[PATCH v5 0/6] block/md/dm: set chunk_sectors from stacked dev stripe size

[PATCH v5 0/6] block/md/dm: set chunk_sectors from stacked dev stripe size

[John Garry]

This value in io_min is used to configure any atomic write limit for the
stacked device. The idea is that the atomic write unit max is a
power-of-2 factor of the stripe size, and the stripe size is available
in io_min.

Using io_min causes issues, as:
a. it may be mutated
b. the check for io_min being set for determining if we are dealing with
a striped device is hard to get right, as reported in [0].

This series now sets chunk_sectors limit to share stripe size.

[0] https://lore.kernel.org/linux-block/888f3b1d-7817-4007-b3b3-1a2ea04df771@linux.ibm.com/T/#mecca17129f72811137d3c2f1e477634e77f06781

Based on 73d9cb37478f (block/for-6.17/block) block: remove pktcdvd driver

This series fixes issues for v6.16, but it's prob better to have this in
v6.17 at this stage.

Differences to v4:
- Use check_shl_overflow() (Nilay)
- Use long long in for chunk bytes in 2/6
- Add tags from Nilay (thanks!)

Differences to v3:
- relocate max_pow_of_two_factor() to common header and rework (Mikulas)
- cater for overflow from chunk sectors (Mikulas)

John Garry (6):
  ilog2: add max_pow_of_two_factor()
  block: sanitize chunk_sectors for atomic write limits
  md/raid0: set chunk_sectors limit
  md/raid10: set chunk_sectors limit
  dm-stripe: limit chunk_sectors to the stripe size
  block: use chunk_sectors when evaluating stacked atomic write limits

 block/blk-settings.c   | 64 +++++++++++++++++++++++++++---------------
 drivers/md/dm-stripe.c |  1 +
 drivers/md/raid0.c     |  1 +
 drivers/md/raid10.c    |  1 +
 fs/xfs/xfs_mount.c     |  5 ----
 include/linux/log2.h   | 14 +++++++++
 6 files changed, 58 insertions(+), 28 deletions(-)

-- 
2.43.5

[PATCH v5 1/6] ilog2: add max_pow_of_two_factor()

[John Garry]

Relocate the function max_pow_of_two_factor() to common ilog2.h from the
xfs code, as it will be used elsewhere.

Also simplify the function, as advised by Mikulas Patocka.

Signed-off-by: John Garry <john.g.garry@oracle.com>
---
 fs/xfs/xfs_mount.c   |  5 -----
 include/linux/log2.h | 14 ++++++++++++++
 2 files changed, 14 insertions(+), 5 deletions(-)

diff --git a/fs/xfs/xfs_mount.c b/fs/xfs/xfs_mount.c
index 29276fe60df9..6c669ae082d4 100644
--- a/fs/xfs/xfs_mount.c
+++ b/fs/xfs/xfs_mount.c
@@ -672,11 +672,6 @@ static inline xfs_extlen_t xfs_calc_atomic_write_max(struct xfs_mount *mp)
 	return rounddown_pow_of_two(XFS_B_TO_FSB(mp, MAX_RW_COUNT));
 }
 
-static inline unsigned int max_pow_of_two_factor(const unsigned int nr)
-{
-	return 1 << (ffs(nr) - 1);
-}
-
 /*
  * If the data device advertises atomic write support, limit the size of data
  * device atomic writes to the greatest power-of-two factor of the AG size so
diff --git a/include/linux/log2.h b/include/linux/log2.h
index 1366cb688a6d..2eac3fc9303d 100644
--- a/include/linux/log2.h
+++ b/include/linux/log2.h
@@ -255,4 +255,18 @@ int __bits_per(unsigned long n)
 	) :					\
 	__bits_per(n)				\
 )
+
+/**
+ * max_pow_of_two_factor - return highest power-of-2 factor
+ * @n: parameter
+ *
+ * find highest power-of-2 which is evenly divisible into n.
+ * 0 is returned for n == 0 or 1.
+ */
+static inline __attribute__((const))
+unsigned int max_pow_of_two_factor(unsigned int n)
+{
+	return n & -n;
+}
+
 #endif /* _LINUX_LOG2_H */
-- 
2.43.5

[PATCH v5 2/6] block: sanitize chunk_sectors for atomic write limits

[John Garry]

Currently we just ensure that a non-zero value in chunk_sectors aligns
with any atomic write boundary, as the blk boundary functionality uses
both these values.

