A write to a kernfs_node is buffered through a kernel buffer. Writes
<= PAGE_SIZE are performed atomically, while larger ones are executed
in PAGE_SIZE chunks. While this is enough for sysfs, cgroup which is
scheduled to be converted to use kernfs needs a bit more control over
it.
This patch adds kernfs_ops->atomic_write_len. If not set (zero), the
behavior stays the same. If set, writes upto the size are executed
atomically and larger writes are rejected with -E2BIG.
A different implementation strategy would be allowing configuring
chunking size while making the original write size available to the
write method; however, such strategy, while being more complicated,
doesn't really buy anything. If the write implementation has to
handle chunking, the specific chunk size shouldn't matter all that
much.
Signed-off-by: Tejun Heo <t...@kernel.org>
---
fs/kernfs/file.c | 49 +++++++++++++++++++++++++++++++------------------
include/linux/kernfs.h | 8 ++++++--
2 files changed, 37 insertions(+), 20 deletions(-)
diff --git a/fs/kernfs/file.c b/fs/kernfs/file.c
index 10a8c91..ddcb471 100644
--- a/fs/kernfs/file.c
+++ b/fs/kernfs/file.c
@@ -252,19 +252,9 @@ static ssize_t kernfs_fop_write(struct file *file, const
char __user *user_buf,
size_t count, loff_t *ppos)
{
struct kernfs_open_file *of = kernfs_of(file);
- ssize_t len = min_t(size_t, count, PAGE_SIZE);
const struct kernfs_ops *ops;
- char *buf;
-
- buf = kmalloc(len + 1, GFP_KERNEL);
- if (!buf)
- return -ENOMEM;
-
- if (copy_from_user(buf, user_buf, len)) {
- len = -EFAULT;
- goto out_free;
- }
- buf[len] = '\0'; /* guarantee string termination */
+ char *buf = NULL;
+ ssize_t len;
/*
* @of->mutex nests outside active ref and is just to ensure that
@@ -273,22 +263,45 @@ static ssize_t kernfs_fop_write(struct file *file, const
char __user *user_buf,
mutex_lock(&of->mutex);
if (!kernfs_get_active(of->kn)) {
mutex_unlock(&of->mutex);
- len = -ENODEV;
- goto out_free;
+ return -ENODEV;
}
ops = kernfs_ops(of->kn);
- if (ops->write)
- len = ops->write(of, buf, len, *ppos);
- else
+ if (!ops->write) {
len = -EINVAL;
+ goto out_unlock;
+ }
+
+ if (ops->atomic_write_len) {
+ len = count;
+ if (len > ops->atomic_write_len) {
+ len = -E2BIG;
+ goto out_unlock;
+ }
+ } else {
+ len = min_t(size_t, count, PAGE_SIZE);
+ }
+
+ buf = kmalloc(len + 1, GFP_KERNEL);
+ if (!buf) {
+ len = -ENOMEM;
+ goto out_unlock;
+ }
+ if (copy_from_user(buf, user_buf, len)) {
+ len = -EFAULT;
+ goto out_unlock;
+ }
+ buf[len] = '\0'; /* guarantee string termination */
+
+ len = ops->write(of, buf, len, *ppos);
+out_unlock:
kernfs_put_active(of->kn);
mutex_unlock(&of->mutex);
if (len > 0)
*ppos += len;
-out_free:
+
kfree(buf);
return len;
}
diff --git a/include/linux/kernfs.h b/include/linux/kernfs.h
index 4520c86..47f5235 100644
--- a/include/linux/kernfs.h
+++ b/include/linux/kernfs.h
@@ -178,9 +178,13 @@ struct kernfs_ops {
loff_t off);
/*
- * write() is bounced through kernel buffer and a write larger than
- * PAGE_SIZE results in partial operation of PAGE_SIZE.
+ * write() is bounced through kernel buffer. If atomic_write_len
+ * is not set, a write larger than PAGE_SIZE results in partial
+ * operations of PAGE_SIZE chunks. If atomic_write_len is set,
+ * writes upto the specified size are executed atomically but
+ * larger ones are rejected with -E2BIG.
*/
+ size_t atomic_write_len;
ssize_t (*write)(struct kernfs_open_file *of, char *buf, size_t bytes,
loff_t off);
--
1.8.5.3
--
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