Re: [PATCH v11 22/24] mm: zswap: Allocate pool batching resources if the compressor supports batching.

[Nhat Pham <nphamcs@gmail.com>]

On Thu, Jul 31, 2025 at 9:36 PM Kanchana P Sridhar
<kanchana.p.sridhar@intel.com> wrote:
>
> This patch sets up zswap for allocating per-CPU resources optimally for
> non-batching and batching compressors.
>
> A new ZSWAP_MAX_BATCH_SIZE constant is defined as 8U, to set an upper
> limit on the number of pages in large folios that will be batch
> compressed.
>
> As per Herbert's comments in [2] in response to the
> crypto_acomp_batch_compress() and crypto_acomp_batch_decompress() API
> proposed in [1], this series does not create new crypto_acomp batching
> API. Instead, zswap compression batching uses the existing
> crypto_acomp_compress() API in combination with the "void *kernel_data"
> member added to "struct acomp_req" earlier in this series.
>
> It is up to the compressor to manage multiple requests, as needed, to
> accomplish batch parallelism. zswap only needs to allocate the per-CPU
> dst buffers according to the batch size supported by the compressor.
>
> A "u8 compr_batch_size" member is added to "struct zswap_pool", as per
> Yosry's suggestion. pool->compr_batch_size is set as the minimum of the
> compressor's max batch-size and ZSWAP_MAX_BATCH_SIZE. Accordingly, it
> proceeds to allocate the necessary compression dst buffers in the
> per-CPU acomp_ctx.
>
> Another "u8 batch_size" member is added to "struct zswap_pool" to store
> the unit for batching large folio stores: for batching compressors, this
> is the pool->compr_batch_size. For non-batching compressors, this is
> ZSWAP_MAX_BATCH_SIZE.
>
> zswap does not use more than one dst buffer yet. Follow-up patches will
> actually utilize the multiple acomp_ctx buffers for batch
> compression/decompression of multiple pages.
>
> Thus, ZSWAP_MAX_BATCH_SIZE limits the amount of extra memory used for
> batching. There is a small extra memory overhead of allocating
> the acomp_ctx->buffers array for compressors that do not support
> batching: On x86_64, the overhead is 1 pointer per-CPU (i.e. 8 bytes).
>
> [1]: https://patchwork.kernel.org/project/linux-mm/patch/20250508194134.28392-11-kanchana.p.sridhar@intel.com/
> [2]: https://patchwork.kernel.org/comment/26382610
>
> Signed-off-by: Kanchana P Sridhar <kanchana.p.sridhar@intel.com>

Mostly LGTM. Just a couple of questions below:

> ---
>  mm/zswap.c | 82 +++++++++++++++++++++++++++++++++++++++++-------------
>  1 file changed, 63 insertions(+), 19 deletions(-)
>
> diff --git a/mm/zswap.c b/mm/zswap.c
> index efd501a7fe294..63a997b999537 100644
> --- a/mm/zswap.c
> +++ b/mm/zswap.c
> @@ -80,6 +80,9 @@ static bool zswap_pool_reached_full;
>
>  #define ZSWAP_PARAM_UNSET ""
>
> +/* Limit the batch size to limit per-CPU memory usage for dst buffers. */
> +#define ZSWAP_MAX_BATCH_SIZE 8U
> +
>  static int zswap_setup(void);
>
>  /* Enable/disable zswap */
> @@ -147,7 +150,7 @@ struct crypto_acomp_ctx {
>         struct crypto_acomp *acomp;
>         struct acomp_req *req;
>         struct crypto_wait wait;
> -       u8 *buffer;
> +       u8 **buffers;
>         struct mutex mutex;
>         bool is_sleepable;
>  };
> @@ -166,6 +169,8 @@ struct zswap_pool {
>         struct work_struct release_work;
>         struct hlist_node node;
>         char tfm_name[CRYPTO_MAX_ALG_NAME];
> +       u8 compr_batch_size;
> +       u8 batch_size;

Apologies if this is explained elsewhere, but I'm very confused - why
do we need both of these two fields?

Seems like batch_size is defined below, and never changed:

      pool->batch_size = (pool->compr_batch_size > 1) ?
                            pool->compr_batch_size : ZSWAP_MAX_BATCH_SIZE;

Can we just determine this in zswap_store() as a local variable?


