Re: [PATCH] sendfile removal

["H. Peter Anvin" <hpa@zytor.com>]

Linus Torvalds wrote:
> And the thing is, neither poll nor select work on regular files. And no, 
> that is _not_ just an implementation issue. It's very fundamental: neither 
> poll nor select get the file offset to wait for!
> 
> And that file offset is _critical_ for a regular file, in a way it 
> obviously is _not_ for a socket, pipe, or other special file. Because 
> without knowing the file offset, you cannot know which page you should be 
> waiting for!
> 
> And no, the file offset is not "f_pos". sendfile(), along with 
> pread/pwrite, uses a totally separate file offset, so if select/poll were 
> to base their decision on f_pos, they'd be _wrong_.

This is obviously correct, although at the time those interfaces were
designed, I don't believe either pread/pwrite nor sendfile() existed,
and they still couldn't wait on real files.  That there isn't a suitable
way to wait for a file at an offset is probably a result of that past
history.

Waiting at f_pos is still a possible interface, of course; it would mean
that pread/pwrite/sendfile users would have to seek before waiting.
However, implementing waiting on files in select/poll is prohibited by
POSIX, so it would at least need some sort of Linux-specific flag anyway.

It seems that being able to do nonblocking I/O on files would be a
useful thing.  This really *does* require proper nonblocking I/O and not
just the ability to wait, since you can never know when the kernel
decides to recycle the page you are just about to want from the cache.

> 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.

	-hpa