Re: [GIT PULL] 9p update for 5.7

[Matthew Wilcox]

On Mon, Apr 06, 2020 at 10:04:11AM -0700, Linus Torvalds wrote:
> On Mon, Apr 6, 2020 at 9:46 AM Matthew Wilcox <willy@xxxxxxxxxxxxx> wrote:
> >
> > POSIX may well "allow" short reads, but userspace programmers basically
> > never check the return value from read().  Short reads aren't actually
> > allowed.  That's why signals are only allowed to interrupt syscalls if
> > they're fatal (and the application will never see the returned value
> > because it's already dead).
> 
> Well, that's true for some applications.
> 
> But look at anybody who ever worked more with NFS mounts, and they got
> used to having the 'intr' mount flag set and incomplete reads and
> -EAGAIN as a result.

That's why you had me implement TASK_KILLABLE ;-)

> Are there apps that react badly? I'm sure - but they also wouldn't
> have O_NONBLOCK set on a regular file. The only reason to set
> O_NONBLOCK is because you think the fd might be a pipe or something,
> and you _are_ ready to get partial reads.
> 
> So the 9p behavior certainly isn't outrageously out of line for a
> network filesystem. In fact, because of O_NONBLOCK rather than a mount
> option, I think it's a lot safer than a fairly standard NFS option.

The NFS option has been a no-op for over a decade ;-)  I agree with you
that O_NONBLOCK is a good indicator the application is willing to handle
short reads (or indeed writes).