pread2/ pwrite2

[Christoph Hellwig]

On Tue, Jan 06, 2015 at 12:47:14AM -0800, Andrew Morton wrote:
> > progress, which is a bit frustrating.
> 
> I took a look at pread2() as well and I have two main issues:
> 
> - The patchset includes a pwrite2() syscall which has nothing to do
>   with nonblocking reads and which was poorly described and had little
>   justification for inclusion.

It allows to do O_SYNC writes on a per-I/O basis.  This is very useful
for file servers (smb, cifs) as well as storage target devices.

Note: that part was my addition, and the complaint about lacking
description ever made it to me.  Can you point to the relevant
questions?

> - We've talked for years about implementing this via fincore+pread
>   and at least two fincore implementations are floating about.  Now
>   along comes pread2() which does it all in one hit.
> 
>   Which approach is best?  I expect fincore+pread is simpler, more
>   flexible and more maintainable.  But pread2() will have lower CPU
>   consumption and lower average-case latency.

fincore+pread is inherently racy and thus entirely unsuitable for the
use case of a non-blockign main thread.

Nevermind that the pread2 path is way simpler than any of the proposed
fincore patches.
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