>  };
>
>  /* Global LRU lists shared by all zswap pools. */
> @@ -258,8 +263,10 @@ static void __zswap_pool_empty(struct percpu_ref *ref);
>   *   zswap_cpu_comp_prepare(), not others.
>   * - Cleanup acomp_ctx resources on all cores in zswap_pool_destroy().
>   */
> -static void acomp_ctx_dealloc(struct crypto_acomp_ctx *acomp_ctx)
> +static void acomp_ctx_dealloc(struct crypto_acomp_ctx *acomp_ctx, u8 nr_buffers)
>  {
> +       u8 i;
> +
>         if (IS_ERR_OR_NULL(acomp_ctx))
>                 return;
>
> @@ -269,7 +276,11 @@ static void acomp_ctx_dealloc(struct crypto_acomp_ctx *acomp_ctx)
>         if (!IS_ERR_OR_NULL(acomp_ctx->acomp))
>                 crypto_free_acomp(acomp_ctx->acomp);
>
> -       kfree(acomp_ctx->buffer);
> +       if (acomp_ctx->buffers) {
> +               for (i = 0; i < nr_buffers; ++i)
> +                       kfree(acomp_ctx->buffers[i]);
> +               kfree(acomp_ctx->buffers);
> +       }
>  }
>
>  static struct zswap_pool *zswap_pool_create(char *type, char *compressor)
> @@ -290,6 +301,7 @@ static struct zswap_pool *zswap_pool_create(char *type, char *compressor)
>                         return NULL;
>         }
>
> +       /* Many things rely on the zero-initialization. */
>         pool = kzalloc(sizeof(*pool), GFP_KERNEL);
>         if (!pool)
>                 return NULL;
> @@ -352,13 +364,28 @@ static struct zswap_pool *zswap_pool_create(char *type, char *compressor)
>                 goto ref_fail;
>         INIT_LIST_HEAD(&pool->list);
>
> +       /*
> +        * Set the unit of compress batching for large folios, for quick
> +        * retrieval in the zswap_compress() fast path:
> +        * If the compressor is sequential (@pool->compr_batch_size is 1),
> +        * large folios will be compressed in batches of ZSWAP_MAX_BATCH_SIZE
> +        * pages, where each page in the batch is compressed sequentially.
> +        * We see better performance by processing the folio in batches of
> +        * ZSWAP_MAX_BATCH_SIZE, due to cache locality of working set
> +        * structures.
> +        */
> +       pool->batch_size = (pool->compr_batch_size > 1) ?
> +                               pool->compr_batch_size : ZSWAP_MAX_BATCH_SIZE;
> +
>         zswap_pool_debug("created", pool);
>
>         return pool;
>
>  ref_fail:
>         for_each_possible_cpu(cpu)
> -               acomp_ctx_dealloc(per_cpu_ptr(pool->acomp_ctx, cpu));
> +               acomp_ctx_dealloc(per_cpu_ptr(pool->acomp_ctx, cpu),
> +                                 pool->compr_batch_size);
> +
>  error:
>         if (pool->acomp_ctx)
>                 free_percpu(pool->acomp_ctx);
> @@ -417,7 +444,8 @@ static void zswap_pool_destroy(struct zswap_pool *pool)
>         zswap_pool_debug("destroying", pool);
>
>         for_each_possible_cpu(cpu)
> -               acomp_ctx_dealloc(per_cpu_ptr(pool->acomp_ctx, cpu));
> +               acomp_ctx_dealloc(per_cpu_ptr(pool->acomp_ctx, cpu),
> +                                 pool->compr_batch_size);
>
>         free_percpu(pool->acomp_ctx);
>
> @@ -876,6 +904,7 @@ 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;
>
>         /*
>          * The per-CPU pool->acomp_ctx is zero-initialized on allocation.
> @@ -888,10 +917,6 @@ static int zswap_cpu_comp_prepare(unsigned int cpu, struct hlist_node *node)
>         if (!IS_ERR_OR_NULL(acomp_ctx->acomp))
>                 return 0;
>
> -       acomp_ctx->buffer = kmalloc_node(PAGE_SIZE * 2, GFP_KERNEL, cpu_to_node(cpu));
> -       if (!acomp_ctx->buffer)
> -               return ret;
> -
>         acomp_ctx->acomp = crypto_alloc_acomp_node(pool->tfm_name, 0, 0, cpu_to_node(cpu));
>         if (IS_ERR_OR_NULL(acomp_ctx->acomp)) {
>                 pr_err("could not alloc crypto acomp %s : %ld\n",
> @@ -904,17 +929,36 @@ static int zswap_cpu_comp_prepare(unsigned int cpu, struct hlist_node *node)
>         acomp_ctx->req = acomp_request_alloc(acomp_ctx->acomp);
>         if (IS_ERR_OR_NULL(acomp_ctx->req)) {
>                 pr_err("could not alloc crypto acomp_request %s\n",
> -                      pool->tfm_name);
> +                       pool->tfm_name);

