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SubjectRe: [PATCH] sendfile removal
Linus Torvalds a écrit :
>
> On Fri, 1 Jun 2007, H. Peter Anvin wrote:
>> Fair enough. Unix has traditionally not acknowledged the possibility of
>> nonblocking I/O on conventional files, for some odd reason.
>
> It's not odd at all.
>
> If you return EAGAIN, you had better have a way to _wait_ for that EAGAIN
> to go away, otherwise the EAGAIN is just a total waste of time.
>
> So the rule about EAGAIN is very simple:
> (a) the file descriptor must be O_NONBLOCK
> (b) the access must otherwise block
> AND
> (c) the condition must be something we can wait for with poll/select
>
> I don't know why people continually ignore that (c) point, even though
> it's obvious and very very important!
>
> If you cannot wait for it, tell me why the kernel should _ever_ return
> EAGAIN? The only option for the user is to just do the operation again
> immediately.
>
> 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 really is very fundamental.
>
> Now, you can argue that you can always just return -EAGAIN anyway, but
> then the calling process will basically be busy-looping, calling
> sendfile() (or splice()) over and over again. That's _horrible_. It's much
> better to just not return EAGAIN, and sleep like a good process should!
>
> 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!

yes, very true, but then some apps do this (and sometimes depends on yield())


>
> - 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!
>
> I have to admit that I didn't look at what raw splice() itself does these
> days. I would not be surprised if Jens also didn't realize this very
> fundamental issue. It seems too easy to miss, because people think
> that EAGAIN stands on its own, and don't realize that EAGAIN must be
> paired with select/poll to make sense.
>

Right now, splice() has one SPLICE_F_NONBLOCK flag, and this flag is applied
on both sides (in & out)

So either :

1) We separate the flag into two flags NONBLOCK_IN & NONBLOCK_OUT, so that the
application is free to chose to busy-loop/yield if it wants.

2) We ignore NONBLOCK flag for regular files in splice() (and sendfile()),
just following current facto

3) We consider select()/poll()/splice() can be extended to regular files on
[f_pos] (select() and related functions have a meaning on non-seekable files,
so consider it can be extended on files only on current file pos)


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