However it is also improper to have atomic write unit max > chunk_sectors
(for non-zero chunk_sectors), as this would lead to splitting of atomic
write bios (which is disallowed).

Sanitize atomic write unit max against chunk_sectors to avoid any
potential problems.

Fixes: d00eea91deaf3 ("block: Add extra checks in blk_validate_atomic_write_limits()")
Reviewed-by: Nilay Shroff <nilay@linux.ibm.com>
Signed-off-by: John Garry <john.g.garry@oracle.com>
---
 block/blk-settings.c | 8 ++++++++
 1 file changed, 8 insertions(+)

diff --git a/block/blk-settings.c b/block/blk-settings.c
index a000daafbfb4..725035376f51 100644
--- a/block/blk-settings.c
+++ b/block/blk-settings.c
@@ -180,6 +180,7 @@ static void blk_atomic_writes_update_limits(struct queue_limits *lim)
 
 static void blk_validate_atomic_write_limits(struct queue_limits *lim)
 {
+	unsigned long long chunk_bytes;
 	unsigned int boundary_sectors;
 
 	if (!(lim->features & BLK_FEAT_ATOMIC_WRITES))
@@ -202,6 +203,13 @@ static void blk_validate_atomic_write_limits(struct queue_limits *lim)
 			 lim->atomic_write_hw_max))
 		goto unsupported;
 
+	chunk_bytes = lim->chunk_sectors << SECTOR_SHIFT;
+	if (chunk_bytes) {
+		if (WARN_ON_ONCE(lim->atomic_write_hw_unit_max >
+			chunk_bytes))
+			goto unsupported;
+	}
+
 	boundary_sectors = lim->atomic_write_hw_boundary >> SECTOR_SHIFT;
 
 	if (boundary_sectors) {
-- 
2.43.5

[PATCH v5 3/6] md/raid0: set chunk_sectors limit

[John Garry]

Currently we use min io size as the chunk size when deciding on the
atomic write size limits - see blk_stack_atomic_writes_head().

The limit min_io size is not a reliable value to store the chunk size, as
this may be mutated by the block stacking code. Such an example would be
for the min io size less than the physical block size, and the min io size
is raised to the physical block size - see blk_stack_limits().

The block stacking limits will rely on chunk_sectors in future,
so set this value (to the chunk size).

Reviewed-by: Nilay Shroff <nilay@linux.ibm.com>
Reviewed-by: Yu Kuai <yukuai3@huawei.com>
Signed-off-by: John Garry <john.g.garry@oracle.com>
---
 drivers/md/raid0.c | 1 +
 1 file changed, 1 insertion(+)

diff --git a/drivers/md/raid0.c b/drivers/md/raid0.c
index d8f639f4ae12..cbe2a9054cb9 100644
--- a/drivers/md/raid0.c
+++ b/drivers/md/raid0.c
@@ -384,6 +384,7 @@ static int raid0_set_limits(struct mddev *mddev)
 	lim.max_write_zeroes_sectors = mddev->chunk_sectors;
 	lim.io_min = mddev->chunk_sectors << 9;
 	lim.io_opt = lim.io_min * mddev->raid_disks;
+	lim.chunk_sectors = mddev->chunk_sectors;
 	lim.features |= BLK_FEAT_ATOMIC_WRITES;
 	err = mddev_stack_rdev_limits(mddev, &lim, MDDEV_STACK_INTEGRITY);
 	if (err)
-- 
2.43.5

[PATCH v5 4/6] md/raid10: set chunk_sectors limit

[John Garry]

Same as done for raid0, set chunk_sectors limit to appropriately set the
atomic write size limit.

