From a08e8b8b3506a9e01fdb2a3b9267187d9e3e4ec7 Mon Sep 17 00:00:00 2001
From: Chesnokov Gleb <Chesnokov.G@raidix.com>
Date: Wed, 8 Dec 2021 20:40:08 +0300
Subject: [PATCH] vdisk_fileio, async mode: Support multi-page sgv's for
 kernels < v5.1

---
 scst/include/scst.h                | 17 +++++++++++
 scst/src/dev_handlers/scst_vdisk.c | 47 ++++++++++++++++++++++++++----
 2 files changed, 59 insertions(+), 5 deletions(-)

diff --git a/scst/include/scst.h b/scst/include/scst.h
index 5cffea355..3442bf2fb 100644
--- a/scst/include/scst.h
+++ b/scst/include/scst.h
@@ -5133,6 +5133,23 @@ static inline int scst_get_buf_count(struct scst_cmd *cmd)
 	return (cmd->sg_cnt == 0) ? 1 : cmd->sg_cnt;
 }
 
+/*
+ * Returns approximate higher rounded buffers count in pages
+ */
+static inline int scst_get_buf_page_count(struct scst_cmd *cmd)
+{
+	struct scatterlist *sg;
+	int page_cnt = 0, i;
+
+	if (unlikely(cmd->sg_cnt == 0))
+		return 1;
+
+	for (i = 0, sg = cmd->sg; i < cmd->sg_cnt; i++, sg = sg_next_inline(sg))
+		page_cnt += PAGE_ALIGN(sg->offset + sg->length) >> PAGE_SHIFT;
+
+	return page_cnt;
+}
+
 /*
  * Returns approximate higher rounded buffers count that
  * scst_get_out_buf_[first|next]() return.
diff --git a/scst/src/dev_handlers/scst_vdisk.c b/scst/src/dev_handlers/scst_vdisk.c
index d7d36dc4f..4a93fea82 100644
--- a/scst/src/dev_handlers/scst_vdisk.c
+++ b/scst/src/dev_handlers/scst_vdisk.c
@@ -3175,7 +3175,11 @@ static bool vdisk_alloc_async_bvec(struct scst_cmd *cmd,
 {
 	int n;
 
+#if LINUX_VERSION_CODE < KERNEL_VERSION(5, 1, 0)
+	n = scst_get_buf_page_count(cmd);
+#else
 	n = scst_get_buf_count(cmd);
+#endif
 	if (n <= ARRAY_SIZE(p->async.small_bvec)) {
 		p->async.bvec = &p->async.small_bvec[0];
 		return true;
@@ -3191,6 +3195,42 @@ static bool vdisk_alloc_async_bvec(struct scst_cmd *cmd,
 	return true;
 }
 
+static inline
+struct bio_vec *vdisk_map_pages_to_bvec(struct bio_vec *bvec, struct page *page,
+					ssize_t length, int offset)
+{
+#if LINUX_VERSION_CODE < KERNEL_VERSION(5, 1, 0)
+	ssize_t page_len = min_t(ssize_t, length, PAGE_SIZE - offset);
+#else
+	ssize_t page_len = length;
+#endif
+
+	*bvec++ = (struct bio_vec) {
+		.bv_page   = page,
+		.bv_offset = offset,
+		.bv_len    = page_len,
+	};
+
+#if LINUX_VERSION_CODE < KERNEL_VERSION(5, 1, 0)
+	length -= page_len;
+
+	while (length > 0) {
+		page++;
+		page_len = min_t(ssize_t, length, PAGE_SIZE);
+
+		*bvec++ = (struct bio_vec) {
+			.bv_page   = page,
+			.bv_offset = 0,
+			.bv_len    = page_len,
+		};
+
+		length -= page_len;
+	}
+#endif
+
+	return bvec;
+}
+
 static void fileio_async_complete(struct kiocb *iocb, long ret
 #if LINUX_VERSION_CODE < KERNEL_VERSION(5, 16, 0)
 				  , long ret2
@@ -3262,11 +3302,8 @@ static enum compl_status_e fileio_exec_async(struct vdisk_cmd_params *p)
 	bvec = p->async.bvec;
 	length = scst_get_sg_page_first(cmd, &page, &offset);
 	while (length) {
-		*bvec++ = (struct bio_vec){
-			.bv_page   = page,
-			.bv_offset = offset,
-			.bv_len    = length,
-		};
+		bvec = vdisk_map_pages_to_bvec(bvec, page, length, offset);
+
 		total += length;
 		sg_cnt++;
 		length = scst_get_sg_page_next(cmd, &page, &offset);