[PATCH] Kill PF_SYNCWRITE flag

A process flag to indicate whether we are doing sync io is incredibly
ugly. It also causes performance problems when one does a lot of async
io and then proceeds to sync it. Part of the io will go out as async,
and the other part as sync. This causes a disconnect between the
previously submitted io and the synced io. For io schedulers such as CFQ,
this will cause us lost merges and suboptimal behaviour in scheduling.

Remove PF_SYNCWRITE completely from the fsync/msync paths, and let
the O_DIRECT path just directly indicate that the writes are sync
by using WRITE_SYNC instead.

Signed-off-by: Jens Axboe <axboe@suse.de>

include/linux/blkdev.h

@@ -241,6 +241,7 @@ enum rq_flag_bits {
 	__REQ_PM_RESUME,	/* resume request */
 	__REQ_PM_SHUTDOWN,	/* shutdown request */
 	__REQ_ORDERED_COLOR,	/* is before or after barrier */
+	__REQ_RW_SYNC,		/* request is sync (O_DIRECT) */
 	__REQ_NR_BITS,		/* stops here */
 };
 
@@ -270,6 +271,7 @@ enum rq_flag_bits {
 #define REQ_PM_RESUME	(1 << __REQ_PM_RESUME)
 #define REQ_PM_SHUTDOWN	(1 << __REQ_PM_SHUTDOWN)
 #define REQ_ORDERED_COLOR	(1 << __REQ_ORDERED_COLOR)
+#define REQ_RW_SYNC	(1 << __REQ_RW_SYNC)
 
 /*
  * State information carried for REQ_PM_SUSPEND and REQ_PM_RESUME