[PATCH V4] block: make segment size limit workable for > 4K PAGE_SIZE

[Ming Lei <ming.lei@redhat.com>]

PAGE_SIZE is applied in validating block device queue limits, this way is
very fragile and is wrong:

- queue limits are read from hardware, which is often one readonly hardware
property

- PAGE_SIZE is one config option which can be changed during build time.

In RH lab, it has been found that max segment size of some mmc card is
less than 64K, then this kind of card can't be probed successfully when
same kernel is re-built with 64K PAGE_SIZE.

Fix this issue by adding BLK_MIN_SEGMENT_SIZE and lim->min_segment_size:

- validate segment limits by BLK_MIN_SEGMENT_SIZE which is 4K(minimized PAGE_SIZE)

- checking if one bvec can be one segment quickly by lim->min_segment_size

commit 6aeb4f836480 ("block: remove bio_add_pc_page")
commit 02ee5d69e3ba ("block: remove blk_rq_bio_prep")
commit b7175e24d6ac ("block: add a dma mapping iterator")

Cc: Daniel Gomez <da.gomez@kernel.org>
Cc: Paul Bunyan <pbunyan@redhat.com>
Cc: Yi Zhang <yi.zhang@redhat.com>
Cc: Luis Chamberlain <mcgrof@kernel.org>
Cc: John Garry <john.g.garry@oracle.com>
Cc: Keith Busch <kbusch@kernel.org>
Reviewed-by: Bart Van Assche <bvanassche@acm.org>
Signed-off-by: Ming Lei <ming.lei@redhat.com>
---
V4:
	- take Daniel's suggestion to add min_segment_size limit
    for avoiding to call into split code in case that max_seg_size
    is > PAGE_SIZE

V3:
	- rephrase commit log & fix patch style(Christoph)
	- more comment log(Christoph)
V2:
	- cover bio_split_rw_at()
	- add BLK_MIN_SEGMENT_SIZE


 block/blk-merge.c      |  2 +-
 block/blk-settings.c   | 14 +++++++++++---
 block/blk.h            |  8 ++++++--
 include/linux/blkdev.h |  3 +++
 4 files changed, 21 insertions(+), 6 deletions(-)

diff --git a/block/blk-merge.c b/block/blk-merge.c
index 15cd231d560c..4fe2dfabfc9d 100644
--- a/block/blk-merge.c
+++ b/block/blk-merge.c
@@ -329,7 +329,7 @@ int bio_split_rw_at(struct bio *bio, const struct queue_limits *lim,
 
 		if (nsegs < lim->max_segments &&
 		    bytes + bv.bv_len <= max_bytes &&
-		    bv.bv_offset + bv.bv_len <= PAGE_SIZE) {
+		    bv.bv_offset + bv.bv_len <= lim->min_segment_size) {
 			nsegs++;
 			bytes += bv.bv_len;
 		} else {
diff --git a/block/blk-settings.c b/block/blk-settings.c
index c44dadc35e1e..703a9217414e 100644
--- a/block/blk-settings.c
+++ b/block/blk-settings.c
@@ -246,6 +246,7 @@ int blk_validate_limits(struct queue_limits *lim)
 {
 	unsigned int max_hw_sectors;
 	unsigned int logical_block_sectors;
+	unsigned long seg_size;
 	int err;
 
 	/*
@@ -303,7 +304,7 @@ int blk_validate_limits(struct queue_limits *lim)
 	max_hw_sectors = min_not_zero(lim->max_hw_sectors,
 				lim->max_dev_sectors);
 	if (lim->max_user_sectors) {
-		if (lim->max_user_sectors < PAGE_SIZE / SECTOR_SIZE)
+		if (lim->max_user_sectors < BLK_MIN_SEGMENT_SIZE / SECTOR_SIZE)
 			return -EINVAL;
 		lim->max_sectors = min(max_hw_sectors, lim->max_user_sectors);
 	} else if (lim->io_opt > (BLK_DEF_MAX_SECTORS_CAP << SECTOR_SHIFT)) {
@@ -341,7 +342,7 @@ int blk_validate_limits(struct queue_limits *lim)
 	 */
 	if (!lim->seg_boundary_mask)
 		lim->seg_boundary_mask = BLK_SEG_BOUNDARY_MASK;
-	if (WARN_ON_ONCE(lim->seg_boundary_mask < PAGE_SIZE - 1))
+	if (WARN_ON_ONCE(lim->seg_boundary_mask < BLK_MIN_SEGMENT_SIZE - 1))
 		return -EINVAL;
 
 	/*
@@ -362,10 +363,17 @@ int blk_validate_limits(struct queue_limits *lim)
 		 */
 		if (!lim->max_segment_size)
 			lim->max_segment_size = BLK_MAX_SEGMENT_SIZE;
-		if (WARN_ON_ONCE(lim->max_segment_size < PAGE_SIZE))
+		if (WARN_ON_ONCE(lim->max_segment_size < BLK_MIN_SEGMENT_SIZE))
 			return -EINVAL;
 	}
 
+	/* setup min segment size for building new segment in fast path */
+	if (lim->seg_boundary_mask > lim->max_segment_size - 1)
+		seg_size = lim->max_segment_size;
+	else
+		seg_size = lim->seg_boundary_mask + 1;
+	lim->min_segment_size = min_t(unsigned, seg_size, PAGE_SIZE);
+
 	/*
 	 * We require drivers to at least do logical block aligned I/O, but
 	 * historically could not check for that due to the separate calls
diff --git a/block/blk.h b/block/blk.h
index 90fa5f28ccab..57fe8261e09f 100644
--- a/block/blk.h
+++ b/block/blk.h
@@ -358,8 +358,12 @@ struct bio *bio_split_zone_append(struct bio *bio,
 static inline bool bio_may_need_split(struct bio *bio,
 		const struct queue_limits *lim)
 {
-	return lim->chunk_sectors || bio->bi_vcnt != 1 ||
-		bio->bi_io_vec->bv_len + bio->bi_io_vec->bv_offset > PAGE_SIZE;
+	if (lim->chunk_sectors)
+		return true;
+	if (bio->bi_vcnt != 1)
+		return true;
+	return bio->bi_io_vec->bv_len + bio->bi_io_vec->bv_offset >
+		lim->min_segment_size;
 }
 
 /**
diff --git a/include/linux/blkdev.h b/include/linux/blkdev.h
index 248416ecd01c..1f7d492975c1 100644
--- a/include/linux/blkdev.h
+++ b/include/linux/blkdev.h
@@ -367,6 +367,7 @@ struct queue_limits {
 	unsigned int		max_sectors;
 	unsigned int		max_user_sectors;
 	unsigned int		max_segment_size;
+	unsigned int		min_segment_size;
 	unsigned int		physical_block_size;
 	unsigned int		logical_block_size;
 	unsigned int		alignment_offset;
@@ -1163,6 +1164,8 @@ static inline bool bdev_is_partition(struct block_device *bdev)
 enum blk_default_limits {
 	BLK_MAX_SEGMENTS	= 128,
 	BLK_SAFE_MAX_SECTORS	= 255,
+	/* use minimized PAGE_SIZE as min segment size hint */
+	BLK_MIN_SEGMENT_SIZE	= 4096,
 	BLK_MAX_SEGMENT_SIZE	= 65536,
 	BLK_SEG_BOUNDARY_MASK	= 0xFFFFFFFFUL,
 };
-- 
2.47.1