion about what
> > hardware actually requires this?  Storage has this weird assumption that
> > I/Os must be at least 512 byte aligned in memory, and I don't know where
> > this idea comes from.  Network devices can do arbitrary byte alignment.
> > Even USB controllers can do arbitrary byte alignment.  Sure, performance
> > is going to suck and there are definite risks on some architectures
> > with doing IOs that are sub-cacheline aligned, but why is storage such a
> > special snowflake that we assume that host controllers are only capable
> > of doing 512-byte aligned DMAs?
> 
> Actually most storage controllers requires 4-byte alignment, but there is
> a significant subset that requires 512-byte alignment.

Can you name one that does require 512-byte alignment, preferably still
in use?  Or even >4-byte alignment.  I just checked AHCI and that requires
only 2-byte alignment.

I have reason to believe that these are uncommon because of the feedback
we got in the NVMe committee after releasing 1.0 which required 4-byte
alignment from people whining that they just couldn't guarantee 4-byte
alignment in their host devices and they absolutely needed to have no
alignment requirements (!)

  reply	other threads:[~2018-10-18 15:06 UTC|newest]

Thread overview: 32+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2018-10-18 13:18 [PATCH 0/5] block: introduce helpers for allocating io buffer from slab Ming Lei
2018-10-18 13:18 ` [PATCH 1/5] block: warn on un-aligned DMA IO buffer Ming Lei
2018-10-18 14:27   ` Jens Axboe
2018-10-18 14:43     ` Christoph Hellwig
2018-10-18 14:46       ` Jens Axboe
2018-10-19  1:28     ` Ming Lei
2018-10-19  1:33       ` Jens Axboe
2018-10-19  1:39         ` Ming Lei
2018-10-19  1:52           ` Jens Axboe
2018-10-19  2:06             ` Ming Lei
2018-10-19  2:10               ` Jens Axboe
2018-10-18 14:28   ` Christoph Hellwig
2018-10-18 13:18 ` [PATCH 2/5] block: move .dma_alignment into q->limits Ming Lei
2018-10-18 14:29   ` Christoph Hellwig
2018-10-18 20:36   ` Bart Van Assche
2018-10-18 20:36     ` Bart Van Assche
2018-10-18 13:18 ` [PATCH 3/5] block: make dma_alignment as stacked limit Ming Lei
2018-10-18 14:31   ` Christoph Hellwig
2018-10-18 13:18 ` [PATCH 4/5] block: introduce helpers for allocating IO buffers from slab Ming Lei
2018-10-18 14:42   ` Christoph Hellwig
2018-10-18 15:11     ` Matthew Wilcox
2018-10-18 15:22       ` Christoph Hellwig
2018-10-19  2:53         ` Ming Lei
2018-10-19  4:06           ` Jens Axboe
2018-10-19  5:43           ` Dave Chinner
2018-10-18 13:18 ` [PATCH 5/5] xfs: use block layer helpers to allocate io buffer " Ming Lei
2018-10-18 14:03 ` [PATCH 0/5] block: introduce helpers for allocating " Matthew Wilcox
2018-10-18 14:05   ` Christoph Hellwig
2018-10-18 15:06     ` Matthew Wilcox [this message]
2018-10-18 15:21       ` Christoph Hellwig
2018-10-18 15:50   ` Bart Van Assche
2018-10-18 15:50     ` Bart Van Assche

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