Reviewed-by: Nilay Shroff <nilay@linux.ibm.com>
Reviewed-by: Yu Kuai <yukuai3@huawei.com>
Signed-off-by: John Garry <john.g.garry@oracle.com>
---
 drivers/md/raid10.c | 1 +
 1 file changed, 1 insertion(+)

diff --git a/drivers/md/raid10.c b/drivers/md/raid10.c
index b74780af4c22..97065bb26f43 100644
--- a/drivers/md/raid10.c
+++ b/drivers/md/raid10.c
@@ -4004,6 +4004,7 @@ static int raid10_set_queue_limits(struct mddev *mddev)
 	md_init_stacking_limits(&lim);
 	lim.max_write_zeroes_sectors = 0;
 	lim.io_min = mddev->chunk_sectors << 9;
+	lim.chunk_sectors = mddev->chunk_sectors;
 	lim.io_opt = lim.io_min * raid10_nr_stripes(conf);
 	lim.features |= BLK_FEAT_ATOMIC_WRITES;
 	err = mddev_stack_rdev_limits(mddev, &lim, MDDEV_STACK_INTEGRITY);
-- 
2.43.5

[PATCH v5 5/6] dm-stripe: limit chunk_sectors to the stripe size

[John Garry]

Same as done for raid0, set chunk_sectors limit to appropriately set the
atomic write size limit.

Setting chunk_sectors limit in this way overrides the stacked limit
already calculated based on the bottom device limits. This is ok, as
when any bios are sent to the bottom devices, the block layer will still
respect the bottom device chunk_sectors.

Reviewed-by: Nilay Shroff <nilay@linux.ibm.com>
Reviewed-by: Mikulas Patocka <mpatocka@redhat.com>
Signed-off-by: John Garry <john.g.garry@oracle.com>
---
 drivers/md/dm-stripe.c | 1 +
 1 file changed, 1 insertion(+)

diff --git a/drivers/md/dm-stripe.c b/drivers/md/dm-stripe.c
index a7dc04bd55e5..5bbbdf8fc1bd 100644
--- a/drivers/md/dm-stripe.c
+++ b/drivers/md/dm-stripe.c
@@ -458,6 +458,7 @@ static void stripe_io_hints(struct dm_target *ti,
 	struct stripe_c *sc = ti->private;
 	unsigned int chunk_size = sc->chunk_size << SECTOR_SHIFT;
 
+	limits->chunk_sectors = sc->chunk_size;
 	limits->io_min = chunk_size;
 	limits->io_opt = chunk_size * sc->stripes;
 }
-- 
2.43.5

[PATCH v5 6/6] block: use chunk_sectors when evaluating stacked atomic write limits

[John Garry]

The atomic write unit max value is limited by any stacked device stripe
size.

It is required that the atomic write unit is a power-of-2 factor of the
stripe size.

Currently we use io_min limit to hold the stripe size, and check for a
io_min <= SECTOR_SIZE when deciding if we have a striped stacked device.

Nilay reports that this causes a problem when the physical block size is
greater than SECTOR_SIZE [0].

Furthermore, io_min may be mutated when stacking devices, and this makes
it a poor candidate to hold the stripe size. Such an example (of when
io_min may change) would be when the io_min is less than the physical
block size.

Use chunk_sectors to hold the stripe size, which is more appropriate.

