Re: [PATCH 10/10] powerpc: remove address space overrides using set_fs()

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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,




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