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

[Badari Pulavarty <pbadari@us.ibm.com>]

Tejun Heo wrote:

> 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.

I can imagine a reason for relaxing the alignment. I keep getting asked
whether we can do "O_DIRECT mount option".  Database folks wants to
make sure all the access to files in a given filesystem are O_DIRECT
(whether they are accessing or some random program like ftp, scp, cp
are acessing them). This was mainly to ensure that buffered accesses to
the file doesn't polute the pagecache (while database is using O_DIRECT
access). Seems like a logical request, but not easy to do :(

Thanks,
Badari

-
To unsubscribe from this list: send the line "unsubscribe linux-kernel" in
the body of a message to majordomo@vger.kernel.org
More majordomo info at  http://vger.kernel.org/majordomo-info.html
Please read the FAQ at  http://www.tux.org/lkml/