Alignment Issue with Direct IO to NVMe Drive

[Matthew Wilcox]

On Tue, Nov 27, 2012 at 01:09:46PM +0100, Jens Axboe wrote:
> On 2012-11-27 01:35, Laine Walker-Avina wrote:
> > Hi all,
> > 
> > We are experiencing an issue with doing direct IO to a NVMe device I'm
> > helping to develop. Every so often, the physical address given by
> > sg_dma_address() is aligned to 0x800 instead of 0x1000 as specified by
> > blk_queue_dma_alignement(queue, 4095) when the queue is initialized.

FYI, this is a modification to the driver that Laine has made; presumably
for a limitation of the prototype hardware he's working with.  The NVMe
spec requires the device to be able to do I/Os to 4 byte boundaries.

Laine, when this occurs, what is the alignment of 'offset' in the sg
entry you're looking at?  If userspace is passing in an unaligned address,
I don't think there's anything we do to try to align it.

> > The request is also split over multiple segments to make up for the
> > missing space (eg: for a 4k IO it's split into two segments 2k in
> > size, and for an 8k IO it's split into 3 segments--2k,4k,2k). Our
> > design requires the physical segments given to the device be aligned
> > to 4k boundaries and be multiples of 4k in size. When not doing direct
> > IO the physical addresses appear to always be 4k aligned as expected.
> > One possible issue is the kernel we're primarily testing against is
> > 2.6.32-220 from CentOS, but we have observed similar behavior from a
> > vanilla 3.3 kernel as well. Any help would be greatly appreciated.
> 
> I'm assuming you set the hardware sector size to 4k as well?
> 
> -- 
> Jens Axboe