Re: [PATCH v2 1/2] libceph: add new iov_iter-based ceph_msg_data_type and ceph_osd_data_type

[Xiubo Li <xiubli@redhat.com>]

On 6/27/22 11:54 PM, Jeff Layton wrote:
> Add an iov_iter to the unions in ceph_msg_data and ceph_msg_data_cursor.
> Instead of requiring a list of pages or bvecs, we can just use an
> iov_iter directly, and avoid extra allocations.
>
> We assume that the pages represented by the iter are pinned such that
> they shouldn't incur page faults, which is the case for the iov_iters
> created by netfs.
>
> While working on this, Al Viro informed me that he was going to change
> iov_iter_get_pages to auto-advance the iterator as that pattern is more
> or less required for ITER_PIPE anyway. We emulate that here for now by
> advancing in the _next op and tracking that amount in the "lastlen"
> field.
>
> In the event that _next is called twice without an intervening
> _advance, we revert the iov_iter by the remaining lastlen before
> calling iov_iter_get_pages.
>
> Cc: Al Viro <viro@zeniv.linux.org.uk>
> Cc: David Howells <dhowells@redhat.com>
> Signed-off-by: Jeff Layton <jlayton@kernel.org>
> ---
>   include/linux/ceph/messenger.h  |  8 ++++
>   include/linux/ceph/osd_client.h |  4 ++
>   net/ceph/messenger.c            | 85 +++++++++++++++++++++++++++++++++
>   net/ceph/osd_client.c           | 27 +++++++++++
>   4 files changed, 124 insertions(+)
>
> diff --git a/include/linux/ceph/messenger.h b/include/linux/ceph/messenger.h
> index 9fd7255172ad..2eaaabbe98cb 100644
> --- a/include/linux/ceph/messenger.h
> +++ b/include/linux/ceph/messenger.h
> @@ -123,6 +123,7 @@ enum ceph_msg_data_type {
>   	CEPH_MSG_DATA_BIO,	/* data source/destination is a bio list */
>   #endif /* CONFIG_BLOCK */
>   	CEPH_MSG_DATA_BVECS,	/* data source/destination is a bio_vec array */
> +	CEPH_MSG_DATA_ITER,	/* data source/destination is an iov_iter */
>   };
>   
>   #ifdef CONFIG_BLOCK
> @@ -224,6 +225,7 @@ struct ceph_msg_data {
>   			bool		own_pages;
>   		};
>   		struct ceph_pagelist	*pagelist;
> +		struct iov_iter		iter;
>   	};
>   };
>   
> @@ -248,6 +250,10 @@ struct ceph_msg_data_cursor {
>   			struct page	*page;		/* page from list */
>   			size_t		offset;		/* bytes from list */
>   		};
> +		struct {
> +			struct iov_iter		iov_iter;
> +			unsigned int		lastlen;
> +		};
>   	};
>   };
>   
> @@ -605,6 +611,8 @@ void ceph_msg_data_add_bio(struct ceph_msg *msg, struct ceph_bio_iter *bio_pos,
>   #endif /* CONFIG_BLOCK */
>   void ceph_msg_data_add_bvecs(struct ceph_msg *msg,
>   			     struct ceph_bvec_iter *bvec_pos);
> +void ceph_msg_data_add_iter(struct ceph_msg *msg,
> +			    struct iov_iter *iter);
>   
>   struct ceph_msg *ceph_msg_new2(int type, int front_len, int max_data_items,
>   			       gfp_t flags, bool can_fail);
> diff --git a/include/linux/ceph/osd_client.h b/include/linux/ceph/osd_client.h
> index 6ec3cb2ac457..ef0ad534b6c5 100644
> --- a/include/linux/ceph/osd_client.h
> +++ b/include/linux/ceph/osd_client.h
> @@ -108,6 +108,7 @@ enum ceph_osd_data_type {
>   	CEPH_OSD_DATA_TYPE_BIO,
>   #endif /* CONFIG_BLOCK */
>   	CEPH_OSD_DATA_TYPE_BVECS,
> +	CEPH_OSD_DATA_TYPE_ITER,
>   };
>   
>   struct ceph_osd_data {
> @@ -131,6 +132,7 @@ struct ceph_osd_data {
>   			struct ceph_bvec_iter	bvec_pos;
>   			u32			num_bvecs;
>   		};
> +		struct iov_iter		iter;
>   	};
>   };
>   
> @@ -501,6 +503,8 @@ void osd_req_op_extent_osd_data_bvecs(struct ceph_osd_request *osd_req,
>   void osd_req_op_extent_osd_data_bvec_pos(struct ceph_osd_request *osd_req,
>   					 unsigned int which,
>   					 struct ceph_bvec_iter *bvec_pos);
> +void osd_req_op_extent_osd_iter(struct ceph_osd_request *osd_req,
> +				unsigned int which, struct iov_iter *iter);
>   
>   extern void osd_req_op_cls_request_data_pagelist(struct ceph_osd_request *,
>   					unsigned int which,
