Re: [PATCH 01/21] block: Add atomic write operations to request_queue limits

[John Garry]

On 09/11/2023 15:10, Christoph Hellwig wrote:
> > Documentation/ABI/stable/sysfs-block | 42 +++++++++++++++++++
> >   block/blk-settings.c                 | 60 ++++++++++++++++++++++++++++
> >   block/blk-sysfs.c                    | 33 +++++++++++++++
> >   include/linux/blkdev.h               | 33 +++++++++++++++
> >   4 files changed, 168 insertions(+)
> >
> > diff --git a/Documentation/ABI/stable/sysfs-block b/Documentation/ABI/stable/sysfs-block
> > index 1fe9a553c37b..05df7f74cbc1 100644
> > --- a/Documentation/ABI/stable/sysfs-block
> > +++ b/Documentation/ABI/stable/sysfs-block
> > @@ -21,6 +21,48 @@ Description:
> >   		device is offset from the internal allocation unit's
> >   		natural alignment.
> >   
> > +What:		/sys/block/<disk>/atomic_write_max_bytes
> > +Date:		May 2023
> > +Contact:	Himanshu Madhani<himanshu.madhani@xxxxxxxxxx>
> > +Description:
> > +		[RO] This parameter specifies the maximum atomic write
> > +		size reported by the device. An atomic write operation
> > +		must not exceed this number of bytes.
> > +What:		/sys/block/<disk>/atomic_write_unit_max_bytes
> > +Date:		January 2023
> > +Contact:	Himanshu Madhani<himanshu.madhani@xxxxxxxxxx>
> > +Description:
> > +		[RO] This parameter defines the largest block which can be
> > +		written atomically with an atomic write operation. This
> > +		value must be a multiple of atomic_write_unit_min and must
> > +		be a power-of-two.
> What is the difference between these two values?

Generally they come from the same device property. Then since 
atomic_write_unit_max_bytes must be a power-of-2 (and 
atomic_write_max_bytes may not be), they may be different. In addition, 
atomic_write_unit_max_bytes is required to be limited by whatever is 
guaranteed to be able to fit in a bio.

atomic_write_max_bytes is really only relevant for merging writes. Maybe 
we should not even expose via sysfs.

BTW, I do still wonder whether all these values should be limited by 
max_sectors_kb (which they aren't currently).

> > +Date:		May 2023
> > +Contact:	Himanshu Madhani<himanshu.madhani@xxxxxxxxxx>
> > +Description:
> > +		[RO] This parameter specifies the smallest block which can
> > +		be written atomically with an atomic write operation. All
> > +		atomic write operations must begin at a
> > +		atomic_write_unit_min boundary and must be multiples of
> > +		atomic_write_unit_min. This value must be a power-of-two.
> How can the minimum unit be anythіng but one logical block?
>
> > +extern void blk_queue_atomic_write_max_bytes(struct request_queue *q,
> > +					     unsigned int bytes);
> Please don't add pointless externs to prototypes in headers.

ok, fine - blkdev.h seems to have a mix for declarations with and 
without extern, so at least we would be consistently inconsistent.

> > +static inline unsigned int queue_atomic_write_unit_max_bytes(const struct request_queue *q)
> .. and please avoid the overly long lines.

ok

Thanks,
John

>