slub: add barn_get_full_sheaf() and refine empty-main sheaf replacement

[Harry Yoo]

On Tue, Dec 02, 2025 at 05:00:08PM +0800, Hao Li wrote:
> Introduce barn_get_full_sheaf(), a helper that detaches a full sheaf from
> the per-node barn without requiring an empty sheaf in exchange.
> 
> Use this helper in __pcs_replace_empty_main() to change how an empty main
> per-CPU sheaf is handled:
> 
>   - If pcs->spare is NULL and pcs->main is empty, first try to obtain a
>     full sheaf from the barn via barn_get_full_sheaf(). On success, park
>     the empty main sheaf in pcs->spare and install the full sheaf as the
>     new pcs->main.
> 
>   - If pcs->spare already exists and has objects, keep the existing
>     behavior of simply swapping pcs->main and pcs->spare.
> 
>   - Only when both pcs->main and pcs->spare are empty do we fall back to
>     barn_replace_empty_sheaf() and trade the empty main sheaf into the
>     barn in exchange for a full one.

Hi Hao,

Yeah this is a very subtle difference between __pcs_replace_full_main()
and __pcs_replace_empty_main(), that the former installs the full main
sheaf in pcs->spare, while the latter replaces the empty main sheaf with
a full sheaf from the barn without populating pcs->spare.

Is it intentional, Vlastimil?

> This makes the empty-main path more symmetric with __pcs_replace_full_main(),
> which for a full main sheaf parks the full sheaf in pcs->spare and pulls an
> empty sheaf from the barn. It also matches the documented design more closely:
> 
>   "When both percpu sheaves are found empty during an allocation, an empty
>    sheaf may be replaced with a full one from the per-node barn."

I'm not convinced that this change is worthwhile by adding more code;
you probably need to make a stronger argument for why it should be done.

> Signed-off-by: Hao Li <haoli.tcs at gmail.com>
> ---

-- 
Cheers,
Harry / Hyeonggon