> diff --git a/net/ceph/messenger.c b/net/ceph/messenger.c
> index 6056c8f7dd4c..604f025034ab 100644
> --- a/net/ceph/messenger.c
> +++ b/net/ceph/messenger.c
> @@ -964,6 +964,69 @@ static bool ceph_msg_data_pagelist_advance(struct ceph_msg_data_cursor *cursor,
>   	return true;
>   }
>   
> +static void ceph_msg_data_iter_cursor_init(struct ceph_msg_data_cursor *cursor,
> +					size_t length)
> +{
> +	struct ceph_msg_data *data = cursor->data;
> +
> +	cursor->iov_iter = data->iter;
> +	cursor->lastlen = 0;
> +	iov_iter_truncate(&cursor->iov_iter, length);
> +	cursor->resid = iov_iter_count(&cursor->iov_iter);
> +}
> +
> +static struct page *ceph_msg_data_iter_next(struct ceph_msg_data_cursor *cursor,
> +						size_t *page_offset,
> +						size_t *length)
> +{
> +	struct page *page;
> +	ssize_t len;
> +
> +	if (cursor->lastlen)
> +		iov_iter_revert(&cursor->iov_iter, cursor->lastlen);
> +
> +	len = iov_iter_get_pages(&cursor->iov_iter, &page, PAGE_SIZE,
> +				 1, page_offset);
> +	BUG_ON(len < 0);
> +
> +	cursor->lastlen = len;
> +
> +	/*
> +	 * FIXME: Al Viro says that he will soon change iov_iter_get_pages
> +	 * to auto-advance the iterator. Emulate that here for now.
> +	 */
> +	iov_iter_advance(&cursor->iov_iter, len);
> +
> +	/*
> +	 * FIXME: The assumption is that the pages represented by the iov_iter
> +	 * 	  are pinned, with the references held by the upper-level
> +	 * 	  callers, or by virtue of being under writeback. Eventually,
> +	 * 	  we'll get an iov_iter_get_pages variant that doesn't take page
> +	 * 	  refs. Until then, just put the page ref.
> +	 */
> +	VM_BUG_ON_PAGE(!PageWriteback(page) && page_count(page) < 2, page);
> +	put_page(page);
> +
> +	*length = min_t(size_t, len, cursor->resid);
> +	return page;
> +}
> +
> +static bool ceph_msg_data_iter_advance(struct ceph_msg_data_cursor *cursor,
> +					size_t bytes)
> +{
> +	BUG_ON(bytes > cursor->resid);
> +	cursor->resid -= bytes;
> +
> +	if (bytes < cursor->lastlen) {
> +		cursor->lastlen -= bytes;
> +	} else {
> +		iov_iter_advance(&cursor->iov_iter, bytes - cursor->lastlen);
> +		cursor->lastlen = 0;
> +	}
> +
> +	return cursor->resid;
> +}
> +
>   /*
>    * Message data is handled (sent or received) in pieces, where each
>    * piece resides on a single page.  The network layer might not
> @@ -991,6 +1054,9 @@ static void __ceph_msg_data_cursor_init(struct ceph_msg_data_cursor *cursor)
>   	case CEPH_MSG_DATA_BVECS:
>   		ceph_msg_data_bvecs_cursor_init(cursor, length);
>   		break;
> +	case CEPH_MSG_DATA_ITER:
> +		ceph_msg_data_iter_cursor_init(cursor, length);
> +		break;
>   	case CEPH_MSG_DATA_NONE:
>   	default:
>   		/* BUG(); */
> @@ -1038,6 +1104,9 @@ struct page *ceph_msg_data_next(struct ceph_msg_data_cursor *cursor,
>   	case CEPH_MSG_DATA_BVECS:
>   		page = ceph_msg_data_bvecs_next(cursor, page_offset, length);
>   		break;
> +	case CEPH_MSG_DATA_ITER:
> +		page = ceph_msg_data_iter_next(cursor, page_offset, length);
> +		break;
>   	case CEPH_MSG_DATA_NONE:
>   	default:
>   		page = NULL;
> @@ -1076,6 +1145,9 @@ void ceph_msg_data_advance(struct ceph_msg_data_cursor *cursor, size_t bytes)
>   	case CEPH_MSG_DATA_BVECS:
>   		new_piece = ceph_msg_data_bvecs_advance(cursor, bytes);
>   		break;
> +	case CEPH_MSG_DATA_ITER:
> +		new_piece = ceph_msg_data_iter_advance(cursor, bytes);
> +		break;
>   	case CEPH_MSG_DATA_NONE:
>   	default:
>   		BUG();
> @@ -1874,6 +1946,19 @@ void ceph_msg_data_add_bvecs(struct ceph_msg *msg,
>   }
>   EXPORT_SYMBOL(ceph_msg_data_add_bvecs);
>   
> +void ceph_msg_data_add_iter(struct ceph_msg *msg,
> +			    struct iov_iter *iter)
> +{
> +	struct ceph_msg_data *data;
> +
> +	data = ceph_msg_data_add(msg);
> +	data->type = CEPH_MSG_DATA_ITER;
> +	data->iter = *iter;
> +
> +	msg->data_length += iov_iter_count(&data->iter);
> +}
> +EXPORT_SYMBOL(ceph_msg_data_add_iter);
> +

