Btrfs: properly honor wbc->nr_to_write changes

When btrfs fills a delayed allocation, it tries to increase
the wbc nr_to_write to cover a big part of allocation.  The
theory is that we're doing contiguous IO and writing a few
more blocks will save seeks overall at a very low cost.

The problem is that extent_write_cache_pages could ignore
the new higher nr_to_write if nr_to_write had already gone
down to zero.  We fix that by rechecking the nr_to_write
for every page that is processed in the pagevec.

This updates the math around bumping the nr_to_write value
to make sure we don't leave a tiny amount of IO hanging
around for the very end of a new extent.

Signed-off-by: Chris Mason <chris.mason@oracle.com>

1 files changed, 27 insertions, 11 deletions

diff --git a/fs/btrfs/extent_io.c b/fs/btrfs/extent_io.c
index a102422cd92e..7e16c6d8153f 100644
--- a/fs/btrfs/extent_io.c
+++ b/fs/btrfs/extent_io.c
@@ -2182,7 +2182,7 @@ static int __extent_writepage(struct page *page, struct writeback_control *wbc,
         delalloc_end = 0;
         page_started = 0;
         if (!epd->extent_locked) {
-                u64 delalloc_to_write;
+                u64 delalloc_to_write = 0;
                 /*
                  * make sure the wbc mapping index is at least updated
                  * to this page.
@@ -2202,16 +2202,24 @@ static int __extent_writepage(struct page *page, struct writeback_control *wbc,
                         tree->ops->fill_delalloc(inode, page, delalloc_start,
                                                  delalloc_end, &page_started,
                                                  &nr_written);
-                        delalloc_to_write = (delalloc_end -
-                                        max_t(u64, page_offset(page),
-                                              delalloc_start) + 1) >>
-                                        PAGE_CACHE_SHIFT;
-                        if (wbc->nr_to_write < delalloc_to_write) {
-                                wbc->nr_to_write = min_t(long, 8192,
-                                                 delalloc_to_write);
-                        }
+                        /*
+                         * delalloc_end is already one less than the total
+                         * length, so we don't subtract one from
+                         * PAGE_CACHE_SIZE
+                         */
+                        delalloc_to_write += (delalloc_end - delalloc_start +
+                                              PAGE_CACHE_SIZE) >>
+                                              PAGE_CACHE_SHIFT;
                         delalloc_start = delalloc_end + 1;
                 }
+                if (wbc->nr_to_write < delalloc_to_write) {
+                        int thresh = 8192;
+
+                        if (delalloc_to_write < thresh * 2)
+                                thresh = delalloc_to_write;
+                        wbc->nr_to_write = min_t(u64, delalloc_to_write,
+                                                 thresh);
+                }
 
                 /* did the fill delalloc function already unlock and start
                  * the IO?
@@ -2388,6 +2396,7 @@ static int extent_write_cache_pages(struct extent_io_tree *tree,
 {
         int ret = 0;
         int done = 0;
+        int nr_to_write_done = 0;
         struct pagevec pvec;
         int nr_pages;
         pgoff_t index;
@@ -2407,7 +2416,7 @@ static int extent_write_cache_pages(struct extent_io_tree *tree,
                 scanned = 1;
         }
 retry:
-        while (!done && (index <= end) &&
+        while (!done && !nr_to_write_done && (index <= end) &&
                (nr_pages = pagevec_lookup_tag(&pvec, mapping, &index,
                               PAGECACHE_TAG_DIRTY, min(end - index,
                                   (pgoff_t)PAGEVEC_SIZE-1) + 1))) {
@@ -2458,8 +2467,15 @@ retry:
                                 unlock_page(page);
                                 ret = 0;
                         }
-                        if (ret || wbc->nr_to_write <= 0)
+                        if (ret)
                                 done = 1;
+
+                        /*
+                         * the filesystem may choose to bump up nr_to_write.
+                         * We have to make sure to honor the new nr_to_write
+                         * at any time
+                         */
+                        nr_to_write_done = wbc->nr_to_write <= 0;
                 }
                 pagevec_release(&pvec);
                 cond_resched();