Le 03/09/2020 à 09:11, Christoph Hellwig a écrit :
Except that we do not actually have such a patch. For normal user writes we only use ->write_iter if ->write is not present. But what shows up in the profile is that /dev/zero only has a read_iter op and not a normal read. I've added a patch below that implements a normal read which might help a tad with this workload, but should not be part of a regression.
With that patch below, throughput is 113.5MB/s (instead of 99.9MB/s). So a 14% improvement. That's not bad. Christophe
--- 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, .write_iter = write_iter_zero, .mmap = mmap_zero, .get_unmapped_area = get_unmapped_area_zero,
