linux-kernel - Re: [PATCH RESEND 0/1] AHCI: Optimize interrupt processing

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Date:	Fri, 19 Jul 2013 14:01:24 -0700
From:	"Nicholas A. Bellinger" <nab@...ux-iscsi.org>
To:	James Bottomley <James.Bottomley@...senPartnership.com>
Cc:	Jens Axboe <axboe@...nel.dk>, Mike Christie <michaelc@...wisc.edu>,
	Alexander Gordeev <agordeev@...hat.com>,
	Tejun Heo <tj@...nel.org>, linux-kernel@...r.kernel.org,
	linux-ide@...r.kernel.org, Jeff Garzik <jgarzik@...ox.com>,
	linux-scsi <linux-scsi@...r.kernel.org>
Subject: Re: [PATCH RESEND 0/1] AHCI: Optimize interrupt processing

On Fri, 2013-07-19 at 08:33 -0700, James Bottomley wrote:
> On Thu, 2013-07-18 at 23:34 -0700, Nicholas A. Bellinger wrote:
> > diff --git a/drivers/ata/libata-scsi.c b/drivers/ata/libata-scsi.c
> > index 0101af5..191bc15 100644
> > --- a/drivers/ata/libata-scsi.c
> > +++ b/drivers/ata/libata-scsi.c
> > @@ -1144,7 +1144,11 @@ static int ata_scsi_dev_config(struct scsi_device *sdev,
> >                         "sector_size=%u > PAGE_SIZE, PIO may malfunction\n",
> >                         sdev->sector_size);
> >  
> > -       blk_queue_update_dma_alignment(q, sdev->sector_size - 1);
> > +       if (!q->mq_ops) {
> > +               blk_queue_update_dma_alignment(q, sdev->sector_size - 1);
> > +       } else {
> > +               printk("Skipping dma_alignment for libata w/ scsi-mq\n");
> > +       }
> 
> Amazingly enough there is a reason for the dma alignment, and it wasn't
> just to annoy you, so you can't blindly do this.
> 
> The email thread is probably lost in the mists of time, but if I
> remember correctly the problem is that some ahci DMA controllers barf if
> the sector they're doing DMA on crosses a page boundary.  Some are
> annoying enough to actually cause silent data corruption.  You won't
> find every ahci DMA controller doing this, so the change will work for
> some, but it will be hard to identify those it won't work for until
> people start losing data.

Thanks for the extra background.

So at least from what I gather thus far this shouldn't be an issue for
initial testing with scsi-mq <-> libata w/ ata_piix.

> 
> The correct fix, obviously, is to do the bio copy on the kernel path for
> unaligned data.  It is OK to assume that REQ_TYPE_FS data is correctly
> aligned (because of the block to page alignment).
> 

Indeed.  Looking into the bio_copy_kern() breakage next..

--nab

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