Re: [PATCH v8 13/14] mm: zswap: Allocate pool batching resources if the compressor supports batching.

Yosry Ahmed
2025-03-06T20:01:22Z

On Mon, Mar 03, 2025 at 12:47:23AM -0800, Kanchana P Sridhar wrote:
> This patch adds support for the per-CPU acomp_ctx to track multiple
> compression/decompression requests and multiple compression destination
> buffers. The zswap_cpu_comp_prepare() CPU onlining code will get the
> maximum batch-size the compressor supports. If so, it will allocate the
> necessary batching resources.
>
> However, zswap does not use more than one request yet. Follow-up patches
> will actually utilize the multiple acomp_ctx requests/buffers for batch
> compression/decompression of multiple pages.
>
> The newly added ZSWAP_MAX_BATCH_SIZE limits the amount of extra memory used
> for batching. There is a small extra memory overhead of allocating the
> "reqs" and "buffers" arrays for compressors that do not support batching.

That's two pointers per-CPU (i.e. 16 bytes on x86_64), right? Please
call that out in the commit log.

>
> Signed-off-by: Kanchana P Sridhar <kanchana.p.sridhar@intel.com>
> ---
> mm/zswap.c | 99 +++++++++++++++++++++++++++++++++++++-----------------
> 1 file changed, 69 insertions(+), 30 deletions(-)
>
> diff --git a/mm/zswap.c b/mm/zswap.c
> index cff96df1df8b..fae59d6d5147 100644
> --- a/mm/zswap.c
> +++ b/mm/zswap.c
> @@ -78,6 +78,16 @@ static bool zswap_pool_reached_full;
>
> #define ZSWAP_PARAM_UNSET ""
>
> +/*
> + * For compression batching of large folios:
> + * Maximum number of acomp compress requests that will be processed
> + * in a batch, iff the zswap compressor supports batching.
> + * This limit exists because we preallocate enough requests and buffers
> + * in the per-cpu acomp_ctx accordingly. Hence, a higher limit means higher
> + * memory usage.
> + */

That's too verbose. Let's do something like:

/* Limit the batch size to limit per-CPU memory usage for reqs and buffers */
#define ZSWAP_MAX_BATCH_SIZE 8U

> +#define ZSWAP_MAX_BATCH_SIZE 8U
> +
> static int zswap_setup(void);
>
> /* Enable/disable zswap */
> @@ -143,8 +153,8 @@ bool zswap_never_enabled(void)
>
> struct crypto_acomp_ctx {
> struct crypto_acomp *acomp;
> - struct acomp_req *req;
> - u8 *buffer;
> + struct acomp_req **reqs;
> + u8 **buffers;
> u8 nr_reqs;
> struct crypto_wait wait;
> struct mutex mutex;
> @@ -251,13 +261,22 @@ static void __zswap_pool_empty(struct percpu_ref *ref);
> static void acomp_ctx_dealloc(struct crypto_acomp_ctx *acomp_ctx)
> {
> if (!IS_ERR_OR_NULL(acomp_ctx) && acomp_ctx->nr_reqs) {
> + u8 i;
> +
> + if (acomp_ctx->reqs) {
> + for (i = 0; i < acomp_ctx->nr_reqs; ++i)
> + if (!IS_ERR_OR_NULL(acomp_ctx->reqs[i]))

Hmm I just realized we check IS_ERR_OR_NULL() here for the requests, but
only a NULL check in zswap_cpu_comp_prepare(). We also check
IS_ERR_OR_NULL here for acomp, but only IS_ERR() in
zswap_cpu_comp_prepare().

This doesn't make sense. Would you be able to include a patch before
this one to make these consistent? I can also send a follow up patch.

> + acomp_request_free(acomp_ctx->reqs[i]);

Please add braces for the for loop here for readability, since the body
has more than one line, even if it's technically not required.

