Re: [PATCH] sendfile removal

[Jens Axboe <jens.axboe@oracle.com>]

On Fri, Jun 01 2007, H. Peter Anvin wrote:
> > So there's a few things to take away from this:
> > 
> >  - regular file access MUST NOT return EAGAIN just because a page isn't 
> >    in the cache. Doing so is simply a bug. No ifs, buts or maybe's about 
> >    it!
> > 
> >    Busy-looping is NOT ACCEPTABLE!
> > 
> >  - you *could* make some alternative conventions:
> > 
> > 	(a) you could make O_NONBLOCK mean that you'll at least 
> > 	    guarantee that you *start* the IO, and while you never return 
> > 	    EAGAIN, you migth validly return a _partial_ result!
> > 
> > 	(b) variation on (a): it's ok to return EAGAIN if _you_ were the 
> > 	    one who started the IO during this particular time aroudn the 
> > 	    loop. But if you find a page that isn't up-to-date yet, and 
> > 	    you didn't start the IO, you *must* wait for it, so that you 
> > 	    end up returning EAGAIN atmost once! Exactly because 
> > 	    busy-looping is simply not acceptable behaviour!
> 
> (b) seems really ugly.  (a) is at least well-defined.  Either seems
> wrong, though.

I totally agree, b) would get nasty. And while a) isn't perfect by any
means, I do follow Linus' logic and agree it's probably the best (only?)
way to handle it.

-- 
Jens Axboe