Re: [PATCH 1/2] xfs: use a seprate member to track space availabe in the GC scatch buffer

[Christoph Hellwig <hch@lst.de>]

On Tue, Jan 27, 2026 at 09:35:11PM -0800, Darrick J. Wong wrote:
> > +	unsigned int			scratch_available;
> >  	unsigned int			scratch_head;
> >  	unsigned int			scratch_tail;
> 
> Hrm.  I did some digging (because clearly I'm not that good at
> ringbuffer) and came up with this gem from akpm:
> 
> "A circular buffer implementation needs only head and tail indices.
> `size' above appears to be redundant.
> 
> "Implementation-wise, the head and tail indices should *not* be
> constrained to be less than the size of the buffer. They should be
> allowed to wrap all the way back to zero. This allows you to distinguish
> between the completely-empty and completely-full states while using 100%
> of the storage."
> 
> https://lkml.iu.edu/hypermail/linux/kernel/0409.1/2709.html
> 
> Can that apply here?

It could, see the version Keith posted.  But this one is actually
slightly simpler, while the unsigned overlflow version requires saves 4
bytes of memory per file system (+/- 4 bytes of padding for either
version which I haven't checked) and a single add and sub instruction
each for every GC operation.

My preference is this simpler version, but the other one should work
just fine as well.