Re: [PATCH v5 15/40] netfs: Add support for DIO buffering

Nathan Chancellor
2023-12-26T16:54:45Z

On Thu, Dec 21, 2023 at 01:23:10PM +0000, 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>
> Reviewed-by: 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 | 4 ++++
> 2 files changed, 14 insertions(+)
>
> diff --git a/fs/netfs/objects.c b/fs/netfs/objects.c
> index 1bd20bdad983..4df5e5eeada6 100644
> --- a/fs/netfs/objects.c
> +++ b/fs/netfs/objects.c
> @@ -76,6 +76,7 @@ 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);
> @@ -84,6 +85,15 @@ 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 3da962e977f5..bbb33ccbf719 100644
> --- a/include/linux/netfs.h
> +++ b/include/linux/netfs.h
> @@ -190,6 +190,9 @@ struct netfs_io_request {
> struct iov_iter iter; /* Unencrypted-side iterator */
> struct iov_iter io_iter; /* I/O (Encrypted-side) iterator */
> void *netfs_priv; /* Private data for the netfs */
> + struct bio_vec *direct_bv /* DIO buffer list (when handling iovec-iter) */
> + __counted_by(direct_bv_count);

This will break the build with versions of clang that have support for
counted_by (as it has been reverted in main but reapplication to main is
being actively worked on) because while annotating pointers with this
attribute is a goal of the counted_by attribute, it is not ready yet.
Please consider removing this and adding a TODO to annotate it when
support is available.

In file included from /builds/linux/fs/ceph/crypto.c:14:
In file included from /builds/linux/fs/ceph/super.h:20:
/builds/linux/include/linux/netfs.h:259:19: error: 'counted_by' only applies to C99 flexible array members
259 | struct bio_vec *direct_bv /* DIO buffer list (when handling iovec-iter) */
| ^~~~~~~~~
1 error generated.

Some additional context from another recent error like this:

https://lore.kernel.org/CAKwvOdkvGTGiWzqEFq=kzqvxSYP5vUj3g9Z-=MZSQROzzSa_dg@mail.gmail.com/

Cheers,
Nathan

> + unsigned int direct_bv_count; /* Number of elements in direct_bv[] */
> unsigned int debug_id;
> atomic_t nr_outstanding; /* Number of ops in progress */
> atomic_t nr_copy_ops; /* Number of copy-to-cache ops in progress */
> @@ -197,6 +200,7 @@ 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 */
>