Re: [PATCH v4 05/12] iomap: Support SW-based atomic writes

[John Garry <john.g.garry@oracle.com>]

On 09/03/2025 21:51, Dave Chinner wrote:
> Mon, Mar 03, 2025 at 05:11:13PM +0000, John Garry wrote:
>> Currently atomic write support requires dedicated HW support. This imposes
>> a restriction on the filesystem that disk blocks need to be aligned and
>> contiguously mapped to FS blocks to issue atomic writes.
>>
>> XFS has no method to guarantee FS block alignment for regular,
>> non-RT files. As such, atomic writes are currently limited to 1x FS block
>> there.
>>
>> To deal with the scenario that we are issuing an atomic write over
>> misaligned or discontiguous data blocks - and raise the atomic write size
>> limit - support a SW-based software emulated atomic write mode. For XFS,
>> this SW-based atomic writes would use CoW support to issue emulated untorn
>> writes.
>>
>> It is the responsibility of the FS to detect discontiguous atomic writes
>> and switch to IOMAP_DIO_ATOMIC_SW mode and retry the write. Indeed,
>> SW-based atomic writes could be used always when the mounted bdev does
>> not support HW offload, but this strategy is not initially expected to be
>> used.
> So now seeing how these are are to be used, these aren't "hardware"
> and "software" atomic IOs. They are block layer vs filesystem atomic
> IOs.
> 
> We can do atomic IOs in software in the block layer drivers (think
> loop or dm-thinp) rather than off-loading to storage hardware.
> 
> Hence I think these really need to be named after the layer that
> will provide the atomic IO guarantees, because "hw" and "sw" as they
> are currently used are not correct. e.g something like
> IOMAP_FS_ATOMIC and IOMAP_BDEV_ATOMIC which indicates which layer
> should be providing the atomic IO constraints and guarantees.

I'd prefer IOMAP_REQ_ATOMIC instead (of IOMAP_BDEV_ATOMIC), as we are 
using REQ_ATOMIC for those BIOs only. Anything which the block layer and 
below does with REQ_ATOMIC is its business, as long as it guarantees 
atomic submission. But I am not overly keen on that name, as it clashes 
with block layer names (naturally).

And IOMAP_FS_ATOMIC seems a bit vague, but I can't think of anything else.

Darrick, any opinion on this?

Cheers,
John