linux-kernel - Re: [PATCH 01/21] block: Add atomic write operations to request

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Date:   Thu, 9 Nov 2023 17:01:10 +0000
From:   John Garry <john.g.garry@...cle.com>
To:     Christoph Hellwig <hch@....de>
Cc:     axboe@...nel.dk, kbusch@...nel.org, sagi@...mberg.me,
        jejb@...ux.ibm.com, martin.petersen@...cle.com, djwong@...nel.org,
        viro@...iv.linux.org.uk, brauner@...nel.org,
        chandan.babu@...cle.com, dchinner@...hat.com,
        linux-block@...r.kernel.org, linux-kernel@...r.kernel.org,
        linux-nvme@...ts.infradead.org, linux-xfs@...r.kernel.org,
        linux-fsdevel@...r.kernel.org, tytso@....edu, jbongio@...gle.com,
        linux-api@...r.kernel.org,
        Himanshu Madhani <himanshu.madhani@...cle.com>
Subject: Re: [PATCH 01/21] block: Add atomic write operations to request_queue
 limits

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@...cle.com>
>> +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@...cle.com>
>> +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@...cle.com>
>> +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

>