[0] https://lore.kernel.org/linux-block/888f3b1d-7817-4007-b3b3-1a2ea04df771@linux.ibm.com/T/#mecca17129f72811137d3c2f1e477634e77f06781

Reviewed-by: Nilay Shroff <nilay@linux.ibm.com>
Tested-by: Nilay Shroff <nilay@linux.ibm.com>
Signed-off-by: John Garry <john.g.garry@oracle.com>
---
 block/blk-settings.c | 56 ++++++++++++++++++++++++++------------------
 1 file changed, 33 insertions(+), 23 deletions(-)

diff --git a/block/blk-settings.c b/block/blk-settings.c
index 725035376f51..2dffd8bd72f0 100644
--- a/block/blk-settings.c
+++ b/block/blk-settings.c
@@ -597,41 +597,50 @@ static bool blk_stack_atomic_writes_boundary_head(struct queue_limits *t,
 	return true;
 }
 
-
-/* Check stacking of first bottom device */
-static bool blk_stack_atomic_writes_head(struct queue_limits *t,
-				struct queue_limits *b)
+static void blk_stack_atomic_writes_chunk_sectors(struct queue_limits *t)
 {
-	if (b->atomic_write_hw_boundary &&
-	    !blk_stack_atomic_writes_boundary_head(t, b))
-		return false;
+	unsigned int chunk_bytes;
 
-	if (t->io_min <= SECTOR_SIZE) {
-		/* No chunk sectors, so use bottom device values directly */
-		t->atomic_write_hw_unit_max = b->atomic_write_hw_unit_max;
-		t->atomic_write_hw_unit_min = b->atomic_write_hw_unit_min;
-		t->atomic_write_hw_max = b->atomic_write_hw_max;
-		return true;
-	}
+	if (!t->chunk_sectors)
+		return;
+
+	/*
+	 * If chunk sectors is so large that its value in bytes overflows
+	 * UINT_MAX, then just shift it down so it definitely will fit.
+	 * We don't support atomic writes of such a large size anyway.
+	 */
+	if (check_shl_overflow(t->chunk_sectors, SECTOR_SHIFT, &chunk_bytes))
+		chunk_bytes = t->chunk_sectors;
 
 	/*
 	 * Find values for limits which work for chunk size.
 	 * b->atomic_write_hw_unit_{min, max} may not be aligned with chunk
-	 * size (t->io_min), as chunk size is not restricted to a power-of-2.
+	 * size, as the chunk size is not restricted to a power-of-2.
 	 * So we need to find highest power-of-2 which works for the chunk
 	 * size.
-	 * As an example scenario, we could have b->unit_max = 16K and
-	 * t->io_min = 24K. For this case, reduce t->unit_max to a value
-	 * aligned with both limits, i.e. 8K in this example.
+	 * As an example scenario, we could have t->unit_max = 16K and
+	 * t->chunk_sectors = 24KB. For this case, reduce t->unit_max to a
+	 * value aligned with both limits, i.e. 8K in this example.
 	 */
-	t->atomic_write_hw_unit_max = b->atomic_write_hw_unit_max;
-	while (t->io_min % t->atomic_write_hw_unit_max)
-		t->atomic_write_hw_unit_max /= 2;
+	t->atomic_write_hw_unit_max = min(t->atomic_write_hw_unit_max,
+					max_pow_of_two_factor(chunk_bytes));
 
-	t->atomic_write_hw_unit_min = min(b->atomic_write_hw_unit_min,
+	t->atomic_write_hw_unit_min = min(t->atomic_write_hw_unit_min,
 					  t->atomic_write_hw_unit_max);
-	t->atomic_write_hw_max = min(b->atomic_write_hw_max, t->io_min);
+	t->atomic_write_hw_max = min(t->atomic_write_hw_max, chunk_bytes);
+}
+
+/* Check stacking of first bottom device */
+static bool blk_stack_atomic_writes_head(struct queue_limits *t,
+				struct queue_limits *b)
+{
+	if (b->atomic_write_hw_boundary &&
+	    !blk_stack_atomic_writes_boundary_head(t, b))
+		return false;
 
