[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>

1 files changed, 10 insertions, 12 deletions

diff --git a/fs/mpage.c b/fs/mpage.c
index 9bf2eb30e6f..1e4598247d0 100644
--- a/fs/mpage.c
+++ b/fs/mpage.c
@@ -707,9 +707,9 @@ mpage_writepages(struct address_space *mapping,
 	struct pagevec pvec;
 	int nr_pages;
 	pgoff_t index;
-	pgoff_t end = -1;		/* Inclusive */
+	pgoff_t end;		/* Inclusive */
 	int scanned = 0;
-	int is_range = 0;
+	int range_whole = 0;
 
 	if (wbc->nonblocking && bdi_write_congested(bdi)) {
 		wbc->encountered_congestion = 1;
@@ -721,16 +721,14 @@ mpage_writepages(struct address_space *mapping,
 		writepage = mapping->a_ops->writepage;
 
 	pagevec_init(&pvec, 0);
-	if (wbc->sync_mode == WB_SYNC_NONE) {
+	if (wbc->range_cyclic) {
 		index = mapping->writeback_index; /* Start from prev offset */
+		end = -1;
 	} else {
-		index = 0;			  /* whole-file sweep */
-		scanned = 1;
-	}
-	if (wbc->start || wbc->end) {
-		index = wbc->start >> PAGE_CACHE_SHIFT;
-		end = wbc->end >> PAGE_CACHE_SHIFT;
-		is_range = 1;
+		index = wbc->range_start >> PAGE_CACHE_SHIFT;
+		end = wbc->range_end >> PAGE_CACHE_SHIFT;
+		if (wbc->range_start == 0 && wbc->range_end == LLONG_MAX)
+			range_whole = 1;
 		scanned = 1;
 	}
 retry:
@@ -759,7 +757,7 @@ retry:
 				continue;
 			}
 
-			if (unlikely(is_range) && page->index > end) {
+			if (!wbc->range_cyclic && page->index > end) {
 				done = 1;
 				unlock_page(page);
 				continue;
@@ -810,7 +808,7 @@ retry:
 		index = 0;
 		goto retry;
 	}
-	if (!is_range)
+	if (wbc->range_cyclic || (range_whole && wbc->nr_to_write > 0))
 		mapping->writeback_index = index;
 	if (bio)
 		mpage_bio_submit(WRITE, bio);