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From: David Laight <David.Laight@ACULAB.COM>
To: 'Al Viro' <viro@zeniv.linux.org.uk>, Jens Axboe <axboe@kernel.dk>
CC: "linux-kernel@vger.kernel.org" <linux-kernel@vger.kernel.org>
Subject: RE: [PATCH 002/437] fs: add generic read/write iterator helpers
Thread-Topic: [PATCH 002/437] fs: add generic read/write iterator helpers
Thread-Index: AQHaj28btiR/XG2LCk2zcJQBF8Y/qLFrUuuQ
Date: Tue, 16 Apr 2024 20:14:37 +0000
Message-ID: <72c1e82b9f274b4f8f217bdd624a1c88@AcuMS.aculab.com>
References: <20240411153126.16201-1-axboe@kernel.dk>
 <20240411153126.16201-3-axboe@kernel.dk> <20240415195504.GU2118490@ZenIV>
In-Reply-To: <20240415195504.GU2118490@ZenIV>
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From: Al Viro
> Sent: 15 April 2024 20:55
>=20
> On Thu, Apr 11, 2024 at 09:12:22AM -0600, Jens Axboe wrote:
>=20
> > +/* generic read side helper for drivers converting to ->read_iter() */
> > +ssize_t vfs_read_iter(struct kiocb *iocb, struct iov_iter *to,
> > +=09=09      ssize_t (*read)(struct file *, char __user *,
> > +=09=09=09=09     size_t, loff_t *))
> > +{
> > +=09return do_loop_readv(iocb->ki_filp, to, &iocb->ki_pos, 0, read);
> > +}
> > +EXPORT_SYMBOL(vfs_read_iter);
> > +
> > +/* generic write side helper for drivers converting to ->write_iter() =
*/
> > +ssize_t vfs_write_iter(struct kiocb *iocb, struct iov_iter *from,
> > +=09=09       ssize_t (*write)(struct file *, const char __user *,
> > +=09=09=09=09       size_t, loff_t *))
> > +{
> > +=09return do_loop_writev(iocb->ki_filp, from, &iocb->ki_pos, 0, write)=
;
> > +}
> > +EXPORT_SYMBOL(vfs_write_iter);
>=20
> Wait a minute; just what do you expect to happen if that ever gets called
> for ITER_BVEC or ITER_XARRAY?

The extra indirect call is also going to be noticeable.
You need a code loop with a direct call.
That probably requires the loop to be a #define.

I was also thinking about drivers that only handle 'user' buffers and
where there really isn't a requirement to do anything else.

I've a driver that basically does:
=09if (!access_ok(....))
=09=09return -EFAULT;
=09for (off =3D 0; off < len; off +=3D 8) {
=09=09if (__put_user(readq(io_addr + off), uaddr + off))
=09=09=09return -EFAULT;
=09}

Any non-trivial change requires a function that return the first/only
user buffer address/length and an error for a non-user address.

=09David

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