target: Fix task count > 1 handling breakage and use max_sector page alignment

This patch addresses recent breakage with multiple se_task (task_count > 1)
operation following backend dev->se_sub_dev->se_dev_attrib.max_sectors in new
transport_allocate_data_tasks() code.  The initial bug here was a bogus
task->task_sg_nents assignment in transport_allocate_data_tasks() based on
the passed parameter, which now uses DIV_ROUND_UP(task_size, PAGE_SIZE) to
determine the proper number of per task SGL entries for the (task_count > 1)
case.

This also means we now need to enforce a PAGE_SIZE aligned max_sector count
value for this to work as expected without bringing back the pre v3.1
transport_map_mem_to_sg() logic to handle SGL offsets across multiple tasks.
So this patch adds se_dev_align_max_sectors() to round down max_sectors as
necessary to ensure this alignment via se_dev_set_default_attribs() and
se_dev_align_max_sectors() and keeps it simple for (task_count > 1)
operation.

So far this bugfix has been tested with (task_count > 1) operation
using iscsi-target and iblock backends.

Reported-by: Chris Boot <bootc@bootc.net>
Cc: Kiran Patil <kiran.patil@intel.com>
Cc: Andy Grover <agrover@redhat.com>
Cc: Christoph Hellwig <hch@lst.de>
Signed-off-by: Nicholas Bellinger <nab@linux-iscsi.org>

1 files changed, 5 insertions, 2 deletions

diff --git a/drivers/target/target_core_transport.c b/drivers/target/target_core_transport.c
index ab61c5550852..efac6a9a7438 100644
--- a/drivers/target/target_core_transport.c
+++ b/drivers/target/target_core_transport.c
@@ -4126,7 +4126,11 @@ static int transport_allocate_data_tasks(
 
 		/* Update new cdb with updated lba/sectors */
 		cmd->transport_split_cdb(task->task_lba, task->task_sectors, cdb);
-
+		/*
+		 * This now assumes that passed sg_ents are in PAGE_SIZE chunks
+		 * in order to calculate the number per task SGL entries
+		 */
+		task->task_sg_nents = DIV_ROUND_UP(task->task_size, PAGE_SIZE);
 		/*
 		 * Check if the fabric module driver is requesting that all
 		 * struct se_task->task_sg[] be chained together..  If so,
@@ -4136,7 +4140,6 @@ static int transport_allocate_data_tasks(
 		 * It's so much easier and only a waste when task_count > 1.
 		 * That is extremely rare.
 		 */
-		task->task_sg_nents = sgl_nents;
 		if (cmd->se_tfo->task_sg_chaining) {
 			task->task_sg_nents++;
 			task->task_padded_sg = 1;