On Mon, Sep 07, 2020 at 12:34:37AM +0200, Rasmus Villemoes wrote: > On 03/09/2020 17.59, Christoph Hellwig wrote: > > 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.le...@csgroup.eu> > > ?-by ?
Suggested-by, > > +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; > > Probably nobody really cares, but currently doing > > read(fd, &unmapped_page - 5, 123); > > returns 5, and those five bytes do get cleared; if I'm reading the above > right you'd return -EFAULT for that case. > > > > + cleared += chunk; > > + count -= chunk; > > + > > + if (signal_pending(current)) > > + return cleared ? cleared : -ERESTARTSYS; > > I can't see how we can get here without 'cleared' being positive, so > this can just be 'return cleared' (and if you fix the above EFAULT case > to more accurately track how much got cleared, there's probably no > longer any code to be symmetric with anyway). Yeah, I'll fix these up and resend.