Re: [PATCH v2] nvmet: Make blksize_shift configurable

[Richard Weinberger <richard@sigma-star.at>]

On Dienstag, 1. Juli 2025 02:34 'Damien Le Moal' via upstream wrote:
> On 7/1/25 4:13 AM, Richard Weinberger wrote:
> > Currently, the block size is automatically configured, and for
> > file-backed namespaces it is likely to be 4K.
> > While this is a reasonable default for modern storage, it can
> > cause confusion if someone wants to export a pre-created disk image
> > that uses a 512-byte block size.
> > As a result, partition parsing will fail.
> > 
> > So, just like we already do for the loop block device, let the user
> > configure the block size if they know better.
> 
> Hmm... That fine with me but this explanation does not match what the patch
> does: you allow configuring the block size bit shift, not the size. That is not
> super user friendly :)
> 
> Even if internally you use the block size bit shift, I think it would be better
> if the user facing interface is the block size as that is much easier to
> manipulate without having to remember the exponent for powers of 2 values :)

The initial intention of this patch was exposing the blksize_shift property.
If we want to expose this as more user friendly, I'm fine with it.
Maybe "minimum_io_size"?

> 
> > 
> > Signed-off-by: Richard Weinberger <richard@nod.at>
> > ---
> > Changes since v1 (RFC)[0]:
> > 
> > - Make sure blksize_shift is in general within reason
> > - In the bdev case and when using direct IO, blksize_shift has to be
> >   smaller than the logical block it the device
> > - In the file case and when using direct IO try to use STATX_DIOALIGN,
> >   just like the loop device does
> > 
> > [0] https://lore.kernel.org/linux-nvme/20250418090834.2755289-1-richard@nod.at/
> > 
> > Thanks,
> > //richard
> > ---
> >  drivers/nvme/target/configfs.c    | 37 +++++++++++++++++++++++++++++++
> >  drivers/nvme/target/io-cmd-bdev.c | 13 ++++++++++-
> >  drivers/nvme/target/io-cmd-file.c | 28 ++++++++++++++++++-----
> >  3 files changed, 71 insertions(+), 7 deletions(-)
> > 
> > diff --git a/drivers/nvme/target/configfs.c b/drivers/nvme/target/configfs.c
> > index e44ef69dffc24..26175c37374ab 100644
> > --- a/drivers/nvme/target/configfs.c
> > +++ b/drivers/nvme/target/configfs.c
> > @@ -797,6 +797,42 @@ static ssize_t nvmet_ns_resv_enable_store(struct config_item *item,
> >  }
> >  CONFIGFS_ATTR(nvmet_ns_, resv_enable);
> >  
> > +static ssize_t nvmet_ns_blksize_shift_show(struct config_item *item, char *page)
> 
> As mentioned above, I think this should be nvmet_ns_blksize_show().
> 
> > +{
> > +	return sysfs_emit(page, "%u\n", to_nvmet_ns(item)->blksize_shift);
> 
> And you can do:
> 
> 	return sysfs_emit(page, "%u\n",
> 			  1U << to_nvmet_ns(item)->blksize_shift);
> 
> > +}
> > +
> > +static ssize_t nvmet_ns_blksize_shift_store(struct config_item *item,
> > +		const char *page, size_t count)
> 
> Similar here: nvmet_ns_blksize_store()
> 
> > +{
> > +	struct nvmet_ns *ns = to_nvmet_ns(item);
> > +	u32 shift;
> > +	int ret;
> > +
> > +	ret = kstrtou32(page, 0, &shift);
> > +	if (ret)
> > +		return ret;
> > +
> > +	/*
> > +	 * Make sure block size is within reason, something between 512 and
> > +	 * BLK_MAX_BLOCK_SIZE.
> > +	 */
> > +	if (shift < 9 || shift > 16)
> > +		return -EINVAL;
> 
> And this would be simpler:
> 
> 	if (blksz < SECTOR_SIZE || blksz > BLK_MAX_BLOCK_SIZE ||
> 	    !is_power_of_2(blksz))
> 		return -EINVAL;
> 
> > +
> > +	mutex_lock(&ns->subsys->lock);
> > +	if (ns->enabled) {
> > +		pr_err("the ns:%d is already enabled.\n", ns->nsid);
> > +		mutex_unlock(&ns->subsys->lock);
> > +		return -EINVAL;
> > +	}
> > +	ns->blksize_shift = shift;
> 
> and here:
> 
> 	ns->blksize_shift = ilog2(blksz);
> 
> > +	mutex_unlock(&ns->subsys->lock);
> > +
> > +	return count;
> > +}
> > +CONFIGFS_ATTR(nvmet_ns_, blksize_shift);
> > +
> >  static struct configfs_attribute *nvmet_ns_attrs[] = {
> >  	&nvmet_ns_attr_device_path,
> >  	&nvmet_ns_attr_device_nguid,
> > @@ -806,6 +842,7 @@ static struct configfs_attribute *nvmet_ns_attrs[] = {
> >  	&nvmet_ns_attr_buffered_io,
> >  	&nvmet_ns_attr_revalidate_size,
> >  	&nvmet_ns_attr_resv_enable,
> > +	&nvmet_ns_attr_blksize_shift,
> >  #ifdef CONFIG_PCI_P2PDMA
> >  	&nvmet_ns_attr_p2pmem,
> >  #endif
> > diff --git a/drivers/nvme/target/io-cmd-bdev.c b/drivers/nvme/target/io-cmd-bdev.c
> > index eba42df2f8215..be39837d4d792 100644
> > --- a/drivers/nvme/target/io-cmd-bdev.c
> > +++ b/drivers/nvme/target/io-cmd-bdev.c
> > @@ -77,6 +77,7 @@ static void nvmet_bdev_ns_enable_integrity(struct nvmet_ns *ns)
> >  
> >  int nvmet_bdev_ns_enable(struct nvmet_ns *ns)
> >  {
> > +	int bdev_blksize_shift;
> >  	int ret;
> >  
> >  	/*
> > @@ -100,7 +101,17 @@ int nvmet_bdev_ns_enable(struct nvmet_ns *ns)
> >  	}
> >  	ns->bdev = file_bdev(ns->bdev_file);
> >  	ns->size = bdev_nr_bytes(ns->bdev);
> > -	ns->blksize_shift = blksize_bits(bdev_logical_block_size(ns->bdev));
> > +	bdev_blksize_shift = blksize_bits(bdev_logical_block_size(ns->bdev));
> > +
> > +	if (ns->blksize_shift) {
> > +		if (ns->blksize_shift < bdev_blksize_shift) {
> > +			pr_err("Configured blksize_shift needs to be at least %d for device %s\n",
> > +				bdev_blksize_shift, ns->device_path);
> > +			return -EINVAL;
> > +		}
> > +	} else {
> > +		ns->blksize_shift = bdev_blksize_shift;
> > +	}
> 
> Nit: to avoid the indented if, may be write this like this: ?
> 
> 	if (!ns->blksize_shift)
> 		ns->blksize_shift = bdev_blksize_shift;
> 
> 	if (ns->blksize_shift < bdev_blksize_shift) {
> 		pr_err("Configured blksize needs to be at least %u for device %s\n",
> 			bdev_logical_block_size(ns->bdev),
> 			ns->device_path);
> 		return -EINVAL;
> 	}