+	t->atomic_write_hw_unit_max = b->atomic_write_hw_unit_max;
+	t->atomic_write_hw_unit_min = b->atomic_write_hw_unit_min;
+	t->atomic_write_hw_max = b->atomic_write_hw_max;
 	return true;
 }
 
@@ -659,6 +668,7 @@ static void blk_stack_atomic_writes_limits(struct queue_limits *t,
 
 	if (!blk_stack_atomic_writes_head(t, b))
 		goto unsupported;
+	blk_stack_atomic_writes_chunk_sectors(t);
 	return;
 
 unsupported:
-- 
2.43.5

Re: [PATCH v5 5/6] dm-stripe: limit chunk_sectors to the stripe size

[Mikulas Patocka]

On Wed, 9 Jul 2025, John Garry wrote:

> Same as done for raid0, set chunk_sectors limit to appropriately set the
> atomic write size limit.
> 
> Setting chunk_sectors limit in this way overrides the stacked limit
> already calculated based on the bottom device limits. This is ok, as
> when any bios are sent to the bottom devices, the block layer will still
> respect the bottom device chunk_sectors.
> 
> Reviewed-by: Nilay Shroff <nilay@linux.ibm.com>
> Reviewed-by: Mikulas Patocka <mpatocka@redhat.com>
> Signed-off-by: John Garry <john.g.garry@oracle.com>
> ---
>  drivers/md/dm-stripe.c | 1 +
>  1 file changed, 1 insertion(+)
> 
> diff --git a/drivers/md/dm-stripe.c b/drivers/md/dm-stripe.c
> index a7dc04bd55e5..5bbbdf8fc1bd 100644
> --- a/drivers/md/dm-stripe.c
> +++ b/drivers/md/dm-stripe.c
> @@ -458,6 +458,7 @@ static void stripe_io_hints(struct dm_target *ti,
>  	struct stripe_c *sc = ti->private;
>  	unsigned int chunk_size = sc->chunk_size << SECTOR_SHIFT;
>  
> +	limits->chunk_sectors = sc->chunk_size;
>  	limits->io_min = chunk_size;
>  	limits->io_opt = chunk_size * sc->stripes;
>  }
> -- 
> 2.43.5

Hi

This will conflict with the current dm code in linux-dm.git. Should I fix 
up the conflict and commit it through the linux-dm git?

Mikulas

Re: [PATCH v5 2/6] block: sanitize chunk_sectors for atomic write limits

[Jens Axboe]

On 7/9/25 4:02 AM, John Garry wrote:
> Currently we just ensure that a non-zero value in chunk_sectors aligns
> with any atomic write boundary, as the blk boundary functionality uses
> both these values.
> 
> However it is also improper to have atomic write unit max > chunk_sectors
> (for non-zero chunk_sectors), as this would lead to splitting of atomic
> write bios (which is disallowed).
> 
> Sanitize atomic write unit max against chunk_sectors to avoid any
> potential problems.
> 
> Fixes: d00eea91deaf3 ("block: Add extra checks in blk_validate_atomic_write_limits()")
> Reviewed-by: Nilay Shroff <nilay@linux.ibm.com>
> Signed-off-by: John Garry <john.g.garry@oracle.com>
> ---
>  block/blk-settings.c | 8 ++++++++
>  1 file changed, 8 insertions(+)
> 
> diff --git a/block/blk-settings.c b/block/blk-settings.c
> index a000daafbfb4..725035376f51 100644
> --- a/block/blk-settings.c
> +++ b/block/blk-settings.c
> @@ -180,6 +180,7 @@ static void blk_atomic_writes_update_limits(struct queue_limits *lim)
>  
>  static void blk_validate_atomic_write_limits(struct queue_limits *lim)
>  {
> +	unsigned long long chunk_bytes;
>  	unsigned int boundary_sectors;
>  
>  	if (!(lim->features & BLK_FEAT_ATOMIC_WRITES))
> @@ -202,6 +203,13 @@ static void blk_validate_atomic_write_limits(struct queue_limits *lim)
>  			 lim->atomic_write_hw_max))
>  		goto unsupported;
>  
> +	chunk_bytes = lim->chunk_sectors << SECTOR_SHIFT;
> +	if (chunk_bytes) {
> +		if (WARN_ON_ONCE(lim->atomic_write_hw_unit_max >
> +			chunk_bytes))
> +			goto unsupported;
> +	}

