Re: [PATCH v2 1/2] mm: zswap: use separate compression and decompression requests

[Nhat Pham]

On Fri, Oct 9, 2026 at 6:31 PM 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, so group it with the compression request. Independent
> requests can share the per-CPU transform; the codecs and drivers
> synchronize their shared state internally. Loads can still wait for
> each other on the decompression mutex, and stores still serialize on
> the compression mutex.
>
> acomp_request_free() ignores NULL requests, so drop the check in
> acomp_ctx_free().
>
> This follows the proposal from Sergey Senozhatsky for the same split
> for zram [1].
>
> [1] https://lore.kernel.org/all/20261005122036.718976-10-senozhatsky@xxxxxxxxxxxx/
>
> Signed-off-by: Usama Arif <usama.arif@xxxxxxxxx>

Acked-by: Nhat Pham <nphamcs@xxxxxxxxx>

Herbert, does this also look good to you?

> ---
>  mm/zswap.c | 108 +++++++++++++++++++++++++++++++----------------------
>  1 file changed, 64 insertions(+), 44 deletions(-)
> +       if (zswap_acomp_req_init(&acomp_ctx->comp.areq, acomp_ctx->acomp) ||
> +           zswap_acomp_req_init(&acomp_ctx->decomp, acomp_ctx->acomp)) {

This really breaks my brain...