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
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