[PATCH v2 0/7] no-copy bvec

@@ -1099,6 +1099,9 @@ static int __bio_iov_append_get_pages(struct bio *bio, struct iov_iter *iter)
  * fit into the bio, or are requested in @iter, whatever is smaller. If
  * MM encounters an error pinning the requested pages, it stops. Error
  * is returned only if 0 pages could be pinned.
+ *
+ * It's intended for direct IO, so doesn't do PSI tracking, the caller is
+ * responsible for setting BIO_WORKINGSET if necessary.
  */
 int bio_iov_iter_get_pages(struct bio *bio, struct iov_iter *iter)
 {

@@ -1123,6 +1126,9 @@ int bio_iov_iter_get_pages(struct bio *bio, struct iov_iter *iter)
 
 	if (is_bvec)
 		bio_set_flag(bio, BIO_NO_PAGE_REF);
+
+	/* don't account direct I/O as memory stall */
+	bio_clear_flag(bio, BIO_WORKINGSET);
 	return bio->bi_vcnt ? 0 : ret;
 }
 EXPORT_SYMBOL_GPL(bio_iov_iter_get_pages);

@@ -426,6 +426,8 @@ static inline void dio_bio_submit(struct dio *dio, struct dio_submit *sdio)
 	unsigned long flags;
 
 	bio->bi_private = dio;
+	/* don't account direct I/O as memory stall */
+	bio_clear_flag(bio, BIO_WORKINGSET);
 
 	spin_lock_irqsave(&dio->bio_lock, flags);
 	dio->refcount++;

@@ -241,6 +241,7 @@ struct target_core_file_cmd {
 	unsigned long	len;
 	struct se_cmd	*cmd;
 	struct kiocb	iocb;
+	struct bio_vec	bvecs[];
 };
 
 static void cmd_rw_aio_complete(struct kiocb *iocb, long ret, long ret2)

@@ -268,29 +269,22 @@ fd_execute_rw_aio(struct se_cmd *cmd, struct scatterlist *sgl, u32 sgl_nents,
 	struct target_core_file_cmd *aio_cmd;
 	struct iov_iter iter = {};
 	struct scatterlist *sg;
-	struct bio_vec *bvec;
 	ssize_t len = 0;
 	int ret = 0, i;
 
-	aio_cmd = kmalloc(sizeof(struct target_core_file_cmd), GFP_KERNEL);
+	aio_cmd = kmalloc(struct_size(aio_cmd, bvecs, sgl_nents), GFP_KERNEL);
 	if (!aio_cmd)
 		return TCM_LOGICAL_UNIT_COMMUNICATION_FAILURE;
 
-	bvec = kcalloc(sgl_nents, sizeof(struct bio_vec), GFP_KERNEL);
-	if (!bvec) {
-		kfree(aio_cmd);
-		return TCM_LOGICAL_UNIT_COMMUNICATION_FAILURE;
-	}
-
 	for_each_sg(sgl, sg, sgl_nents, i) {
-		bvec[i].bv_page = sg_page(sg);
-		bvec[i].bv_len = sg->length;
-		bvec[i].bv_offset = sg->offset;
+		aio_cmd->bvecs[i].bv_page = sg_page(sg);
+		aio_cmd->bvecs[i].bv_len = sg->length;
+		aio_cmd->bvecs[i].bv_offset = sg->offset;
 
 		len += sg->length;
 	}
 
-	iov_iter_bvec(&iter, is_write, bvec, sgl_nents, len);
+	iov_iter_bvec(&iter, is_write, aio_cmd->bvecs, sgl_nents, len);
 
 	aio_cmd->cmd = cmd;
 	aio_cmd->len = len;

@@ -307,8 +301,6 @@ fd_execute_rw_aio(struct se_cmd *cmd, struct scatterlist *sgl, u32 sgl_nents,
 	else
 		ret = call_read_iter(file, &aio_cmd->iocb, &iter);
 
-	kfree(bvec);
-
 	if (ret != -EIOCBQUEUED)
 		cmd_rw_aio_complete(&aio_cmd->iocb, ret, 0);
 

@@ -865,3 +865,10 @@ no matter what.  Everything is handled by the caller.
 
 clone_private_mount() returns a longterm mount now, so the proper destructor of
 its result is kern_unmount() or kern_unmount_array().
+
+---
+
+**mandatory**
+
+zero-length bvec segments are disallowed, they must be filtered out before
+passed on to an iterator.

