library: fallback MemAvailable evaluation if missing

This commit adds support for fallback calculation
of the MemAvailable field if not exported by the
kernel. The MemAvailable field appeared in kernel
3.14, but it's possible to calculate it from other
fields since 2.6.27 (splitLRU changes).

proc/sysinfo.c

@@ -54,6 +54,8 @@ static int loadavg_fd = -1;
 static int meminfo_fd = -1;
 #define VMINFO_FILE "/proc/vmstat"
 static int vminfo_fd = -1;
+#define VM_MIN_FREE_FILE "/proc/sys/vm/min_free_kbytes"
+static int vm_min_free_fd = -1;
 
 // As of 2.6.24 /proc/meminfo seems to need 888 on 64-bit,
 // and would need 1258 if the obsolete fields were there.
@@ -84,6 +86,8 @@ static char buf[8192];
 /* evals 'x' twice */
 #define SET_IF_DESIRED(x,y) do{  if(x) *(x) = (y); }while(0)
 
+/* return minimum of two values */
+#define MIN(x,y) ((x) < (y) ? (x) : (y))
 
 /***********************************************************************/
 int uptime(double *restrict uptime_secs, double *restrict idle_secs) {
@@ -651,6 +655,8 @@ void meminfo(void){
   {"Writeback",    &kb_writeback},    // kB version of vmstat nr_writeback
   };
   const int mem_table_count = sizeof(mem_table)/sizeof(mem_table_struct);
+  unsigned long watermark_low;
+  signed long mem_available;
 
   FILE_TO_BUF(MEMINFO_FILE,meminfo_fd);
 
@@ -686,6 +692,20 @@ nextline:
   }
   kb_swap_used = kb_swap_total - kb_swap_free;
   kb_main_used = kb_main_total - kb_main_free;
+
+  /* zero? might need fallback for 2.6.27 <= kernel <? 3.14 */
+  if (!kb_main_available && linux_version_code >= 20627) {
+    vminfo();
+    watermark_low = vm_min_free * 5 / 4; /* should be equal to sum of all 'low' fields in /proc/zoneinfo */
+
+    mem_available = (signed long)vm_nr_free_pages + vm_nr_inactive_file + vm_nr_active_file
+    - MIN((vm_nr_inactive_file + vm_nr_active_file) / 2, watermark_low)
+    + vm_nr_slab_reclaimable - MIN(vm_nr_slab_reclaimable / 2, watermark_low)
+    - watermark_low;
+
+    if (mem_available < 0) mem_available = 0;
+    kb_main_available = (unsigned long)((unsigned long long)mem_available * sysconf(_SC_PAGESIZE) / 1024ull);
+  }
 }
 
 /*****************************************************************/
@@ -709,6 +729,12 @@ unsigned long vm_nr_page_table_pages;// pages used for pagetables
 unsigned long vm_nr_reverse_maps;    // includes PageDirect
 unsigned long vm_nr_mapped;          // mapped into pagetables
 unsigned long vm_nr_slab;            // in slab
+unsigned long vm_nr_slab_reclaimable;  // 2.6.19+ kernels
+unsigned long vm_nr_slab_unreclaimable;// 2.6.19+ kernels
+unsigned long vm_nr_active_file;       // 2.6.27+ kernels
+unsigned long vm_nr_inactive_file;     // 2.6.27+ kernels
+unsigned long vm_nr_free_pages;        // 2.6.21+ kernels
+unsigned long vm_min_free;             // calculated from /proc/sys/vm/min_free_kbytes
 unsigned long vm_pgpgin;             // kB disk reads  (same as 1st num on /proc/stat page line)
 unsigned long vm_pgpgout;            // kB disk writes (same as 2nd num on /proc/stat page line)
 unsigned long vm_pswpin;             // swap reads     (same as 1st num on /proc/stat swap line)
@@ -759,12 +785,17 @@ void vminfo(void){
   {"allocstall",          &vm_allocstall},
   {"kswapd_inodesteal",   &vm_kswapd_inodesteal},
   {"kswapd_steal",        &vm_kswapd_steal},
+  {"nr_active_file",      &vm_nr_active_file},     // 2.6.27+ kernels
   {"nr_dirty",            &vm_nr_dirty},           // page version of meminfo Dirty
+  {"nr_free_pages",       &vm_nr_free_pages},      // 2.6.21+ kernels
+  {"nr_inactive_file",    &vm_nr_inactive_file},   // 2.6.27+ kernels
   {"nr_mapped",           &vm_nr_mapped},          // page version of meminfo Mapped
   {"nr_page_table_pages", &vm_nr_page_table_pages},// same as meminfo PageTables
   {"nr_pagecache",        &vm_nr_pagecache},       // gone in 2.5.66+ kernels
   {"nr_reverse_maps",     &vm_nr_reverse_maps},    // page version of meminfo ReverseMaps GONE
-  {"nr_slab",             &vm_nr_slab},            // page version of meminfo Slab
+  {"nr_slab",             &vm_nr_slab},            // page version of meminfo Slab (gone in 2.6.19+)
+  {"nr_slab_reclaimable", &vm_nr_slab_reclaimable},// 2.6.19+ kernels
+ {"nr_slab_unreclaimable",&vm_nr_slab_unreclaimable},// 2.6.19+ kernels
   {"nr_unstable",         &vm_nr_unstable},
   {"nr_writeback",        &vm_nr_writeback},       // page version of meminfo Writeback
   {"pageoutrun",          &vm_pageoutrun},
@@ -855,6 +886,9 @@ nextline:
                 + vm_pgscan_kswapd_dma + vm_pgscan_kswapd_high + vm_pgscan_kswapd_normal;
   if(!vm_pgsteal)
     vm_pgsteal  = vm_pgsteal_dma + vm_pgsteal_high + vm_pgsteal_normal;
+
+  FILE_TO_BUF(VM_MIN_FREE_FILE, vm_min_free_fd);
+  vm_min_free = (unsigned long) (strtoull(buf,&tail,10) * 1024ull / sysconf(_SC_PAGESIZE));
 }
 
 ///////////////////////////////////////////////////////////////////////

proc/sysinfo.h

@@ -73,6 +73,12 @@ extern unsigned long vm_nr_page_table_pages;
 extern unsigned long vm_nr_reverse_maps;
 extern unsigned long vm_nr_mapped;
 extern unsigned long vm_nr_slab;
+extern unsigned long vm_nr_slab_reclaimable;
+extern unsigned long vm_nr_slab_unreclaimable;
+extern unsigned long vm_nr_active_file;
+extern unsigned long vm_nr_inactive_file;
+extern unsigned long vm_nr_free_pages;
+extern unsigned long vm_min_free;
 extern unsigned long vm_pgpgin;
 extern unsigned long vm_pgpgout;
 extern unsigned long vm_pswpin;