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    SubjectRe: [PATCH rc8-mm1] hotfix libata-scsi corruption
    On Tue, 22 Jan 2008, James Bottomley wrote:
    > > --- 2.6.24-rc8-mm1/drivers/ata/libata-scsi.c 2008-01-17 16:49:47.000000000 +0000
    > > +++ linux/drivers/ata/libata-scsi.c 2008-01-22 15:45:40.000000000 +0000
    > > @@ -826,7 +826,7 @@ static void ata_scsi_sdev_config(struct
    > > sdev->max_device_blocked = 1;
    > >
    > > /* set the min alignment */
    > > - blk_queue_update_dma_alignment(sdev->request_queue, ATA_DMA_PAD_SZ - 1);
    > > + blk_queue_update_dma_alignment(sdev->request_queue, ATA_SECT_SIZE - 1);
    > > }
    > >
    > > static void ata_scsi_dev_config(struct scsi_device *sdev,
    >
    > Unfortunately, that's likely not the entire hot fix ... the implication
    > is that we have some mapping error in the way we do direct SG_IO.

    Quite possibly, I'm not sure.

    > What the fix you propose does is make it far more likely that block will
    > copy, perform I/O then uncopy (almost certain, since most smartd data
    > transfers are well under ATA_SECT_SIZE, which is 512). However,
    > implicating a generic path like this implies that we would get the same
    > problem for SCSI commands as well, so the correct hot fix is below.

    I've not noticed any problems from the normal activity of the system,
    only from smartd's sg_ioctl. My impression was that it's a libata
    issue, because it's going through ata_pio_sector, which does

    ap->ops->data_xfer(qc->dev, buf + offset, qc->sect_size, do_write);

    referring to sect_size, without considering the possibility of any smaller
    I/O size. (Me, I don't even know why it's going PIO rather than DMA:
    I'm assuming smartd does things that way, but there's no limit to my
    ignorance here.)

    > However, I'd like to see if we can track the problem through the SG_IO
    > direct path ... how many adjacent page bytes are corrupt? Just a few or
    > a large number (I'm wondering if it's an off by one or off by alignment
    > type bug)?

    I've assumed it's just the one next page: because ata_pio_sector is
    doing a data_xfer of sect_size ATA_SECT_SIZE 512 to an offset above
    0xe00 in the smartd stack page. The time I actually saw corruption
    rather than an oops at startup, it was in a tmpfs swap vector page
    running 64-bit kernel, and I didn't examine any further pages (just
    checked the page before and matched it up to smartd's stack, already
    suspecting that).

    I don't believe it's an off-by-one at your SCSI end.

    Hugh


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