On 10/18/18 8:06 PM, Ming Lei wrote: > On Thu, Oct 18, 2018 at 07:52:59PM -0600, Jens Axboe wrote: >> On 10/18/18 7:39 PM, Ming Lei wrote: >>> On Thu, Oct 18, 2018 at 07:33:50PM -0600, Jens Axboe wrote: >>>> 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. >>> >>> It is just a WARN_ON_ONCE() which exactly shows something which need >>> further attention, then related people may take a look and we can move >>> on. >>> >>> So I think it is correct thing to do. >> >> It most certainly is NOT the right thing to do, when we know that: >> >> 1) We currently have drivers setting an alignment that we don't meet >> 2) We have drivers not setting an alignment, and getting 512 by default > > The 512 default should have been removed given it isn't respected at > all in normal io path, but it is included from the beginning of 2.6.12 > >> 3) We have drivers setting an alignment that seems incorrect > > Then WARN_ON() is helpful for both 1) and 2) after the default 512 > limit is removed.
I'm not saying it's not useful (though even that is doubtful), I'm saying it's exactly the wrong order. You cut the very paragraph where I stated that. Drop this patch, focus on the other bits. Once that is done, then we can debate whether it's useful or not. Right now it definitely isn't. -- Jens Axboe
