Re: [PATCH 27/45] xfs: implement buffered writes to zoned RT devices

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

On Thu, Feb 20, 2025 at 07:16:05AM +0100, Christoph Hellwig wrote:
> On Wed, Feb 19, 2025 at 01:47:27PM -0800, Darrick J. Wong wrote:
> > I don't want to go adding opencoded logic loops all over the place, that
> > would be pretty horrid.  But what if xfs_zone_alloc_ctx were instead a
> > general freecounter reservation context?  Then we could hide all this
> > "try to reserve space, push a garbage collector once if we can't, and
> > try again once" logic into an xfs_reserve_space() function, and pass
> > that reservation through the iomap functions to ->iomap_begin.
> > 
> > But a subtlety here is that the code under iomap_file_buffered_write
> > might not actually need to create any delalloc reservations, in which
> > case a worst case reservation could fail with ENOSPC even though we
> > don't actually need to allocate a single byte.
> 
> I think the idea of per-reserving space before starting transactions
> is generally a good idea, and I'd be happy to look into reworking the
> conventional code towards that.  But I'd rather not do that as part
> of this series if I can avoid it.
> 
> > Having said that, this is like the 5th time I've read through this patch
> > and I don't see anything obviously wrong now, so my last question is:
> > Zoned files cannot have preallocations so that's why we don't do this
> > for FALLOC_FL_ALLOCATE_RANGE, right?
> 
> Exactly.  Should I add this to the commit log?  It's mentioned mostly
> in the cover letter at the moment.

That seems like a good idea.

--D