Re: [rfc patch 2/2] direct-io: remove address alignment check

[Tejun Heo <htejun@gmail.com>]

Daniel McNeil wrote:
> This patch relaxes the direct i/o alignment check so that user addresses
> do not have to be a multiple of the device block size.
> 
> I've done some preliminary testing and it mostly works on an ext3
> file system on a ide disk.  I have seen trouble when the user address
> is on an odd byte boundary.  Sometimes the data is read back incorrectly
> on read and sometimes I get these kernel error messages:
> 	hda: dma_timer_expiry: dma status == 0x60
> 	hda: DMA timeout retry
> 	hda: timeout waiting for DMA
> 	hda: status error: status=0x58 { DriveReady SeekComplete DataRequest }
> 	ide: failed opcode was: unknown
> 	hda: drive not ready for command
> 
> Doing direct-io with user addresses on even, non-512 boundaries appears
> to be working correctly.
> 
> Any additional testing and/or comments welcome.
> 

  Hi, Daniel.

  I don't think the change is a good idea.  We may be able to relax 
alignment contraints on some hardware to certain levels, but IMHO it 
will be very difficult to verify.  All internal block IO code follows 
strict block boundary alignment.  And as raw IOs (especially unaligned 
ones) aren't very common operations, they won't get tested much.  Then 
when some rare (probably not an open source one) application uses it on 
some rare buggy hardware, it may cause *very* strange things.

  Also, I don't think it will improve application programmer's 
convenience.  As each hardware employs different DMA alignemnt, we need 
to implement a way to export the alignment to user space and enforce it. 
   So, in the end, user application must do aligned allocation 
accordingly.  Just following block boundary will be easier.

  I don't know why you wanna relax the alignment requirement, but 
wouldn't it be easier to just write/use block-aligned allocator for such 
buffers?  It will even make the program more portable.

-- 
tejun