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

["Sridhar, Kanchana P" <kanchana.p.sridhar@intel.com>]

> -----Original Message-----
> From: Barry Song <21cnbao@gmail.com>
> Sent: Monday, August 25, 2025 8:48 PM
> To: Sridhar, Kanchana P <kanchana.p.sridhar@intel.com>
> Cc: linux-kernel@vger.kernel.org; linux-mm@kvack.org;
> hannes@cmpxchg.org; yosry.ahmed@linux.dev; nphamcs@gmail.com;
> chengming.zhou@linux.dev; usamaarif642@gmail.com;
> ryan.roberts@arm.com; ying.huang@linux.alibaba.com; akpm@linux-
> foundation.org; senozhatsky@chromium.org; linux-crypto@vger.kernel.org;
> herbert@gondor.apana.org.au; davem@davemloft.net;
> clabbe@baylibre.com; ardb@kernel.org; ebiggers@google.com;
> surenb@google.com; Accardi, Kristen C <kristen.c.accardi@intel.com>;
> Gomes, Vinicius <vinicius.gomes@intel.com>; Feghali, Wajdi K
> <wajdi.k.feghali@intel.com>; Gopal, Vinodh <vinodh.gopal@intel.com>
> Subject: Re: [PATCH v11 22/24] mm: zswap: Allocate pool batching resources
> if the compressor supports batching.
> 
> Hi Kanchana,
> 
> 
> [...]
> >
> > +       /*
> > +        * 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;
> >
> 
> It’s hard to follow — you add batch_size and compr_batch_size in this
> patch, but only use them in another. Could we merge the related changes
> into one patch instead of splitting them into several that don’t work
> independently?

Hi Barry,

Thanks for reviewing the code and for your comments! Sure, I can merge
this patch with the next one. I was trying to keep the changes modularized
to a) zswap_cpu_comp_prepare(), b) zswap_store() and c) zswap_compress()
so the changes are broken into smaller parts, but I can see how this can
make the changes appear disjointed.

One thing though: the commit logs for each of the patches will
also probably need to be merged, since I have tried to explain the
changes in detail.

Thanks,
Kanchana




> 
> > -
> >         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);
> >                 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;
> > +       }
> 
> It’s hard to follow — memory is allocated here but only used in another
> patch. Could we merge the related changes into a single patch instead of
> splitting them into several that don’t work independently?
> 
> >
> >         /*
> >          * 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];
> 
> Hard to understand what is going on if related changes are not kept in
> one self-contained patch.
> 
> Thanks
> Barry