> + kfree(acomp_ctx->reqs);
> + acomp_ctx->reqs = NULL;
> + }
>
> - if (!IS_ERR_OR_NULL(acomp_ctx->req))
> - acomp_request_free(acomp_ctx->req);
> - acomp_ctx->req = NULL;
> -
> - kfree(acomp_ctx->buffer);
> - acomp_ctx->buffer = NULL;
> + if (acomp_ctx->buffers) {
> + for (i = 0; i < acomp_ctx->nr_reqs; ++i)
> + kfree(acomp_ctx->buffers[i]);
> + kfree(acomp_ctx->buffers);
> + acomp_ctx->buffers = NULL;
> + }
>
> if (!IS_ERR_OR_NULL(acomp_ctx->acomp))
> crypto_free_acomp(acomp_ctx->acomp);
> @@ -271,6 +290,7 @@ static int zswap_cpu_comp_prepare(unsigned int cpu, struct hlist_node *node)
> struct zswap_pool *pool = hlist_entry(node, struct zswap_pool, node);
> struct crypto_acomp_ctx *acomp_ctx = per_cpu_ptr(pool->acomp_ctx, cpu);
> int ret = -ENOMEM;
> + u8 i;
>
> /*
> * Just to be even more fail-safe against changes in assumptions and/or
> @@ -292,22 +312,41 @@ static int zswap_cpu_comp_prepare(unsigned int cpu, struct hlist_node *node)
> goto fail;
> }
>
> - acomp_ctx->nr_reqs = 1;
> + acomp_ctx->nr_reqs = min(ZSWAP_MAX_BATCH_SIZE,
> + crypto_acomp_batch_size(acomp_ctx->acomp));
>
> - acomp_ctx->req = acomp_request_alloc(acomp_ctx->acomp);
> - if (!acomp_ctx->req) {
> - pr_err("could not alloc crypto acomp_request %s\n",
> - pool->tfm_name);
> - ret = -ENOMEM;
> + acomp_ctx->reqs = kcalloc_node(acomp_ctx->nr_reqs, sizeof(struct acomp_req *),
> + GFP_KERNEL, cpu_to_node(cpu));
> + if (!acomp_ctx->reqs)
> goto fail;
> +
> + for (i = 0; i < acomp_ctx->nr_reqs; ++i) {
> + acomp_ctx->reqs[i] = acomp_request_alloc(acomp_ctx->acomp);
> + if (!acomp_ctx->reqs[i]) {
> + pr_err("could not alloc crypto acomp_request reqs[%d] %s\n",
> + i, pool->tfm_name);
> + goto fail;
> + }
> }
>
> - acomp_ctx->buffer = kmalloc_node(PAGE_SIZE * 2, GFP_KERNEL, cpu_to_node(cpu));
> - if (!acomp_ctx->buffer) {
> - ret = -ENOMEM;
> + acomp_ctx->buffers = kcalloc_node(acomp_ctx->nr_reqs, sizeof(u8 *),
> + GFP_KERNEL, cpu_to_node(cpu));
> + if (!acomp_ctx->buffers)
> goto fail;
> +
> + for (i = 0; i < acomp_ctx->nr_reqs; ++i) {
> + acomp_ctx->buffers[i] = kmalloc_node(PAGE_SIZE * 2, GFP_KERNEL,
> + cpu_to_node(cpu));
> + if (!acomp_ctx->buffers[i])
> + goto fail;
> }
>
> + /*
> + * The crypto_wait is used only in fully synchronous, i.e., with scomp
> + * or non-poll mode of acomp, hence there is only one "wait" per
> + * acomp_ctx, with callback set to reqs[0], under the assumption that
> + * there is at least 1 request per acomp_ctx.
> + */

I am not sure I understand. Does this say that we assume that scomp or
non-poll acomp will never use batching so having a single "wait" is
fine?

If so, this needs to be enforced at runtime or at least have a warning,
and not just mentioned in a comment, in case batching support is ever
added for these. Please clarify.

We should also probably merge the comments above crypto_init_wait() and
acomp_request_set_callback() now.

> crypto_init_wait(&acomp_ctx->wait);
>
> /*