The real chunk_exponent maximum is 30 (FIO_MAX == 1<<30)

src/fellow_cache.c

@@ -1836,8 +1836,6 @@ fellow_busy_region_alloc(struct fellow_busy *fbo, size_t size, int8_t cram)
 	CHECK_OBJ_NOTNULL(fbo, FELLOW_BUSY_MAGIC);
 
 	AN(size);
-	if (size > FIO_MAX)
-		size = FIO_MAX;
 
 	fc = fbo->fc;
 	CHECK_OBJ_NOTNULL(fc, FELLOW_CACHE_MAGIC);
@@ -3553,6 +3551,7 @@ fellow_busy_obj_getspace(struct fellow_busy *fbo, size_t *sz, uint8_t **ptr)
 	CHECK_OBJ_NOTNULL(fbo->fc, FELLOW_CACHE_MAGIC);
 	CHECK_OBJ_NOTNULL(fbo->fc->tune, STVFE_TUNE_MAGIC);
 	max = (size_t)1 << fbo->fc->tune->chunk_exponent;
+	assert(max <= FIO_MAX);
 
 	/*
 	 * for content-length, varnish-cache will ask for less and less.

src/tbl/fellow_tunables.h

@@ -40,7 +40,7 @@ TUNE(float, logbuffer_flush_interval, 2.0, 0.0, FLT_MAX);
 TUNE(float, log_rewrite_ratio, 0.5, 0.001, FLT_MAX);
 
 // reserve chunk is the larger of chunk_exponent and result from logbuffer size
-TUNE(unsigned, chunk_exponent, 20 /* 1MB*/, 12 /* 4KB */, 28 /* 256MB */);
+TUNE(unsigned, chunk_exponent, 20 /* 1MB*/, 12 /* 4KB */, 30 /* 1GB */);
 TUNE(uint8_t, wait_table_exponent, 10, 6, 32);
 TUNE(unsigned, dsk_reserve_chunks, 4, 2, UINT_MAX);
 TUNE(unsigned, mem_reserve_chunks, 1, 0, UINT_MAX);