mm: zswap: tie per-CPU acomp_ctx lifetime to the pool

Currently, per-CPU acomp_ctx are allocated on pool creation and/or CPU
hotplug, and destroyed on pool destruction or CPU hotunplug. This
complicates the lifetime management to save memory while a CPU is
offlined, which is not very common.

Simplify lifetime management by allocating per-CPU acomp_ctx once on pool
creation (or CPU hotplug for CPUs onlined later), and keeping them
allocated until the pool is destroyed.

Refactor cleanup code from zswap_cpu_comp_dead() into acomp_ctx_free() to
be used elsewhere.

The main benefit of using the CPU hotplug multi state instance startup
callback to allocate the acomp_ctx resources is that it prevents the cores
from being offlined until the multi state instance addition call returns.

From Documentation/core-api/cpu_hotplug.rst:

"The node list add/remove operations and the callback invocations are
serialized against CPU hotplug operations."

Furthermore, zswap_[de]compress() cannot contend with
zswap_cpu_comp_prepare() because:

- During pool creation/deletion, the pool is not in the zswap_pools
list.

- During CPU hot[un]plug, the CPU is not yet online, as Yosry pointed
out. zswap_cpu_comp_prepare() will be run on a control CPU,
since CPUHP_MM_ZSWP_POOL_PREPARE is in the PREPARE section of "enum
cpuhp_state".

In both these cases, any recursions into zswap reclaim from
zswap_cpu_comp_prepare() will be handled by the old pool.

The above two observations enable the following simplifications:

1) zswap_cpu_comp_prepare():

a) acomp_ctx mutex locking:

If the process gets migrated while zswap_cpu_comp_prepare() is
running, it will complete on the new CPU. In case of failures, we
pass the acomp_ctx pointer obtained at the start of
zswap_cpu_comp_prepare() to acomp_ctx_free(), which again, can
only undergo migration. There appear to be no contention
scenarios that might cause inconsistent values of acomp_ctx's
members. Hence, it seems there is no need for
mutex_lock(&acomp_ctx->mutex) in zswap_cpu_comp_prepare().

b) acomp_ctx mutex initialization:

Since the pool is not yet on zswap_pools list, we don't need to
initialize the per-CPU acomp_ctx mutex in
zswap_pool_create(). This has been restored to occur in
zswap_cpu_comp_prepare().

c) Subsequent CPU offline-online transitions:

zswap_cpu_comp_prepare() checks upfront if acomp_ctx->acomp is
valid. If so, it returns success. This should handle any CPU
hotplug online-offline transitions after pool creation is done.

2) CPU offline vis-a-vis zswap ops:

Let's suppose the process is migrated to another CPU before the
current CPU is dysfunctional. If zswap_[de]compress() holds the
acomp_ctx->mutex lock of the offlined CPU, that mutex will be
released once it completes on the new CPU. Since there is no
teardown callback, there is no possibility of UAF.

3) Pool creation/deletion and process migration to another CPU:

During pool creation/deletion, the pool is not in the zswap_pools
list. Hence it cannot contend with zswap ops on that CPU. However,
the process can get migrated.

a) Pool creation --> zswap_cpu_comp_prepare()
--> process migrated:
* Old CPU offline: no-op.
* zswap_cpu_comp_prepare() continues
to run on the new CPU to finish
allocating acomp_ctx resources for
the offlined CPU.

b) Pool deletion --> acomp_ctx_free()
--> process migrated:
* Old CPU offline: no-op.
* acomp_ctx_free() continues
to run on the new CPU to finish
de-allocating acomp_ctx resources
for the offlined CPU.

4) Pool deletion vis-a-vis CPU onlining:

The call to cpuhp_state_remove_instance() cannot race with
zswap_cpu_comp_prepare() because of hotplug synchronization.

The current acomp_ctx_get_cpu_lock()/acomp_ctx_put_unlock() are deleted.
Instead, zswap_[de]compress() directly call
mutex_[un]lock(&acomp_ctx->mutex).

The per-CPU memory cost of not deleting the acomp_ctx resources upon CPU
offlining, and only deleting them when the pool is destroyed, is 8.28 KB
on x86_64. This cost is only paid when a CPU is offlined, until it is
onlined again.

Link: https://lore.kernel.org/20260331183351.29844-3-kanchanapsridhar2026@gmail.com
Co-developed-by: Kanchana P. Sridhar <kanchanapsridhar2026@gmail.com>
Signed-off-by: Kanchana P. Sridhar <kanchanapsridhar2026@gmail.com>
Signed-off-by: Kanchana P Sridhar <kanchana.p.sridhar@intel.com>
Acked-by: Yosry Ahmed <yosry@kernel.org>
Cc: Chengming Zhou <chengming.zhou@linux.dev>
Cc: Herbert Xu <herbert@gondor.apana.org.au>
Cc: Johannes Weiner <hannes@cmpxchg.org>
Cc: Nhat Pham <nphamcs@gmail.com>
Cc: Sergey Senozhatsky <senozhatsky@chromium.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>