zram: pass estimated src size hint to zstd

zram works with PAGE_SIZE buffers, so we always know exact size of the
source buffer and hence can pass estimated_src_size to zstd_get_params().

This hint on x86_64, for example, reduces the size of the work memory
buffer from 1303520 bytes down to 90080 bytes.  Given that compression
streams are per-CPU that's quite some memory saving.

Link: https://lkml.kernel.org/r/20240902105656.1383858-10-senozhatsky@chromium.orgSigned-off-by: Sergey Senozhatsky <senozhatsky@chromium.org>
Cc: Minchan Kim <minchan@kernel.org>
Cc: Nick Terrell <terrelln@fb.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>

drivers/block/zram/backend_zstd.c

@@ -35,7 +35,7 @@ static void *zstd_create(void)
 		return NULL;
 
 	ctx->level = zstd_default_clevel();
-	params = zstd_get_params(ctx->level, 0);
+	params = zstd_get_params(ctx->level, PAGE_SIZE);
 	sz = zstd_cctx_workspace_bound(&params.cParams);
 	ctx->cctx_mem = vzalloc(sz);
 	if (!ctx->cctx_mem)
@@ -65,7 +65,7 @@ static int zstd_compress(void *ctx, const unsigned char *src, size_t src_len,
 			 unsigned char *dst, size_t *dst_len)
 {
 	struct zstd_ctx *zctx = ctx;
-	const zstd_parameters params = zstd_get_params(zctx->level, 0);
+	const zstd_parameters params = zstd_get_params(zctx->level, PAGE_SIZE);
 	size_t ret;
 
 	ret = zstd_compress_cctx(zctx->cctx, dst, *dst_len,