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

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Date:	Fri, 19 Jul 2013 08:33:20 -0700
From:	James Bottomley <James.Bottomley@...senPartnership.com>
To:	"Nicholas A. Bellinger" <nab@...ux-iscsi.org>
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 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.

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).

James

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