On 10/18/18 7:28 PM, Ming Lei wrote:

On Thu, Oct 18, 2018 at 08:27:28AM -0600, Jens Axboe wrote:

quoted

On 10/18/18 7:18 AM, Ming Lei wrote:

quoted

Now we only check if DMA IO buffer is aligned to queue_dma_alignment()
for pass-through request, and it isn't done for normal IO request.

Given the check has to be done on each bvec, it isn't efficient to add the
check in generic_make_request_checks().

This patch addes one WARN in blk_queue_split() for capturing this issue.

I don't want to do this, because then we are forever doomed to
have something that fully loops a bio at submission time. I
absolutely hate the splitting we have and the need for it,
hopefully it can go away for a subset of IOs at some point.

In many ways, this seems to be somewhat of a made-up problem, I don't
recall a single bug report for something like this over decades of
working with the IO stack. 512b alignment restrictions for DMA seems
absolutely insane. I know people claim they exist, but clearly that
isn't a hard requirement or we would have been boned years ago.

There are still some drivers with this requirement:

drivers/ata/libata-scsi.c:1308: blk_queue_update_dma_alignment(q, sdev->sector_size - 1);
drivers/ata/pata_macio.c:812:           blk_queue_update_dma_alignment(sdev->request_queue, 31);
drivers/ata/pata_macio.c:827:           blk_queue_update_dma_alignment(sdev->request_queue, 15);
drivers/block/ps3disk.c:470:    blk_queue_dma_alignment(queue, dev->blk_size-1);
drivers/block/rsxx/dev.c:282:           blk_queue_dma_alignment(card->queue, blk_size - 1);
drivers/block/xen-blkfront.c:957:       blk_queue_dma_alignment(rq, 511);
drivers/ide/ide-cd.c:1512:      blk_queue_dma_alignment(q, 31);
drivers/message/fusion/mptscsih.c:2388: blk_queue_dma_alignment (sdev->request_queue, 512 - 1);
drivers/staging/rts5208/rtsx.c:94:      blk_queue_dma_alignment(sdev->request_queue, (512 - 1));
drivers/usb/image/microtek.c:329:       blk_queue_dma_alignment(s->request_queue, (512 - 1));
drivers/usb/storage/scsiglue.c:92:      blk_queue_update_dma_alignment(sdev->request_queue, (512 - 1));
drivers/usb/storage/uas.c:818:  blk_queue_update_dma_alignment(sdev->request_queue, (512 - 1));

Of course, I too can grep :-)

My point is that these settings might not match reality. And the
WARN_ON(), as implemented, is going to trigger on any device that
DOESN'T set the alignment, as Bart pointed out.

-- 
Jens Axboe