[PATCH v4 07/33] netfs: Abstract out a rolling folio buffer implementation

[Kees Bakker]

Op 08-11-2024 om 18:32 schreef David Howells:
> A rolling buffer is a series of folios held in a list of folio_queues.  New
> folios and folio_queue structs may be inserted at the head simultaneously
> with spent ones being removed from the tail without the need for locking.
>
> The rolling buffer includes an iov_iter and it has to be careful managing
> this as the list of folio_queues is extended such that an oops doesn't
> incurred because the iterator was pointing to the end of a folio_queue
> segment that got appended to and then removed.
>
> We need to use the mechanism twice, once for read and once for write, and,
> in future patches, we will use a second rolling buffer to handle bounce
> buffering for content encryption.
>
> Signed-off-by: David Howells <dhowells at redhat.com>
> cc: Jeff Layton <jlayton at kernel.org>
> cc: netfs at lists.linux.dev
> cc: linux-fsdevel at vger.kernel.org
> ---
>   fs/netfs/Makefile              |   1 +
>   fs/netfs/buffered_read.c       | 119 ++++-------------
>   fs/netfs/direct_read.c         |  14 +-
>   fs/netfs/direct_write.c        |  10 +-
>   fs/netfs/internal.h            |   4 -
>   fs/netfs/misc.c                | 147 ---------------------
>   fs/netfs/objects.c             |   2 +-
>   fs/netfs/read_pgpriv2.c        |  32 ++---
>   fs/netfs/read_retry.c          |   2 +-
>   fs/netfs/rolling_buffer.c      | 225 +++++++++++++++++++++++++++++++++
>   fs/netfs/write_collect.c       |  19 +--
>   fs/netfs/write_issue.c         |  26 ++--
>   include/linux/netfs.h          |  10 +-
>   include/linux/rolling_buffer.h |  61 +++++++++
>   include/trace/events/netfs.h   |   2 +
>   15 files changed, 375 insertions(+), 299 deletions(-)
>   create mode 100644 fs/netfs/rolling_buffer.c
>   create mode 100644 include/linux/rolling_buffer.h
> [...]
> diff --git a/fs/netfs/direct_write.c b/fs/netfs/direct_write.c
> index 88f2adfab75e..0722fb9919a3 100644
> --- a/fs/netfs/direct_write.c
> +++ b/fs/netfs/direct_write.c
> @@ -68,19 +68,19 @@ ssize_t netfs_unbuffered_write_iter_locked(struct kiocb *iocb, struct iov_iter *
>   		 * request.
>   		 */
>   		if (async || user_backed_iter(iter)) {
> -			n = netfs_extract_user_iter(iter, len, &wreq->iter, 0);
> +			n = netfs_extract_user_iter(iter, len, &wreq->buffer.iter, 0);
>   			if (n < 0) {
>   				ret = n;
>   				goto out;
>   			}
> -			wreq->direct_bv = (struct bio_vec *)wreq->iter.bvec;
> +			wreq->direct_bv = (struct bio_vec *)wreq->buffer.iter.bvec;
>   			wreq->direct_bv_count = n;
>   			wreq->direct_bv_unpin = iov_iter_extract_will_pin(iter);
>   		} else {
> -			wreq->iter = *iter;
> +			wreq->buffer.iter = *iter;
>   		}
>   
> -		wreq->io_iter = wreq->iter;
> +		wreq->buffer.iter = wreq->buffer.iter;
Is this correct, an assignment to itself?
>   	}
>   
>   	__set_bit(NETFS_RREQ_USE_IO_ITER, &wreq->flags);
> [...]