Re: [PATCH 06/10] zram: lz4hc: use LZ4_decompress_safe_usingDict()

[Brian Geffon]

On Mon, Oct 5, 2026 at 9:21 PM Sergey Senozhatsky
<senozhatsky@xxxxxxxxxxxx> wrote:
>
> Use LZ4_decompress_safe_usingDict() in lz4hc_decompress() to avoid
> needing a per-CPU LZ4_streamDecode_t structure.  Drop struct lz4hc_ctx
> as well and store the compression workspace directly in ctx->context,
> matching backend_lz4.
>
> Using vmalloc(LZ4HC_MEM_COMPRESS) instead of kzalloc() for cstrm when a
> dictionary is configured also avoids power-of-2 page rounding (~256 KB
> rounded up to 512 KB), saving ~252 KB per CPU.
>
> Signed-off-by: Sergey Senozhatsky <senozhatsky@xxxxxxxxxxxx>

Acked-by: Brian Geffon <bgeffon@xxxxxxxxxx>

> ---
>  drivers/block/zram/backend_lz4hc.c | 70 +++++-------------------------
>  1 file changed, 11 insertions(+), 59 deletions(-)
>
> diff --git a/drivers/block/zram/backend_lz4hc.c b/drivers/block/zram/backend_lz4hc.c
> index f3656c27af21..b62736b26622 100644
> --- a/drivers/block/zram/backend_lz4hc.c
> +++ b/drivers/block/zram/backend_lz4hc.c
> @@ -4,18 +4,10 @@
>
>  #include <linux/kernel.h>
>  #include <linux/lz4.h>
> -#include <linux/slab.h>
>  #include <linux/vmalloc.h>
>
>  #include "backend_lz4hc.h"
>
> -struct lz4hc_ctx {
> -       void *mem;
> -
> -       LZ4_streamDecode_t *dstrm;
> -       LZ4_streamHC_t *cstrm;
> -};
> -
>  static void lz4hc_release_params(struct zcomp_params *params)
>  {
>         LZ4_streamHC_t *dict_stream = params->drv_data;
> @@ -65,61 +57,32 @@ static int lz4hc_setup_params(struct zcomp_params *params)
>
>  static void lz4hc_destroy(struct zcomp_ctx *ctx)
>  {
> -       struct lz4hc_ctx *zctx = ctx->context;
> -
> -       if (!zctx)
> -               return;
> -
> -       kfree(zctx->dstrm);
> -       kfree(zctx->cstrm);
> -       vfree(zctx->mem);
> -       kfree(zctx);
> +       vfree(ctx->context);
>  }
>
>  static int lz4hc_create(struct zcomp_params *params, struct zcomp_ctx *ctx)
>  {
> -       struct lz4hc_ctx *zctx;
> +       BUILD_BUG_ON(sizeof(LZ4_streamHC_t) != LZ4HC_MEM_COMPRESS);
>
> -       zctx = kzalloc_obj(*zctx);
> -       if (!zctx)
> +       ctx->context = vmalloc(LZ4HC_MEM_COMPRESS);
> +       if (!ctx->context)
>                 return -ENOMEM;
> -
> -       ctx->context = zctx;
> -       if (params->dict_sz == 0) {
> -               zctx->mem = vmalloc(LZ4HC_MEM_COMPRESS);
> -               if (!zctx->mem)
> -                       goto error;
> -       } else {
> -               zctx->dstrm = kzalloc_obj(*zctx->dstrm);
> -               if (!zctx->dstrm)
> -                       goto error;
> -
> -               zctx->cstrm = kzalloc_obj(*zctx->cstrm);
> -               if (!zctx->cstrm)
> -                       goto error;
> -       }
> -
>         return 0;
> -
> -error:
> -       lz4hc_destroy(ctx);
> -       return -EINVAL;
>  }
>
>  static int lz4hc_compress(struct zcomp_params *params, struct zcomp_ctx *ctx,
>                           struct zcomp_req *req)
>  {
> -       struct lz4hc_ctx *zctx = ctx->context;
>         int ret;
>
> -       if (!zctx->cstrm) {
> +       if (!params->drv_data) {
>                 ret = LZ4_compress_HC(req->src, req->dst, req->src_len,
>                                       req->dst_len, params->level,
> -                                     zctx->mem);
> +                                     ctx->context);
>         } else {
>                 /* Cstrm needs to be reset */
> -               memcpy(zctx->cstrm, params->drv_data, sizeof(*zctx->cstrm));
> -               ret = LZ4_compress_HC_continue(zctx->cstrm, req->src, req->dst,
> +               memcpy(ctx->context, params->drv_data, sizeof(LZ4_streamHC_t));
> +               ret = LZ4_compress_HC_continue(ctx->context, req->src, req->dst,
>                                                req->src_len, req->dst_len);
>         }
>         if (!ret)
> @@ -131,22 +94,11 @@ static int lz4hc_compress(struct zcomp_params *params, struct zcomp_ctx *ctx,
>  static int lz4hc_decompress(struct zcomp_params *params, struct zcomp_ctx *ctx,
>                             struct zcomp_req *req)
>  {
> -       struct lz4hc_ctx *zctx = ctx->context;
>         int ret;
>
> -       if (!zctx->dstrm) {
> -               ret = LZ4_decompress_safe(req->src, req->dst, req->src_len,
> -                                         req->dst_len);
> -       } else {
> -               /* Dstrm needs to be reset */
> -               ret = LZ4_setStreamDecode(zctx->dstrm, params->dict,
> -                                         params->dict_sz);
> -               if (!ret)
> -                       return -EINVAL;
> -               ret = LZ4_decompress_safe_continue(zctx->dstrm, req->src,
> -                                                  req->dst, req->src_len,
> -                                                  req->dst_len);
> -       }
> +       ret = LZ4_decompress_safe_usingDict(req->src, req->dst, req->src_len,
> +                                           req->dst_len, params->dict,
> +                                           params->dict_sz);
>         if (ret < 0)
>                 return -EINVAL;
>         return 0;
> --
> 2.56.0.rc1.315.gc6ed9934b7-goog
>