--- a/src/core/ngx_slab.c
+++ b/src/core/ngx_slab.c
@@ -154,6 +154,7 @@ ngx_slab_init(ngx_slab_pool_t *pool)
     }
 
     pool->last = pool->pages + pages;
+    pool->pfree = pages;
 
     pool->log_nomem = 1;
     pool->log_ctx = &pool->zero;
@@ -691,6 +692,8 @@ ngx_slab_alloc_pages(ngx_slab_pool_t *po
             page->next = NULL;
             page->prev = NGX_SLAB_PAGE;
 
+            pool->pfree -= pages;
+
             if (--pages == 0) {
                 return page;
             }
@@ -721,6 +724,8 @@ ngx_slab_free_pages(ngx_slab_pool_t *poo
 {
     ngx_slab_page_t  *prev, *join;
 
+    pool->pfree += pages;
+
     page->slab = pages--;
 
     if (pages) {