On 07/09/2020 08.20, Christoph Hellwig wrote: > On Mon, Sep 07, 2020 at 12:34:37AM +0200, Rasmus Villemoes wrote: >> On 03/09/2020 17.59, Christoph Hellwig wrote: > >>> +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. >
Actually, while you're micro-optimizing it, AFAIK VFS already handles count==0, so you can avoid the initial branch and the last cond_resched() by writing it something like while (1) { size_t chunk = min_t(size_t, count, PAGE_SIZE), c; c = chunk - clear_user(buf + cleared, chunk); if (unlikely(!c)) return cleared ?: -EFAULT; cleared += c; count -= c; if (!count || signal_pending()) return cleared; cond_resched(); } For the dd test case with the default bs=512 that avoids cond_resched() and signal_pending() altogether. Rasmus