Re: [PATCH v8 12/14] mm: zswap: Simplify acomp_ctx resource allocation/deletion and mutex lock usage.

[Yosry Ahmed <yosry.ahmed@linux.dev>]

On Mon, Mar 03, 2025 at 12:47:22AM -0800, Kanchana P Sridhar wrote:
> This patch modifies the acomp_ctx resources' lifetime to be from pool
> creation to deletion. A "bool __online" and "u8 nr_reqs" are added to
> "struct crypto_acomp_ctx" which simplify a few things:
> 
> 1) zswap_pool_create() will initialize all members of each percpu acomp_ctx
>    to 0 or NULL and only then initialize the mutex.
> 2) CPU hotplug will set nr_reqs to 1, allocate resources and set __online
>    to true, without locking the mutex.
> 3) CPU hotunplug will lock the mutex before setting __online to false. It
>    will not delete any resources.
> 4) acomp_ctx_get_cpu_lock() will lock the mutex, then check if __online
>    is true, and if so, return the mutex for use in zswap compress and
>    decompress ops.
> 5) CPU onlining after offlining will simply check if either __online or
>    nr_reqs are non-0, and return 0 if so, without re-allocating the
>    resources.
> 6) zswap_pool_destroy() will call a newly added zswap_cpu_comp_dealloc() to
>    delete the acomp_ctx resources.
> 7) Common resource deletion code in case of zswap_cpu_comp_prepare()
>    errors, and for use in zswap_cpu_comp_dealloc(), is factored into a new
>    acomp_ctx_dealloc().
> 
> The CPU hot[un]plug callback functions are moved to "pool functions"
> accordingly.
> 
> 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 as follows:
> 
>     IAA with batching: 64.8 KB
>     Software compressors: 8.2 KB

I am assuming this is specifically on x86_64, so let's call that out.