Re: [PATCH 1/2] mm: zswap: use separate compression and decompression requests

[Nhat Pham]

On Tue, Oct 6, 2026 at 2:23 AM Usama Arif <usama.arif@xxxxxxxxx> wrote:
>
> Stores and loads serialize on the same per-CPU acomp request and mutex.
> A low-priority store can be preempted as soon as the compressor drops
> its stream lock, while it still holds the mutex. A higher-priority load
> on that CPU then waits until the store runs again, which can take a
> long time when other tasks are runnable.
>
> Give compression and decompression their own request, completion wait
> and mutex. Since commit e2c3b6b21c77f ("mm: zswap: use SG list
> decompression APIs from zsmalloc"), the per-CPU buffer is only used for
> compression. The two requests can share the per-CPU transform: no
> in-tree implementation modifies transform state while (de)compressing,
> and shared codec state has its own locking. Loads can still wait for
> each other on the decompression mutex, and stores still serialize on
> the compression mutex.
>
> This follows the proposal from Sergey Senozhatsky for the same split
> for zram [1].

Thanks, zram peeps :P

>
> [1] https://lore.kernel.org/all/20261005122036.718976-10-senozhatsky@xxxxxxxxxxxx/
>
> Signed-off-by: Usama Arif <usama.arif@xxxxxxxxx>

Code mostly LGTM. Just one question:

[...]

> -        * If there was an error in allocating @acomp_ctx->req, it
> -        * would be set to NULL.
> -        */
> -       if (acomp_ctx->req)
> -               acomp_request_free(acomp_ctx->req);
> -
> -       acomp_ctx->req = NULL;
> +       acomp_request_free(acomp_ctx->comp.req);
> +       acomp_ctx->comp.req = NULL;
> +       acomp_request_free(acomp_ctx->decomp.req);
> +       acomp_ctx->decomp.req = NULL;

Hmm do we not have to null check here anymore? Does
acomp_request_free() handle NULL itself too?

For instance, taking the code blob below:

> -       /* acomp_request_alloc() returns NULL in case of an error. */
> -       acomp_ctx->req = acomp_request_alloc(acomp_ctx->acomp);
> -       if (!acomp_ctx->req) {
> +       if (zswap_acomp_req_init(&acomp_ctx->comp, acomp_ctx->acomp) ||
> +           zswap_acomp_req_init(&acomp_ctx->decomp, acomp_ctx->acomp)) {
>                 pr_err("could not alloc crypto acomp_request %s\n",
>                        pool->tfm_name);

Here, we can success with the comp's req but fail with the decomp's req, right?