Re: [PATCH v5] generic: add a test for atomic writes

["Darrick J. Wong" <djwong@kernel.org>]

On Sun, May 04, 2025 at 10:37:07PM -0700, Christoph Hellwig wrote:
> On Fri, May 02, 2025 at 12:39:42PM -0700, Darrick J. Wong wrote:
> > Me neither.  We can't write 512b blocks to the rt device obviously, but
> > I think the whole point of the separate "sector" size is that's the
> > maximum size that the fs knows it can write to the device without
> > tearing.
> 
> The sector size is really the minimum addressable unit.
> 
> > Maybe there's a way out of this: the only metadata on the realtime
> > volume is the rt superblock, whose size is a full fsblock.  Perhaps we
> > could set/validate the block size of the rt dev with the fsblock size
> > instead?
> 
> We still allow subsector dio to the rt device, so this would be a bit
> of a sketchy change.

I don't understand the 'subsector' in this sentence -- we allow
sub-fsblock dio, but not sub-LBA dio, right?

So the only thing we need to validate for the rt device is that
fsblock >= lbasize to avoid confusing the pagecache when it does IO, as
well as user programs that aren't expecting such things.

--D