mm: Add NR_WRITEBACK_TEMP counter

Fuse will use temporary buffers to write back dirty data from memory mappings
(normal writes are done synchronously). This is needed, because there cannot
be any guarantee about the time in which a write will complete.

By using temporary buffers, from the MM's point if view the page is written
back immediately. If the writeout was due to memory pressure, this
effectively migrates data from a full zone to a less full zone.

This patch adds a new counter (NR_WRITEBACK_TEMP) for the number of pages used
as temporary buffers.

[[e-mail]: add vmstat_text for NR_WRITEBACK_TEMP]
Signed-off-by: Miklos Szeredi <[e-mail]>
Cc: Christoph Lameter <[e-mail]>
Signed-off-by: Lee Schermerhorn <[e-mail]>
Signed-off-by: Andrew Morton <[e-mail]>
Signed-off-by: Linus Torvalds <[e-mail]>

drivers/base/node.c

···
 		       "Node %d PageTables:   %8lu kB\n"
 		       "Node %d NFS_Unstable: %8lu kB\n"
 		       "Node %d Bounce:       %8lu kB\n"
+		       "Node %d WritebackTmp: %8lu kB\n"
 		       "Node %d Slab:         %8lu kB\n"
 		       "Node %d SReclaimable: %8lu kB\n"
 		       "Node %d SUnreclaim:   %8lu kB\n",
···
 		       nid, K(node_page_state(nid, NR_PAGETABLE)),
 		       nid, K(node_page_state(nid, NR_UNSTABLE_NFS)),
 		       nid, K(node_page_state(nid, NR_BOUNCE)),
+		       nid, K(node_page_state(nid, NR_WRITEBACK_TEMP)),
 		       nid, K(node_page_state(nid, NR_SLAB_RECLAIMABLE) +
 				node_page_state(nid, NR_SLAB_UNRECLAIMABLE)),
 		       nid, K(node_page_state(nid, NR_SLAB_RECLAIMABLE)),

fs/proc/proc_misc.c

···
 		"PageTables:   %8lu kB\n"
 		"NFS_Unstable: %8lu kB\n"
 		"Bounce:       %8lu kB\n"
+		"WritebackTmp: %8lu kB\n"
 		"CommitLimit:  %8lu kB\n"
 		"Committed_AS: %8lu kB\n"
 		"VmallocTotal: %8lu kB\n"
···
 		K(global_page_state(NR_PAGETABLE)),
 		K(global_page_state(NR_UNSTABLE_NFS)),
 		K(global_page_state(NR_BOUNCE)),
+		K(global_page_state(NR_WRITEBACK_TEMP)),
 		K(allowed),
 		K(committed),
 		(unsigned long)VMALLOC_TOTAL >> 10,

include/linux/mmzone.h

···
 	NR_UNSTABLE_NFS,	/* NFS unstable pages */
 	NR_BOUNCE,
 	NR_VMSCAN_WRITE,
+	NR_WRITEBACK_TEMP,	/* Writeback using temporary buffers */
 #ifdef CONFIG_NUMA
 	NUMA_HIT,		/* allocated in intended node */
 	NUMA_MISS,		/* allocated in non intended node */

mm/page-writeback.c

···
 	avail_dirty = dirty -
 		(global_page_state(NR_FILE_DIRTY) +
 		 global_page_state(NR_WRITEBACK) +
-		 global_page_state(NR_UNSTABLE_NFS));
+		 global_page_state(NR_UNSTABLE_NFS) +
+		 global_page_state(NR_WRITEBACK_TEMP));
 
 	if (avail_dirty < 0)
 		avail_dirty = 0;

mm/vmstat.c

···
 	"nr_unstable",
 	"nr_bounce",
 	"nr_vmscan_write",
+	"nr_writeback_temp",
 
 #ifdef CONFIG_NUMA
 	"numa_hit",