[RFC PATCH 07/13] fs/userfaultfd: support read_iter to use io_uring

Nadav Amit
2020-11-29T00:51:08Z

iouring with userfaultfd cannot currently be used fixed buffers since
userfaultfd does not provide read_iter(). This is required to allow
asynchronous (queued) reads from userfaultfd.

To support async-reads of userfaultfd provide read_iter() instead of
read().

Cc: Jens Axboe <axboe@kernel.dk>
Cc: Andrea Arcangeli <aarcange@redhat.com>
Cc: Peter Xu <peterx@redhat.com>
Cc: Alexander Viro <viro@zeniv.linux.org.uk>
Cc: io-uring@vger.kernel.org
Cc: linux-fsdevel@vger.kernel.org
Cc: linux-kernel@vger.kernel.org
Cc: linux-mm@kvack.org
Signed-off-by: Nadav Amit <namit@vmware.com>
---
fs/userfaultfd.c | 18 ++++++++++--------
1 file changed, 10 insertions(+), 8 deletions(-)

diff --git a/fs/userfaultfd.c b/fs/userfaultfd.c
index b6a04e526025..6333b4632742 100644
--- a/fs/userfaultfd.c
+++ b/fs/userfaultfd.c
@@ -1195,9 +1195,9 @@ static ssize_t userfaultfd_ctx_read(struct userfaultfd_ctx *ctx, int no_wait,
return ret;
}

-static ssize_t userfaultfd_read(struct file *file, char __user *buf,
- size_t count, loff_t *ppos)
+static ssize_t userfaultfd_read_iter(struct kiocb *iocb, struct iov_iter *to)
{
+ struct file *file = iocb->ki_filp;
struct userfaultfd_ctx *ctx = file->private_data;
ssize_t _ret, ret = 0;
struct uffd_msg msg;
@@ -1207,16 +1207,18 @@ static ssize_t userfaultfd_read(struct file *file, char __user *buf,
return -EINVAL;

for (;;) {
- if (count < sizeof(msg))
+ if (iov_iter_count(to) < sizeof(msg))
return ret ? ret : -EINVAL;
_ret = userfaultfd_ctx_read(ctx, no_wait, &msg);
if (_ret < 0)
return ret ? ret : _ret;
- if (copy_to_user((__u64 __user *) buf, &msg, sizeof(msg)))
- return ret ? ret : -EFAULT;
+
+ _ret = copy_to_iter(&msg, sizeof(msg), to);
+ if (_ret != sizeof(msg))
+ return ret ? ret : -EINVAL;
+
ret += sizeof(msg);
- buf += sizeof(msg);
- count -= sizeof(msg);
+
/*
* Allow to read more than one fault at time but only
* block if waiting for the very first one.
@@ -1980,7 +1982,7 @@ static const struct file_operations userfaultfd_fops = {
#endif
.release = userfaultfd_release,
.poll = userfaultfd_poll,
- .read = userfaultfd_read,
+ .read_iter = userfaultfd_read_iter,
.unlocked_ioctl = userfaultfd_ioctl,
.compat_ioctl = compat_ptr_ioctl,
.llseek = noop_llseek,
--
2.25.1