Re: How to do DIRECT IO on kernel buffer

[Rajat Sharma <fs.rajat@xxxxxxxxx>]

> Doesn't that mean user space right to block device? Simply based on
> that, I can  see you fail all these times... because you do it from
> kernel space..and that's against the very  basic meaning of direct I/O

You can visualize direct I/O for two purpose:

1. Zero copy directly from user buffers to block device as you pointed out.
2. Don't make an extra effort to cache the data in page-cache, as it might have already be done by applications.

In some cases, you may not afford to solve 1., e.g. you may want to manipulate data before writing it do target. Although this can be done on the same user space buffer, but you may not want to do that because it may change the working data set of application.

Anyways, to cut it short again, I had put my requirements in my original mail, and I have one solution where I map pages in user space, I am looking for a better solution, or the verification that there is none, where I can directly user kernel buffers for doing non-cached I/O.

Rajat