Message-ID: <CAGsJ_4xMYUHo_SRc=1jgP6jMNosA_NP3HCitSMKwx=5ScJ0Cng@mail.gmail.com>
Date: Fri, 29 Aug 2025 11:54:19 +1200
From: Barry Song <21cnbao@...il.com>
To: Kanchana P Sridhar <kanchana.p.sridhar@...el.com>
Cc: linux-kernel@...r.kernel.org, linux-mm@...ck.org, hannes@...xchg.org, 
	yosry.ahmed@...ux.dev, nphamcs@...il.com, chengming.zhou@...ux.dev, 
	usamaarif642@...il.com, ryan.roberts@....com, ying.huang@...ux.alibaba.com, 
	akpm@...ux-foundation.org, senozhatsky@...omium.org, 
	linux-crypto@...r.kernel.org, herbert@...dor.apana.org.au, 
	davem@...emloft.net, clabbe@...libre.com, ardb@...nel.org, 
	ebiggers@...gle.com, surenb@...gle.com, kristen.c.accardi@...el.com, 
	vinicius.gomes@...el.com, wajdi.k.feghali@...el.com, vinodh.gopal@...el.com
Subject: Re: [PATCH v11 24/24] mm: zswap: Batched zswap_compress() with
 compress batching of large folios.

> +static bool zswap_compress(struct folio *folio, long start, unsigned int nr_pages,
> +                          struct zswap_entry *entries[], struct zswap_pool *pool,
> +                          int node_id)
>  {
>         struct crypto_acomp_ctx *acomp_ctx;
>         struct scatterlist input, output;
> -       int comp_ret = 0, alloc_ret = 0;
> -       unsigned int dlen = PAGE_SIZE;
> -       unsigned long handle;
> -       struct zpool *zpool;
> +       struct zpool *zpool = pool->zpool;
> +
> +       unsigned int dlens[ZSWAP_MAX_BATCH_SIZE];
> +       int errors[ZSWAP_MAX_BATCH_SIZE];
> +
> +       unsigned int nr_comps = min(nr_pages, pool->compr_batch_size);
> +       unsigned int i, j;
> +       int err;
>         gfp_t gfp;
> -       u8 *dst;
> +
> +       gfp = GFP_NOWAIT | __GFP_NORETRY | __GFP_HIGHMEM | __GFP_MOVABLE;
>
>         acomp_ctx = raw_cpu_ptr(pool->acomp_ctx);
>
>         mutex_lock(&acomp_ctx->mutex);
>
> -       dst = acomp_ctx->buffers[0];
> -       sg_init_table(&input, 1);
> -       sg_set_page(&input, page, PAGE_SIZE, 0);
> -
>         /*
> -        * We need PAGE_SIZE * 2 here since there maybe over-compression case,
> -        * and hardware-accelerators may won't check the dst buffer size, so
> -        * giving the dst buffer with enough length to avoid buffer overflow.
> +        * Note:
> +        * [i] refers to the incoming batch space and is used to
> +        *     index into the folio pages, @entries and @errors.
>          */
> -       sg_init_one(&output, dst, PAGE_SIZE * 2);
> -       acomp_request_set_params(acomp_ctx->req, &input, &output, PAGE_SIZE, dlen);
> +       for (i = 0; i < nr_pages; i += nr_comps) {
> +               if (nr_comps == 1) {
> +                       sg_init_table(&input, 1);
> +                       sg_set_page(&input, folio_page(folio, start + i), PAGE_SIZE, 0);
>
> -       /*
> -        * it maybe looks a little bit silly that we send an asynchronous request,
> -        * then wait for its completion synchronously. This makes the process look
> -        * synchronous in fact.
> -        * Theoretically, acomp supports users send multiple acomp requests in one
> -        * acomp instance, then get those requests done simultaneously. but in this
> -        * case, zswap actually does store and load page by page, there is no
> -        * existing method to send the second page before the first page is done
> -        * in one thread doing zwap.
> -        * but in different threads running on different cpu, we have different
> -        * acomp instance, so multiple threads can do (de)compression in parallel.
> -        */
> -       comp_ret = crypto_wait_req(crypto_acomp_compress(acomp_ctx->req), &acomp_ctx->wait);
> -       dlen = acomp_ctx->req->dlen;
> -       if (comp_ret)
> -               goto unlock;
> +                       /*
> +                        * We need PAGE_SIZE * 2 here since there maybe over-compression case,
> +                        * and hardware-accelerators may won't check the dst buffer size, so
> +                        * giving the dst buffer with enough length to avoid buffer overflow.
> +                        */
> +                       sg_init_one(&output, acomp_ctx->buffers[0], PAGE_SIZE * 2);
> +                       acomp_request_set_params(acomp_ctx->req, &input,
> +                                                &output, PAGE_SIZE, PAGE_SIZE);
> +
> +                       errors[i] = crypto_wait_req(crypto_acomp_compress(acomp_ctx->req),
> +                                                   &acomp_ctx->wait);
> +                       if (unlikely(errors[i]))
> +                               goto compress_error;
> +
> +                       dlens[i] = acomp_ctx->req->dlen;
> +               } else {
> +                       struct page *pages[ZSWAP_MAX_BATCH_SIZE];
> +                       unsigned int k;
> +
> +                       for (k = 0; k < nr_pages; ++k)
> +                               pages[k] = folio_page(folio, start + k);
> +
> +                       struct swap_batch_comp_data batch_comp_data = {
> +                               .pages = pages,
> +                               .dsts = acomp_ctx->buffers,
> +                               .dlens = dlens,
> +                               .errors = errors,
> +                               .nr_comps = nr_pages,
> +                       };

Why would this work given that nr_pages might be larger than
pool->compr_batch_size?

unsigned int nr_comps = min(nr_pages, pool->compr_batch_size);

So this actually doesn’t happen unless pool->compr_batch_size == 1,
but the code is confusing, right?

> +
> +                       acomp_ctx->req->kernel_data = &batch_comp_data;

Can you actually pass a request larger than pool->compr_batch_size
to the crypto driver?

By the way, swap_batch_comp_data seems like a poor name. Why should
crypto drivers know anything about swap_? kernel_data isn’t ideal either;
maybe batch_data would be better ?

> +
> +                       if (unlikely(crypto_acomp_compress(acomp_ctx->req)))
> +                               goto compress_error;
> +               }

Thanks
Barry

Powered by blists - more mailing lists