diff mbox series

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

Message ID 20250219024409.901186-1-ming.lei@redhat.com (mailing list archive)
State New
Headers show
Series [V4] block: make segment size limit workable for > 4K PAGE_SIZE | expand

Commit Message

Ming Lei Feb. 19, 2025, 2:44 a.m. UTC 
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(-)

Comments

Christoph Hellwig Feb. 20, 2025, 6:13 a.m. UTC | #1 
On Wed, Feb 19, 2025 at 10:44:09AM +0800, Ming Lei wrote:
> PAGE_SIZE is applied in validating block device queue limits, this way is
> very fragile and is wrong:

It's neither very fragily nor wrong.  If you want to change it to suit
your needs that might or might not be ok but this language isn't.

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

queues limits aren't read from hardware per definition.  Very often they
are software limits.
Ming Lei Feb. 20, 2025, 11:38 a.m. UTC | #2 
On Wed, Feb 19, 2025 at 10:13:24PM -0800, Christoph Hellwig wrote:
> On Wed, Feb 19, 2025 at 10:44:09AM +0800, Ming Lei wrote:
> > PAGE_SIZE is applied in validating block device queue limits, this way is
> > very fragile and is wrong:
> 
> It's neither very fragily nor wrong.  If you want to change it to suit
> your needs that might or might not be ok but this language isn't.

max segment size is fixed value since it is read from hardware, now
kernel validates it by variable PAGE_SIZE, not fragile?

Well, it isn't my need only, there are lots of device which max segment
size is < 64K, they work just fine in 4K page size kernel, however,
they become unusable in 64K page size kernel.

> 
> > - queue limits are read from hardware, which is often one readonly hardware
> > property
> 
> queues limits aren't read from hardware per definition.  Very often they
> are software limits.

Fine, I will update the commit log to just mention max segment size limit.


Thanks,
Ming
diff mbox series

Patch

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,
 };