@@ -72,8 +72,6 @@
 	__start.bi_bvec_done = skip;			\
 	__start.bi_idx = 0;				\
 	for_each_bvec(__v, i->bvec, __bi, __start) {	\
-		if (!__v.bv_len)			\
-			continue;			\
 		(void)(STEP);				\
 	}						\
 }

@@ -644,7 +644,6 @@ iter_file_splice_write(struct pipe_inode_info *pipe, struct file *out,
 		ret = splice_from_pipe_next(pipe, &sd);
 		if (ret <= 0)
 			break;
-
 		if (unlikely(nbufs < pipe->max_usage)) {
 			kfree(array);
 			nbufs = pipe->max_usage;

@@ -662,12 +661,13 @@ iter_file_splice_write(struct pipe_inode_info *pipe, struct file *out,
 
 		/* build the vector */
 		left = sd.total_len;
-		for (n = 0; !pipe_empty(head, tail) && left && n < nbufs; tail++, n++) {
+		for (n = 0; !pipe_empty(head, tail) && left && n < nbufs; tail++) {
 			struct pipe_buffer *buf = &pipe->bufs[tail & mask];
 			size_t this_len = buf->len;
 
-			if (this_len > left)
-				this_len = left;
+			if (!this_len)
+				continue;
+			this_len = min(this_len, left);
 
 			ret = pipe_buf_confirm(pipe, buf);
 			if (unlikely(ret)) {

@@ -680,6 +680,7 @@ iter_file_splice_write(struct pipe_inode_info *pipe, struct file *out,
 			array[n].bv_len = this_len;
 			array[n].bv_offset = buf->offset;
 			left -= this_len;
+			n++;
 		}
 
 		iov_iter_bvec(&from, WRITE, array, n, sd.total_len - left);

@@ -416,7 +416,7 @@ __blkdev_direct_IO(struct kiocb *iocb, struct iov_iter *iter, int nr_pages)
 		dio->size += bio->bi_iter.bi_size;
 		pos += bio->bi_iter.bi_size;
 
-		nr_pages = iov_iter_npages(iter, BIO_MAX_PAGES);
+		nr_pages = bio_iov_vecs_to_alloc(iter, BIO_MAX_PAGES);
 		if (!nr_pages) {
 			bool polled = false;
 

@@ -481,9 +481,10 @@ blkdev_direct_IO(struct kiocb *iocb, struct iov_iter *iter)
 {
 	int nr_pages;
 
-	nr_pages = iov_iter_npages(iter, BIO_MAX_PAGES + 1);
-	if (!nr_pages)
+	if (!iov_iter_count(iter))
 		return 0;
+
+	nr_pages = bio_iov_vecs_to_alloc(iter, BIO_MAX_PAGES + 1);
 	if (is_sync_kiocb(iocb) && nr_pages <= BIO_MAX_PAGES)
 		return __blkdev_direct_IO_simple(iocb, iter, nr_pages);
 

@@ -250,11 +250,8 @@ iomap_dio_bio_actor(struct inode *inode, loff_t pos, loff_t length,
 	orig_count = iov_iter_count(dio->submit.iter);
 	iov_iter_truncate(dio->submit.iter, length);
 
-	nr_pages = iov_iter_npages(dio->submit.iter, BIO_MAX_PAGES);
-	if (nr_pages <= 0) {
-		ret = nr_pages;
+	if (!iov_iter_count(dio->submit.iter))
 		goto out;
-	}
 
 	if (need_zeroout) {
 		/* zero out from the start of the block to the write offset */

@@ -263,6 +260,7 @@ iomap_dio_bio_actor(struct inode *inode, loff_t pos, loff_t length,
 			iomap_dio_zero(dio, iomap, pos - pad, pad);
 	}
 
+	nr_pages = bio_iov_vecs_to_alloc(dio->submit.iter, BIO_MAX_PAGES);
 	do {
 		size_t n;
 		if (dio->error) {

