[RFC,11/53] netfs: Add support for DIO buffering

Add a bvec array pointer and an iterator to netfs_io_request for either
holding a copy of a DIO iterator or a list of all the bits of buffer
pointed to by a DIO iterator.

There are two problems:  Firstly, if an iovec-class iov_iter is passed to
->read_iter() or ->write_iter(), this cannot be passed directly to
kernel_sendmsg() or kernel_recvmsg() as that may cause locking recursion if
a fault is generated, so we need to keep track of the pages involved
separately.

Secondly, if the I/O is asynchronous, we must copy the iov_iter describing
the buffer before returning to the caller as it may be immediately
deallocated.

Signed-off-by: David Howells <dhowells@redhat.com>
cc: Jeff Layton <jlayton@kernel.org>
cc: linux-cachefs@redhat.com
cc: linux-fsdevel@vger.kernel.org
cc: linux-mm@kvack.org
---
 fs/netfs/objects.c    | 10 ++++++++++
 include/linux/netfs.h |  3 +++
 2 files changed, 13 insertions(+)

On Fri, 2023-10-13 at 17:03 +0100, David Howells wrote:
> Add a bvec array pointer and an iterator to netfs_io_request for either
> holding a copy of a DIO iterator or a list of all the bits of buffer
> pointed to by a DIO iterator.
> 
> There are two problems:  Firstly, if an iovec-class iov_iter is passed to
> ->read_iter() or ->write_iter(), this cannot be passed directly to
> kernel_sendmsg() or kernel_recvmsg() as that may cause locking recursion if
> a fault is generated, so we need to keep track of the pages involved
> separately.
> 
> Secondly, if the I/O is asynchronous, we must copy the iov_iter describing
> the buffer before returning to the caller as it may be immediately
> deallocated.
> 
> Signed-off-by: David Howells <dhowells@redhat.com>
> cc: Jeff Layton <jlayton@kernel.org>
> cc: linux-cachefs@redhat.com
> cc: linux-fsdevel@vger.kernel.org
> cc: linux-mm@kvack.org
> ---
>  fs/netfs/objects.c    | 10 ++++++++++
>  include/linux/netfs.h |  3 +++
>  2 files changed, 13 insertions(+)
> 
> diff --git a/fs/netfs/objects.c b/fs/netfs/objects.c
> index 8e92b8401aaa..4396318081bf 100644
> --- a/fs/netfs/objects.c
> +++ b/fs/netfs/objects.c
> @@ -78,6 +78,7 @@ static void netfs_free_request(struct work_struct *work)
>  {
>  	struct netfs_io_request *rreq =
>  		container_of(work, struct netfs_io_request, work);
> +	unsigned int i;
>  
>  	trace_netfs_rreq(rreq, netfs_rreq_trace_free);
>  	netfs_proc_del_rreq(rreq);
> @@ -86,6 +87,15 @@ static void netfs_free_request(struct work_struct *work)
>  		rreq->netfs_ops->free_request(rreq);
>  	if (rreq->cache_resources.ops)
>  		rreq->cache_resources.ops->end_operation(&rreq->cache_resources);
> +	if (rreq->direct_bv) {
> +		for (i = 0; i < rreq->direct_bv_count; i++) {
> +			if (rreq->direct_bv[i].bv_page) {
> +				if (rreq->direct_bv_unpin)
> +					unpin_user_page(rreq->direct_bv[i].bv_page);
> +			}
> +		}
> +		kvfree(rreq->direct_bv);
> +	}
>  	kfree_rcu(rreq, rcu);
>  	netfs_stat_d(&netfs_n_rh_rreq);
>  }
> diff --git a/include/linux/netfs.h b/include/linux/netfs.h
> index bd0437088f0e..66479a61ad00 100644
> --- a/include/linux/netfs.h
> +++ b/include/linux/netfs.h
> @@ -191,7 +191,9 @@ struct netfs_io_request {
>  	struct list_head	subrequests;	/* Contributory I/O operations */
>  	struct iov_iter		iter;		/* Unencrypted-side iterator */
>  	struct iov_iter		io_iter;	/* I/O (Encrypted-side) iterator */
> +	struct bio_vec		*direct_bv;	/* DIO buffer list (when handling iovec-iter) */
>  	void			*netfs_priv;	/* Private data for the netfs */
> +	unsigned int		direct_bv_count; /* Number of elements in bv[] */

nit: "number of elements in direct_bv[]"

Also, just for better readability, can you swap direct_bv and
netfs_priv? Then at least the array and count are together.

>  	unsigned int		debug_id;
>  	unsigned int		rsize;		/* Maximum read size (0 for none) */
>  	atomic_t		nr_outstanding;	/* Number of ops in progress */
> @@ -200,6 +202,7 @@ struct netfs_io_request {
>  	size_t			len;		/* Length of the request */
>  	short			error;		/* 0 or error that occurred */
>  	enum netfs_io_origin	origin;		/* Origin of the request */
> +	bool			direct_bv_unpin; /* T if direct_bv[] must be unpinned */
>  	loff_t			i_size;		/* Size of the file */
>  	loff_t			start;		/* Start position */
>  	pgoff_t			no_unlock_folio; /* Don't unlock this folio after read */
>

Jeff Layton <jlayton@kernel.org> wrote:

> > +	struct bio_vec		*direct_bv;	/* DIO buffer list (when handling iovec-iter) */
> >  	void			*netfs_priv;	/* Private data for the netfs */
> > +	unsigned int		direct_bv_count; /* Number of elements in bv[] */
> 
> nit: "number of elements in direct_bv[]"
> 
> Also, just for better readability, can you swap direct_bv and
> netfs_priv? Then at least the array and count are together.

Yeah - and stick a __counted_by() on too.

David