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Date: Fri, 12 Apr 2024 05:37:03 +0100
From: Al Viro <viro@zeniv.linux.org.uk>
To: Jens Axboe <axboe@kernel.dk>
Cc: linux-kernel@vger.kernel.org
Subject: Re: [PATCHSET RFC 0/437] Kill off old fops ->read() and ->write()
Message-ID: <20240412043703.GA2606434@ZenIV>
References: <20240411153126.16201-1-axboe@kernel.dk>
 <20240412042910.GK2118490@ZenIV>
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Sender: Al Viro <viro@ftp.linux.org.uk>

On Fri, Apr 12, 2024 at 05:29:10AM +0100, Al Viro wrote:
> On Thu, Apr 11, 2024 at 09:12:20AM -0600, Jens Axboe wrote:
> > Hi,
> > 
> > This patchset will obviously be split, commit messages updated, and
> > specific driver patches targeted to where they belong. But I figured
> > it'd be useful to blast out the full set at least once for reference,
> > and then I'll continue down the right path for the next one.
> > 
> > Subject line says it all, really. 10 years ago we added ->read_iter()
> > and ->write_iter() to struct file_operations. These are great, as they
> > pass in an iov_iter rather than a user buffer + length, and they also
> > take a struct kiocb rather than just a file. Since then we've had two
> > paths for any read or write - one legacy one that can't do per-IO hints
> > like "This read should be non-blocking", they strictly only work with
> > O_NONBLOCK on the file, and a newer one that supports everything the
> > old path does and a bunch more. We've had a few issues with the
> > iov_iter based path being slower, but those have basically been
> > resolved with solutions like ITER_UBUF to optimize the single segment
> > case that is often the fast path.
> > 
> > There are basically three parts to this series:
> > 
> > 1) Add generic helpers that we need to convert drivers.
> > 2) Convert any use of fops->read() and ->write()
> > 3) Kill off old cruft.
> > 3a) Profit.
> 
> The fundamental problem with that is that a bunch of drivers
> do care about the vector boundaries.  Very much so.  It's very
> common to have this kind of situation:
> 	write() parses the buffer sloppily, and ignores the junk in
> the end, claiming that everything that been written.
> 	writev() feeds each vector to write().
> 
> From a cursory look through that pile, you seem to have broken
> writev() on at least some (if not all) of those.

OK, it's 'some', not 'all', but AFAICS you'd been far too
optimistic about the number of instances that do not need
your vfs_write_iter() wrappers...