@@ -308,7 +306,8 @@ iomap_dio_bio_actor(struct inode *inode, loff_t pos, loff_t length,
 		dio->size += n;
 		copied += n;
 
-		nr_pages = iov_iter_npages(dio->submit.iter, BIO_MAX_PAGES);
+		nr_pages = bio_iov_vecs_to_alloc(dio->submit.iter,
+						 BIO_MAX_PAGES);
 		iomap_dio_submit_bio(dio, iomap, bio, pos);
 		pos += n;
 	} while (nr_pages);

@@ -10,6 +10,7 @@
 #include <linux/ioprio.h>
 /* struct bio, bio_vec and BIO_* flags are defined in blk_types.h */
 #include <linux/blk_types.h>
+#include <linux/uio.h>
 
 #define BIO_DEBUG
 

@@ -441,6 +442,15 @@ static inline void bio_wouldblock_error(struct bio *bio)
 	bio_endio(bio);
 }
 
+/*
+ * Calculate number of bvec segments that should be allocated to fit data
+ * pointed by @iter.
+ */
+static inline int bio_iov_vecs_to_alloc(struct iov_iter *iter, int max_segs)
+{
+	return iov_iter_npages(iter, max_segs);
+}
+
 struct request_queue;
 
 extern int submit_bio_wait(struct bio *bio);

@@ -1065,6 +1065,21 @@ static void pipe_advance(struct iov_iter *i, size_t size)
 	pipe_truncate(i);
 }
 
+static void iov_iter_bvec_advance(struct iov_iter *i, size_t size)
+{
+	struct bvec_iter bi;
+
+	bi.bi_size = i->count;
+	bi.bi_bvec_done = i->iov_offset;
+	bi.bi_idx = 0;
+	bvec_iter_advance(i->bvec, &bi, size);
+
+	i->bvec += bi.bi_idx;
+	i->nr_segs -= bi.bi_idx;
+	i->count = bi.bi_size;
+	i->iov_offset = bi.bi_bvec_done;
+}
+
 void iov_iter_advance(struct iov_iter *i, size_t size)
 {
 	if (unlikely(iov_iter_is_pipe(i))) {

@@ -1075,6 +1090,10 @@ void iov_iter_advance(struct iov_iter *i, size_t size)
 		i->count -= size;
 		return;
 	}
+	if (iov_iter_is_bvec(i)) {
+		iov_iter_bvec_advance(i, size);
+		return;
+	}
 	iterate_and_advance(i, size, v, 0, 0, 0)
 }
 EXPORT_SYMBOL(iov_iter_advance);

@@ -872,3 +872,12 @@ its result is kern_unmount() or kern_unmount_array().
 
 zero-length bvec segments are disallowed, they must be filtered out before
 passed on to an iterator.
+
+---
+
+**mandatory**
+
+For bvec based itererators bio_iov_iter_get_pages() now doesn't copy bvecs but
+uses the one provided. Anyone issuing kiocb-I/O should ensure that the bvec and
+page references stay until I/O has completed, i.e. until ->ki_complete() has
+been called or returned with non -EIOCBQUEUED code.

@@ -960,21 +960,17 @@ void bio_release_pages(struct bio *bio, bool mark_dirty)
 }
 EXPORT_SYMBOL_GPL(bio_release_pages);
 
-static int __bio_iov_bvec_add_pages(struct bio *bio, struct iov_iter *iter)
+static int bio_iov_bvec_set(struct bio *bio, struct iov_iter *iter)
 {
-	const struct bio_vec *bv = iter->bvec;
-	unsigned int len;
-	size_t size;
-
-	if (WARN_ON_ONCE(iter->iov_offset > bv->bv_len))
-		return -EINVAL;
-
-	len = min_t(size_t, bv->bv_len - iter->iov_offset, iter->count);
-	size = bio_add_page(bio, bv->bv_page, len,
-				bv->bv_offset + iter->iov_offset);
-	if (unlikely(size != len))
-		return -EINVAL;
-	iov_iter_advance(iter, size);
+	WARN_ON_ONCE(BVEC_POOL_IDX(bio) != 0);
+
+	bio->bi_vcnt = iter->nr_segs;
+	bio->bi_max_vecs = iter->nr_segs;
+	bio->bi_io_vec = (struct bio_vec *)iter->bvec;
+	bio->bi_iter.bi_bvec_done = iter->iov_offset;
+	bio->bi_iter.bi_size = iter->count;
+
+	iov_iter_advance(iter, iter->count);
 	return 0;
 }
 