Is this intentional? :)

>                 goto fail;
>         }
>
> -       crypto_init_wait(&acomp_ctx->wait);
> +       /*
> +        * Allocate up to ZSWAP_MAX_BATCH_SIZE dst buffers if the
> +        * compressor supports batching.
> +        */
> +       pool->compr_batch_size = min(ZSWAP_MAX_BATCH_SIZE,
> +                                    crypto_acomp_batch_size(acomp_ctx->acomp));
> +
> +       acomp_ctx->buffers = kcalloc_node(pool->compr_batch_size, sizeof(u8 *),
> +                                         GFP_KERNEL, cpu_to_node(cpu));
> +       if (!acomp_ctx->buffers)
> +               goto fail;
> +
> +       for (i = 0; i < pool->compr_batch_size; ++i) {
> +               acomp_ctx->buffers[i] = kmalloc_node(PAGE_SIZE * 2, GFP_KERNEL,
> +                                                    cpu_to_node(cpu));
> +               if (!acomp_ctx->buffers[i])
> +                       goto fail;
> +       }
>
>         /*
>          * if the backend of acomp is async zip, crypto_req_done() will wakeup
>          * crypto_wait_req(); if the backend of acomp is scomp, the callback
>          * won't be called, crypto_wait_req() will return without blocking.
>          */
> +       crypto_init_wait(&acomp_ctx->wait);
> +
>         acomp_request_set_callback(acomp_ctx->req, CRYPTO_TFM_REQ_MAY_BACKLOG,
>                                    crypto_req_done, &acomp_ctx->wait);
>
> @@ -922,7 +966,7 @@ static int zswap_cpu_comp_prepare(unsigned int cpu, struct hlist_node *node)
>         return 0;
>
>  fail:
> -       acomp_ctx_dealloc(acomp_ctx);
> +       acomp_ctx_dealloc(acomp_ctx, pool->compr_batch_size);
>         return ret;
>  }
>
> @@ -942,7 +986,7 @@ static bool zswap_compress(struct page *page, struct zswap_entry *entry,
>
>         mutex_lock(&acomp_ctx->mutex);
>
> -       dst = acomp_ctx->buffer;
> +       dst = acomp_ctx->buffers[0];
>         sg_init_table(&input, 1);
>         sg_set_page(&input, page, PAGE_SIZE, 0);
>
> @@ -1003,19 +1047,19 @@ static bool zswap_decompress(struct zswap_entry *entry, struct folio *folio)
>
>         acomp_ctx = raw_cpu_ptr(entry->pool->acomp_ctx);
>         mutex_lock(&acomp_ctx->mutex);
> -       obj = zpool_obj_read_begin(zpool, entry->handle, acomp_ctx->buffer);
> +       obj = zpool_obj_read_begin(zpool, entry->handle, acomp_ctx->buffers[0]);
>
>         /*
>          * zpool_obj_read_begin() might return a kmap address of highmem when
> -        * acomp_ctx->buffer is not used.  However, sg_init_one() does not
> -        * handle highmem addresses, so copy the object to acomp_ctx->buffer.
> +        * acomp_ctx->buffers[0] is not used.  However, sg_init_one() does not
> +        * handle highmem addresses, so copy the object to acomp_ctx->buffers[0].
>          */
>         if (virt_addr_valid(obj)) {
>                 src = obj;
>         } else {
> -               WARN_ON_ONCE(obj == acomp_ctx->buffer);
> -               memcpy(acomp_ctx->buffer, obj, entry->length);
> -               src = acomp_ctx->buffer;
> +               WARN_ON_ONCE(obj == acomp_ctx->buffers[0]);
> +               memcpy(acomp_ctx->buffers[0], obj, entry->length);
> +               src = acomp_ctx->buffers[0];
>         }
>
>         sg_init_one(&input, src, entry->length);
> --
> 2.27.0
>