[PATCH] writeback: fix range handling

When a writeback_control's `start' and `end' fields are used to
indicate a one-byte-range starting at file offset zero, the required
values of .start=0,.end=0 mean that the ->writepages() implementation
has no way of telling that it is being asked to perform a range
request.  Because we're currently overloading (start == 0 && end == 0)
to mean "this is not a write-a-range request".

To make all this sane, the patch changes range of writeback_control.

So caller does: If it is calling ->writepages() to write pages, it
sets range (range_start/end or range_cyclic) always.

And if range_cyclic is true, ->writepages() thinks the range is
cyclic, otherwise it just uses range_start and range_end.

This patch does,

    - Add LLONG_MAX, LLONG_MIN, ULLONG_MAX to include/linux/kernel.h
      -1 is usually ok for range_end (type is long long). But, if someone did,

		range_end += val;		range_end is "val - 1"
		u64val = range_end >> bits;	u64val is "~(0ULL)"

      or something, they are wrong. So, this adds LLONG_MAX to avoid nasty
      things, and uses LLONG_MAX for range_end.

    - All callers of ->writepages() sets range_start/end or range_cyclic.

    - Fix updates of ->writeback_index. It seems already bit strange.
      If it starts at 0 and ended by check of nr_to_write, this last
      index may reduce chance to scan end of file.  So, this updates
      ->writeback_index only if range_cyclic is true or whole-file is
      scanned.

Signed-off-by: OGAWA Hirofumi <hirofumi@mail.parknet.co.jp>
Cc: Nathan Scott <nathans@sgi.com>
Cc: Anton Altaparmakov <aia21@cantab.net>
Cc: Steven French <sfrench@us.ibm.com>
Cc: "Vladimir V. Saveliev" <vs@namesys.com>
Signed-off-by: Andrew Morton <akpm@osdl.org>
Signed-off-by: Linus Torvalds <torvalds@osdl.org>

2 files changed, 6 insertions, 2 deletions

diff --git a/include/linux/kernel.h b/include/linux/kernel.h
index f4fc576ed4c4..25fccd859fbf 100644
--- a/include/linux/kernel.h
+++ b/include/linux/kernel.h
@@ -24,6 +24,9 @@ extern const char linux_banner[];
 #define LONG_MAX	((long)(~0UL>>1))
 #define LONG_MIN	(-LONG_MAX - 1)
 #define ULONG_MAX	(~0UL)
+#define LLONG_MAX	((long long)(~0ULL>>1))
+#define LLONG_MIN	(-LLONG_MAX - 1)
+#define ULLONG_MAX	(~0ULL)
 
 #define STACK_MAGIC	0xdeadbeef
 
diff --git a/include/linux/writeback.h b/include/linux/writeback.h
index 56f92fcbe94a..9e38b566d0e7 100644
--- a/include/linux/writeback.h
+++ b/include/linux/writeback.h
@@ -50,14 +50,15 @@ struct writeback_control {
 	 * a hint that the filesystem need only write out the pages inside that
 	 * byterange.  The byte at `end' is included in the writeout request.
 	 */
-	loff_t start;
-	loff_t end;
+	loff_t range_start;
+	loff_t range_end;
 
 	unsigned nonblocking:1;		/* Don't get stuck on request queues */
 	unsigned encountered_congestion:1; /* An output: a queue is full */
 	unsigned for_kupdate:1;		/* A kupdate writeback */
 	unsigned for_reclaim:1;		/* Invoked from the page allocator */
 	unsigned for_writepages:1;	/* This is a writepages() call */
+	unsigned range_cyclic:1;	/* range_start is cyclic */
 };
 
 /*