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

[Darrick J. Wong]

On Mon, Mar 10, 2025 at 10:44:47AM +0000, John Garry wrote:
> 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).

I don't like encoding "REQ_ATOMIC" in iomap flags.  If we're changing
the names, they ought to reflect who's making the guarantees:

IOMAP_DIO_BDEV_ATOMIC vs. IOMAP_DIO_FS_ATOMIC.

Not sure why the flags lost the "_DIO" part.

--D

> And IOMAP_FS_ATOMIC seems a bit vague, but I can't think of anything else.
> 
> Darrick, any opinion on this?
> 
> Cheers,
> John
>