It's a matter of taste, but yes...

> Also, if the backend is an HDD, do we want to allow the user to configure a
> block size that is less than the *physical* block size ? Performance will
> suffer on regular HDDs and writes may fail with SMR HDDs.

I'm not sure whether it's worth putting more smartness into this logic.
 
> >  
> >  	ns->pi_type = 0;
> >  	ns->metadata_size = 0;
> > diff --git a/drivers/nvme/target/io-cmd-file.c b/drivers/nvme/target/io-cmd-file.c
> > index 2d068439b129c..a4066b5a1dc97 100644
> > --- a/drivers/nvme/target/io-cmd-file.c
> > +++ b/drivers/nvme/target/io-cmd-file.c
> > @@ -49,12 +49,28 @@ int nvmet_file_ns_enable(struct nvmet_ns *ns)
> >  
> >  	nvmet_file_ns_revalidate(ns);
> >  
> > -	/*
> > -	 * i_blkbits can be greater than the universally accepted upper bound,
> > -	 * so make sure we export a sane namespace lba_shift.
> > -	 */
> > -	ns->blksize_shift = min_t(u8,
> > -			file_inode(ns->file)->i_blkbits, 12);
> > +	if (ns->blksize_shift) {
> > +		if (!ns->buffered_io) {
> > +			struct block_device *sb_bdev = ns->file->f_mapping->host->i_sb->s_bdev;
> > +			struct kstat st;
> > +
> > +			if (!vfs_getattr(&ns->file->f_path, &st, STATX_DIOALIGN, 0) &&
> > +			    (st.result_mask & STATX_DIOALIGN) &&
> > +			    (1 << ns->blksize_shift) < st.dio_offset_align)
> > +				return -EINVAL;
> > +
> > +			if (sb_bdev && (1 << ns->blksize_shift < bdev_logical_block_size(sb_bdev)))
> > +				return -EINVAL;
> 
> I am confused... This is going to check both... But if you got STATX_DIOALIGN
> and it is OK, you do not need (and probably should not) do the second if, no ?

I was not sure about that.
Is it guaranteed that STATX_DIOALIGN returns something sane?

> 
> Also, the second condition of the second if is essentially the same check as
> for the block dev case. So maybe reuse that by creating a small helper function ?

Ok.
 
> > +		}
> > +	} else {
> > +		/*
> > +		 * i_blkbits can be greater than the universally accepted
> > +		 * upper bound, so make sure we export a sane namespace
> > +		 * lba_shift.
> > +		 */
> > +		ns->blksize_shift = min_t(u8,
> > +				file_inode(ns->file)->i_blkbits, 12);
> > +	}
> 
> It feels like this entire hunk should be a helper function as that would allow
> making it a lot more readable with early returns. This code here whould be
> something like:

Ok.

Thanks,
//richard

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