On 10/18/18 7:28 PM, Ming Lei wrote: > On Thu, Oct 18, 2018 at 08:27:28AM -0600, Jens Axboe wrote: >> On 10/18/18 7:18 AM, Ming Lei wrote: >>> 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
