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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