@@ -1088,12 +1084,12 @@ static int __bio_iov_append_get_pages(struct bio *bio, struct iov_iter *iter)
  * This takes either an iterator pointing to user memory, or one pointing to
  * kernel pages (BVEC iterator). If we're adding user pages, we pin them and
  * map them into the kernel. On IO completion, the caller should put those
- * pages. If we're adding kernel pages, and the caller told us it's safe to
- * do so, we just have to add the pages to the bio directly. We don't grab an
- * extra reference to those pages (the user should already have that), and we
- * don't put the page on IO completion. The caller needs to check if the bio is
- * flagged BIO_NO_PAGE_REF on IO completion. If it isn't, then pages should be
- * released.
+ * pages. For bvec based iterators bio_iov_iter_get_pages() uses the provided
+ * bvecs rather than copying them. Hence anyone issuing kiocb based IO needs
+ * to ensure the bvecs and pages stay referenced until the submitted I/O is
+ * completed by a call to ->ki_complete() or returns with an error other than
+ * -EIOCBQUEUED. The caller needs to check if the bio is flagged BIO_NO_PAGE_REF
+ * on IO completion. If it isn't, then pages should be released.
  *
  * The function tries, but does not guarantee, to pin as many pages as
  * fit into the bio, or are requested in @iter, whatever is smaller. If

@@ -1105,27 +1101,22 @@ static int __bio_iov_append_get_pages(struct bio *bio, struct iov_iter *iter)
  */
 int bio_iov_iter_get_pages(struct bio *bio, struct iov_iter *iter)
 {
-	const bool is_bvec = iov_iter_is_bvec(iter);
-	int ret;
-
-	if (WARN_ON_ONCE(bio->bi_vcnt))
-		return -EINVAL;
+	int ret = 0;
 
-	do {
-		if (bio_op(bio) == REQ_OP_ZONE_APPEND) {
-			if (WARN_ON_ONCE(is_bvec))
-				return -EINVAL;
-			ret = __bio_iov_append_get_pages(bio, iter);
-		} else {
-			if (is_bvec)
-				ret = __bio_iov_bvec_add_pages(bio, iter);
+	if (iov_iter_is_bvec(iter)) {
+		if (WARN_ON_ONCE(bio_op(bio) == REQ_OP_ZONE_APPEND))
+			return -EINVAL;
+		bio_iov_bvec_set(bio, iter);
+		bio_set_flag(bio, BIO_NO_PAGE_REF);
+		return 0;
+	} else {
+		do {
+			if (bio_op(bio) == REQ_OP_ZONE_APPEND)
+				ret = __bio_iov_append_get_pages(bio, iter);
 			else
 				ret = __bio_iov_iter_get_pages(bio, iter);
-		}
-	} while (!ret && iov_iter_count(iter) && !bio_full(bio, 0));
-
-	if (is_bvec)
-		bio_set_flag(bio, BIO_NO_PAGE_REF);
+		} while (!ret && iov_iter_count(iter) && !bio_full(bio, 0));
+	}
 
 	/* don't account direct I/O as memory stall */
 	bio_clear_flag(bio, BIO_WORKINGSET);

@@ -444,10 +444,13 @@ static inline void bio_wouldblock_error(struct bio *bio)
 
 /*
  * Calculate number of bvec segments that should be allocated to fit data
- * pointed by @iter.
+ * pointed by @iter. If @iter is backed by bvec it's going to be reused
+ * instead of allocating a new one.
  */
 static inline int bio_iov_vecs_to_alloc(struct iov_iter *iter, int max_segs)
 {
+	if (iov_iter_is_bvec(iter))
+		return 0;
 	return iov_iter_npages(iter, max_segs);
 }