Re: [PATCH 1/5] block: warn on un-aligned DMA IO buffer

[Jens Axboe <axboe@kernel.dk>]

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