Slab: free pages statistics.

src/core/ngx_slab.c

diff --git a/src/core/ngx_slab.c b/src/core/ngx_slab.c
index cc2e0b6..6695bb3 100644
--- a/src/core/ngx_slab.c
+++ b/src/core/ngx_slab.c
@@ -154,6 +154,7 @@
     }
 
     pool->last = pool->pages + pages;
+    pool->pfree = pages;
 
     pool->log_nomem = 1;
     pool->log_ctx = &pool->zero;
@@ -691,6 +692,8 @@
             page->next = NULL;
             page->prev = NGX_SLAB_PAGE;
 
+            pool->pfree -= pages;
+
             if (--pages == 0) {
                 return page;
             }
@@ -721,6 +724,8 @@
 {
     ngx_slab_page_t  *prev, *join;
 
+    pool->pfree += pages;
+
     page->slab = pages--;
 
     if (pages) {

src/core/ngx_slab.h

diff --git a/src/core/ngx_slab.h b/src/core/ngx_slab.h
index bd19321..eff893c 100644
--- a/src/core/ngx_slab.h
+++ b/src/core/ngx_slab.h
@@ -42,6 +42,7 @@
     ngx_slab_page_t   free;
 
     ngx_slab_stat_t  *stats;
+    ngx_uint_t        pfree;
 
     u_char           *start;
     u_char           *end;