Will this be used outside the libceph.ko ? It seem will never ? And also 
some other existing ones like 'ceph_msg_data_add_bvecs'.

>   /*
>    * construct a new message with given type, size
>    * the new msg has a ref count of 1.
> diff --git a/net/ceph/osd_client.c b/net/ceph/osd_client.c
> index 75761537c644..2a7e46524e71 100644
> --- a/net/ceph/osd_client.c
> +++ b/net/ceph/osd_client.c
> @@ -171,6 +171,13 @@ static void ceph_osd_data_bvecs_init(struct ceph_osd_data *osd_data,
>   	osd_data->num_bvecs = num_bvecs;
>   }
>   
> +static void ceph_osd_iter_init(struct ceph_osd_data *osd_data,
> +			       struct iov_iter *iter)
> +{
> +	osd_data->type = CEPH_OSD_DATA_TYPE_ITER;
> +	osd_data->iter = *iter;
> +}
> +
>   static struct ceph_osd_data *
>   osd_req_op_raw_data_in(struct ceph_osd_request *osd_req, unsigned int which)
>   {
> @@ -264,6 +271,22 @@ void osd_req_op_extent_osd_data_bvec_pos(struct ceph_osd_request *osd_req,
>   }
>   EXPORT_SYMBOL(osd_req_op_extent_osd_data_bvec_pos);
>   
> +/**
> + * osd_req_op_extent_osd_iter - Set up an operation with an iterator buffer
> + * @osd_req: The request to set up
> + * @which: ?

For this could you explain more ?


> + * @iter: The buffer iterator
> + */
> +void osd_req_op_extent_osd_iter(struct ceph_osd_request *osd_req,
> +				unsigned int which, struct iov_iter *iter)
> +{
> +	struct ceph_osd_data *osd_data;
> +
> +	osd_data = osd_req_op_data(osd_req, which, extent, osd_data);
> +	ceph_osd_iter_init(osd_data, iter);
> +}
> +EXPORT_SYMBOL(osd_req_op_extent_osd_iter);
> +
>   static void osd_req_op_cls_request_info_pagelist(
>   			struct ceph_osd_request *osd_req,
>   			unsigned int which, struct ceph_pagelist *pagelist)
> @@ -346,6 +369,8 @@ static u64 ceph_osd_data_length(struct ceph_osd_data *osd_data)
>   #endif /* CONFIG_BLOCK */
>   	case CEPH_OSD_DATA_TYPE_BVECS:
>   		return osd_data->bvec_pos.iter.bi_size;
> +	case CEPH_OSD_DATA_TYPE_ITER:
> +		return iov_iter_count(&osd_data->iter);
>   	default:
>   		WARN(true, "unrecognized data type %d\n", (int)osd_data->type);
>   		return 0;
> @@ -954,6 +979,8 @@ static void ceph_osdc_msg_data_add(struct ceph_msg *msg,
>   #endif
>   	} else if (osd_data->type == CEPH_OSD_DATA_TYPE_BVECS) {
>   		ceph_msg_data_add_bvecs(msg, &osd_data->bvec_pos);
> +	} else if (osd_data->type == CEPH_OSD_DATA_TYPE_ITER) {
> +		ceph_msg_data_add_iter(msg, &osd_data->iter);
>   	} else {
>   		BUG_ON(osd_data->type != CEPH_OSD_DATA_TYPE_NONE);
>   	}