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

[Greg KH <gregkh@linuxfoundation.org>]

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