[PATCH 2/2] sysfs/kernfs: make read requests on pre-alloc files use the buffer.

[NeilBrown <neilb@suse.de>]

To match the previous patch which used the pre-alloc buffer for
writes, this patch causes reads to use the same buffer.
This is not strictly necessary as the current seq_read() will allocate
on first read, so user-space can trigger the required pre-alloc.  But
consistency is valuable.

The read function is somewhat simpler than seq_read() and, for example,
does not support reading from an offset into the file: reads must be
at the start of the file.

As the buffer is shared with writes and other reads, the mutex is
extended to cover the copy_to_user.

Signed-off-by: NeilBrown <neilb@suse.de>
---
 fs/kernfs/file.c |   17 ++++++++++-------
 fs/sysfs/file.c  |   28 ++++++++++++++++++++++++----
 2 files changed, 34 insertions(+), 11 deletions(-)

diff --git a/fs/kernfs/file.c b/fs/kernfs/file.c
index 73bd5ed143cd..7072240604f5 100644
--- a/fs/kernfs/file.c
+++ b/fs/kernfs/file.c
@@ -189,7 +189,9 @@ static ssize_t kernfs_file_direct_read(struct kernfs_open_file *of,
 	const struct kernfs_ops *ops;
 	char *buf;
 
-	buf = kmalloc(len, GFP_KERNEL);
+	buf = of->buf;
+	if (!buf)
+		buf = kmalloc(len, GFP_KERNEL);
 	if (!buf)
 		return -ENOMEM;
 
@@ -210,21 +212,22 @@ static ssize_t kernfs_file_direct_read(struct kernfs_open_file *of,
 	else
 		len = -EINVAL;
 
-	kernfs_put_active(of->kn);
-	mutex_unlock(&of->mutex);
-
 	if (len < 0)
-		goto out_free;
+		goto out_unlock;
 
 	if (copy_to_user(user_buf, buf, len)) {
 		len = -EFAULT;
-		goto out_free;
+		goto out_unlock;
 	}
 
 	*ppos += len;
 
+ out_unlock:
+	kernfs_put_active(of->kn);
+	mutex_unlock(&of->mutex);
  out_free:
-	kfree(buf);
+	if (buf != of->buf)
+		kfree(buf);
 	return len;
 }
 
diff --git a/fs/sysfs/file.c b/fs/sysfs/file.c
index 4a959d231b43..3f7e4d341546 100644
--- a/fs/sysfs/file.c
+++ b/fs/sysfs/file.c
@@ -102,6 +102,18 @@ static ssize_t sysfs_kf_bin_read(struct kernfs_open_file *of, char *buf,
 	return battr->read(of->file, kobj, battr, buf, pos, count);
 }
 
+/* kernfs read callback for regular sysfs files with pre-alloc */
+static ssize_t sysfs_kf_read(struct kernfs_open_file *of, char *buf,
+                            size_t count, loff_t pos)
+{
+       const struct sysfs_ops *ops = sysfs_file_ops(of->kn);
+       struct kobject *kobj = of->kn->parent->priv;
+
+       if (pos)
+               return 0;
+       return ops->show(kobj, of->kn->priv, buf);
+}
+
 /* kernfs write callback for regular sysfs files */
 static ssize_t sysfs_kf_write(struct kernfs_open_file *of, char *buf,
 			      size_t count, loff_t pos)
@@ -184,13 +196,18 @@ static const struct kernfs_ops sysfs_file_kfops_rw = {
 	.write		= sysfs_kf_write,
 };
 
+static const struct kernfs_ops sysfs_prealloc_kfops_ro = {
+	.read		= sysfs_kf_read,
+	.prealloc	= true,
+};
+
 static const struct kernfs_ops sysfs_prealloc_kfops_wo = {
 	.write		= sysfs_kf_write,
 	.prealloc	= true,
 };
 
 static const struct kernfs_ops sysfs_prealloc_kfops_rw = {
-	.seq_show	= sysfs_kf_seq_show,
+	.read		= sysfs_kf_read,
 	.write		= sysfs_kf_write,
 	.prealloc	= true,
 };
@@ -238,9 +255,12 @@ int sysfs_add_file_mode_ns(struct kernfs_node *parent,
 				ops = &sysfs_prealloc_kfops_rw;
 			else
 				ops = &sysfs_file_kfops_rw;
-		} else if (sysfs_ops->show)
-			ops = &sysfs_file_kfops_ro;
-		else if (sysfs_ops->store) {
+		} else if (sysfs_ops->show) {
+			if (mode & SYSFS_PREALLOC)
+				ops = &sysfs_prealloc_kfops_ro;
+			else
+				ops = &sysfs_file_kfops_ro;
+		} else if (sysfs_ops->store) {
 			if (mode & SYSFS_PREALLOC)
 				ops = &sysfs_prealloc_kfops_wo;
 			else