Re: [PATCH] /dev/zero: also implement ->read

Greg KH
2020-09-03T18:02:00Z

On Thu, Sep 03, 2020 at 07:51:07PM +0200, Christophe Leroy wrote:
>
>
> Le 03/09/2020 à 17:59, Christoph Hellwig a écrit :
> > Christophe reported a major speedup due to avoiding the iov_iter
> > overhead, so just add this trivial function. Note that /dev/zero
> > already implements both an iter and non-iter writes so this just
> > makes it more symmetric.
> >
> > Christophe Leroy <christophe.leroy@csgroup.eu>
> > Signed-off-by: Christoph Hellwig <hch@lst.de>
> > ---
> > drivers/char/mem.c | 22 ++++++++++++++++++++++
> > 1 file changed, 22 insertions(+)
> >
> > diff --git a/drivers/char/mem.c b/drivers/char/mem.c
> > index abd4ffdc8cdebc..1dc99ab158457a 100644
> > --- a/drivers/char/mem.c
> > +++ b/drivers/char/mem.c
> > @@ -726,6 +726,27 @@ static ssize_t read_iter_zero(struct kiocb *iocb, struct iov_iter *iter)
> > return written;
> > }
> > +static ssize_t read_zero(struct file *file, char __user *buf,
> > + size_t count, loff_t *ppos)
> > +{
> > + size_t cleared = 0;
> > +
> > + while (count) {
> > + size_t chunk = min_t(size_t, count, PAGE_SIZE);
> > +
> > + if (clear_user(buf + cleared, chunk))
> > + return cleared ? cleared : -EFAULT;
> > + cleared += chunk;
> > + count -= chunk;
> > +
> > + if (signal_pending(current))
> > + return cleared ? cleared : -ERESTARTSYS;
> > + cond_resched();
> > + }
> > +
> > + return cleared;
> > +}
> > +
> > static int mmap_zero(struct file *file, struct vm_area_struct *vma)
> > {
> > #ifndef CONFIG_MMU
> > @@ -921,6 +942,7 @@ static const struct file_operations zero_fops = {
> > .llseek = zero_lseek,
> > .write = write_zero,
> > .read_iter = read_iter_zero,
> > + .read = read_zero,
>
> Wondering if .read should be before .write, so that we get in order read,
> write, read_iter, write_iter.

It really does not matter :)

thanks,

greg k-h