[PATCH v5 0/3] mm/zswap: shrink zswap_entry via a pool id

[Jianyue Wu]

Every stored page has a struct zswap_entry, so its size is pure per-page
overhead.  On 64-bit it is currently 56 bytes, of which 8 bytes are a
pointer to the owning zswap_pool.

Only a handful of pools are ever live: a new pool is created only when
the compressor is (re)set, and pools are reused across compressor
switches.  That makes a per-entry pool pointer more expensive than it
needs to be, and the RCU list that currently tracks pools is more
machinery than this needs once each pool already has a stable id.

This series:

  1. Releases retired pools with queue_rcu_work() instead of a worker
     calling synchronize_rcu(), so the release worker no longer blocks
     on an RCU grace period.
  2. Replaces the zswap_pools list with an allocating xarray
     (XA_FLAGS_ALLOC1 | XA_FLAGS_LOCK_BH) and a separate RCU-protected
     current-pool pointer, giving each pool a stable small id.  Ids
     start at 1.  The reserved id 0 is never allocated, so looking it
     up resolves to NULL.  The table grows as needed up to 255 live
     pools (u8 pool_idx), not a fixed slot array.
  3. Stores that u8 pool id in each zswap_entry instead of the pool
     pointer.  The u8 fits in padding after the bool referenced field,
     so the entry shrinks from 56 to 48 bytes on 64-bit (~2MiB of
     metadata saved per 1GiB of data held in zswap).

Runtime compressor switching is preserved.  Pool ids are bounded to
1..255 because struct zswap_entry stores the id in a u8.  The cap
counts every id still in the xarray, including a killed pool that still
has entries.  Ids are reused when a pool is erased.  Switching back to
a compressor that still has a pool in the xarray resurrects it rather
than allocating a new id.  If all usable ids are full, creating a pool
for another compressor fails and the compressor switch is rejected.

On 64-bit, struct zswap_entry is 56 -> 48 bytes, which fits 73 -> 85
objects in a 4K slab.

Benchmark (x86_64, compressor=lzo, MADV_PAGEOUT store + fault-in load):

  - e2e store+load median latency: no measurable regression vs baseline
    at matched stored_delta

Each store, free, and decompress looks up the pool with xa_load()
instead of following a pointer.  With only a handful of live pools the
xarray walk is short.

Testing
=======

- Boot with DEBUG_ATOMIC_SLEEP + lockdep/PROVE_RCU + KASAN:
  zswap store/load, shrinker writeback, and compressor switch
  (retire, then switch back to resurrect) pass

This series is based on akpm/mm-unstable as of 2026-09-04
(20cab322c95c).

To: Johannes Weiner <hannes@xxxxxxxxxxx>
To: Yosry Ahmed <yosry@xxxxxxxxxx>
To: Nhat Pham <nphamcs@xxxxxxxxx>
To: Chengming Zhou <chengming.zhou@xxxxxxxxx>
To: Andrew Morton <akpm@xxxxxxxxxxxxxxxxxxxx>
Cc: Chris Li <chrisl@xxxxxxxxxx>
Cc: linux-mm@xxxxxxxxx
Cc: linux-kernel@xxxxxxxxxxxxxxx
Signed-off-by: Jianyue Wu <wujianyue000@xxxxxxxxx>

Changes since RFC v4:
  - Replace the RCU pool list with an allocating xarray
    (XA_FLAGS_ALLOC1 | XA_FLAGS_LOCK_BH) and a separate RCU-protected
    current-pool pointer.
  - Resolve entry->pool_idx with xa_load() under rcu_read_lock().
  - Write pool_idx before the entry is stored in the swap tree, so a
    lookup cannot see a stale pool_idx left over from slab reuse.
  - Retire pools with queue_rcu_work() on system_percpu_wq.
  - Drop the RFC tag.

Link: https://lore.kernel.org/all/20260830114731.8322-1-wujianyue000@xxxxxxxxx/
Link: https://lore.kernel.org/all/20260815-shrink_zswap_entry_0815_v2-v3-3-0171bd86a667@xxxxxxxxx/
Link: https://lore.kernel.org/all/20260731-shrink_zswap_entry_v2-0-0-v2-0-e72083aa8734@xxxxxxxxx/
Link: https://lore.kernel.org/all/20260726-shrink_zswap_entry_v1-0-0-v1-1-30957e4d0cb6@xxxxxxxxx/

---
Jianyue Wu (3):
  mm/zswap: release retired pools via queue_rcu_work() instead of
    synchronize_rcu()
  mm/zswap: replace the zswap_pools list with an allocating xarray
  mm/zswap: reference the pool by id to shrink struct zswap_entry

 mm/zswap.c | 144 ++++++++++++++++++++++++++++++++++++-----------------
 1 file changed, 97 insertions(+), 47 deletions(-)


base-commit: 20cab322c95cea0327215bb81a05df69336032dd
-- 
2.43.0