linux-kernel - Re: [GIT PULL] 9p update for 5.7

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Message-ID: <20200406173957.GI21484@bombadil.infradead.org>
Date:   Mon, 6 Apr 2020 10:39:57 -0700
From:   Matthew Wilcox <willy@...radead.org>
To:     Linus Torvalds <torvalds@...ux-foundation.org>
Cc:     Dominique Martinet <asmadeus@...ewreck.org>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        linux-fsdevel <linux-fsdevel@...r.kernel.org>,
        v9fs-developer@...ts.sourceforge.net,
        Sergey Alirzaev <l29ah@...k.li>
Subject: Re: [GIT PULL] 9p update for 5.7

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@...radead.org> 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).