RE: [PATCH v11 24/24] mm: zswap: Batched zswap_compress() with compress batching of large folios.

[Sridhar, Kanchana P]

> -----Original Message-----
> From: Barry Song <21cnbao@xxxxxxxxx>
> Sent: Thursday, August 28, 2025 4:54 PM
> To: Sridhar, Kanchana P <kanchana.p.sridhar@xxxxxxxxx>
> Cc: linux-kernel@xxxxxxxxxxxxxxx; linux-mm@xxxxxxxxx;
> hannes@xxxxxxxxxxx; yosry.ahmed@xxxxxxxxx; nphamcs@xxxxxxxxx;
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> ryan.roberts@xxxxxxx; ying.huang@xxxxxxxxxxxxxxxxx; akpm@linux-
> foundation.org; senozhatsky@xxxxxxxxxxxx; linux-crypto@xxxxxxxxxxxxxxx;
> herbert@xxxxxxxxxxxxxxxxxxx; davem@xxxxxxxxxxxxx;
> clabbe@xxxxxxxxxxxx; ardb@xxxxxxxxxx; ebiggers@xxxxxxxxxx;
> surenb@xxxxxxxxxx; Accardi, Kristen C <kristen.c.accardi@xxxxxxxxx>;
> Gomes, Vinicius <vinicius.gomes@xxxxxxxxx>; Feghali, Wajdi K
> <wajdi.k.feghali@xxxxxxxxx>; Gopal, Vinodh <vinodh.gopal@xxxxxxxxx>
> 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?

You mean the batching call? For batching compressors, nr_pages 
is always <= pool->batch_size. For batching compressors, pool->batch_size
is the 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?

Clarification above..

> 
> 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 ?

This will be changing in v12 to use an SG list based on Herbert's suggestions.

Thanks,
Kanchana

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