Unnecessary indentation here. Why not just:

	chunk_bytes = lim->chunk_sectors << SECTOR_SHIFT;
	if (WARN_ON_ONCE(chunk_bytes &&
			 lim->atomic_write_hw_unit_max > chunk_bytes))
		goto unsupposed.

Also avoids splitting a comparison over multiple lines, which is always
annoying to read.

-- 
Jens Axboe

Re: [PATCH v5 2/6] block: sanitize chunk_sectors for atomic write limits

[John Garry]

On 10/07/2025 16:08, Jens Axboe wrote:
>> +	chunk_bytes = lim->chunk_sectors << SECTOR_SHIFT;
>> +	if (chunk_bytes) {
>> +		if (WARN_ON_ONCE(lim->atomic_write_hw_unit_max >
>> +			chunk_bytes))
>> +			goto unsupported;
>> +	}
> Unnecessary indentation here. Why not just:
> 
> 	chunk_bytes = lim->chunk_sectors << SECTOR_SHIFT;
> 	if (WARN_ON_ONCE(chunk_bytes &&
> 			 lim->atomic_write_hw_unit_max > chunk_bytes))
> 		goto unsupposed.
> 
> Also avoids splitting a comparison over multiple lines, which is always
> annoying to read.

ok, I can tidy that up.

Thanks,
John

Re: [PATCH v5 5/6] dm-stripe: limit chunk_sectors to the stripe size

[John Garry]

On 10/07/2025 16:03, Mikulas Patocka wrote:
> 
> 
> On Wed, 9 Jul 2025, John Garry wrote:
> 
>> Same as done for raid0, set chunk_sectors limit to appropriately set the
>> atomic write size limit.
>>
>> Setting chunk_sectors limit in this way overrides the stacked limit
>> already calculated based on the bottom device limits. This is ok, as
>> when any bios are sent to the bottom devices, the block layer will still
>> respect the bottom device chunk_sectors.
>>
>> Reviewed-by: Nilay Shroff <nilay@linux.ibm.com>
>> Reviewed-by: Mikulas Patocka <mpatocka@redhat.com>
>> Signed-off-by: John Garry <john.g.garry@oracle.com>
>> ---
>>   drivers/md/dm-stripe.c | 1 +
>>   1 file changed, 1 insertion(+)
>>
>> diff --git a/drivers/md/dm-stripe.c b/drivers/md/dm-stripe.c
>> index a7dc04bd55e5..5bbbdf8fc1bd 100644
>> --- a/drivers/md/dm-stripe.c
>> +++ b/drivers/md/dm-stripe.c
>> @@ -458,6 +458,7 @@ static void stripe_io_hints(struct dm_target *ti,
>>   	struct stripe_c *sc = ti->private;
>>   	unsigned int chunk_size = sc->chunk_size << SECTOR_SHIFT;
>>   
>> +	limits->chunk_sectors = sc->chunk_size;
>>   	limits->io_min = chunk_size;
>>   	limits->io_opt = chunk_size * sc->stripes;
>>   }
>> -- 
>> 2.43.5
> 
> Hi
> 
> This will conflict with the current dm code in linux-dm.git. Should I fix
> up the conflict and commit it through the linux-dm git?

I was hoping that Jens would take this series through the block tree, so 
I will let him comment.

But I think that taking this patch separately though linux-dm would 
create an intermediate breakage for atomics functionality for dm-stripe 
on the block tree. Not many are using it yet, so not an utterly terrible 
a way to go.

Thanks,
John