mm/zswap: reference the pool by id to shrink struct zswap_entry

struct zswap_entry is one allocation per stored page, so its size is pure
overhead.  It currently embeds an 8-byte pool pointer, even though the
live pools now sit in an allocating xarray keyed by a small integer id
that fits in a u8.

Replace the per-entry pool pointer with that u8 id and resolve it through
the xarray with xa_load().  xa_load() does its own RCU-protected lookup,
so the caller needs no rcu_read_lock() section of its own.  The resolved
pool stays valid because a live entry pins it via percpu_ref (taken in
zswap_store_page()), so its id cannot be reused.  A live entry never uses
the reserved id 0, so a zeroed id resolves to NULL and trips a WARN rather
than aliasing a live pool.

The u8 fits in the padding after the bool referenced field, shrinking the
entry from 56 to 48 bytes on 64-bit.  This raises objs_per_slab from 73 to
85 and saves about 2MiB of metadata per 1GiB of data held in zswap.

Link: https://lore.kernel.org/20260906-shrink_zswap_entry_v6-v6-3-ac4cf61565fb@gmail.com
Signed-off-by: Jianyue Wu <wujianyue000@gmail.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Suggested-by: Chris Li <chrisl@kernel.org>
Acked-by: Yosry Ahmed <yosry@kernel.org>
Cc: Chengming Zhou <chengming.zhou@linux.dev>
Cc: Johannes Weiner <hannes@cmpxchg.org>
Cc: Nhat Pham <nphamcs@gmail.com>

0 files changed, 0 insertions, 0 deletions