Re: Possible O_DIRECT problems ?

[CJ <cj@cjcj.com>]

Shouldn't O_DIRECT's requirements come from the hardware?  If we can 
ASPI or CAM DMA SCSI devices to odd addresses and lengths, why not 
O_DIRECT?  Do ape drives DMA to user buffers?  Are O_DIRECT's 
current limits gratuitous?


Andrea Arcangeli wrote:

>On Fri, Dec 21, 2001 at 12:39:42AM +0000, Dave Jones wrote:
>
>>On Fri, Dec 21, 2001 at 01:23:45AM +0100, Trond Myklebust wrote:
>>
>> >    O_DIRECT for NFS isn't yet merged into the kernel. Are these Chuck
>> > Lever's NFS patches you've been testing?
>>
>>Nope, stock 2.4.17rc2 & 2.5.1. 
>>I thought NFS might just ignore the O_DIRECT flag if it didn't
>>understand it yet, I wasn't expecting such a dramatic failure.
>>
>
>The point of O_DIRECT is to do DMA directly into the userspace memory
>(and to avoid the VM overhead but that's a secondary issue and with data
>journaling we may need to put an anchor into the VM to serialize the
>direct I/O with the pagecache I/O in a secondary - slower - direct_IO
>callback for the data journaling fs).
>
>But to avoid the mem copies you're required to use strict alignment and
>size of the userspace buffers, just like rawio.
>
>If you don't you will get -EINVAL. This ensures people will use O_DIRECT
>correctly in their apps. In short every single bugreport like this about
>this -EINVAL strict behaviour is the proof we need to be strict and to
>return -EINVAL :)
>
>Andrea
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