btrfs: improve the delalloc_to_write calculation in writepage_delalloc

Currently writepage_delalloc adds to delalloc_to_write in every loop
operation.  That is not only more work than doing it once after the
loop, but can also over-increment the counter due to rounding errors
when a new loop iteration starts with an offset into a page.

Add a new page_start variable instead of recaculation that value over
and over, move the delalloc_to_write calculation out of the loop, use
the DIV_ROUND_UP helper instead of open coding it and remove the pointless
found local variable.

Reviewed-by: Josef Bacik <josef@toxicpanda.com>
Signed-off-by: Christoph Hellwig <hch@lst.de>
Reviewed-by: David Sterba <dsterba@suse.com>
Signed-off-by: David Sterba <dsterba@suse.com>

diff --git a/fs/btrfs/extent_io.c b/fs/btrfs/extent_io.c
index 4981058..148d289 100644 (file)
--- a/fs/btrfs/extent_io.c
+++ b/fs/btrfs/extent_io.c
@@ -1185,8 +1185,10 @@ static inline void contiguous_readpages(struct page *pages[], int nr_pages,
 static noinline_for_stack int writepage_delalloc(struct btrfs_inode *inode,
                struct page *page, struct writeback_control *wbc)
 {
-       const u64 page_end = page_offset(page) + PAGE_SIZE - 1;
-       u64 delalloc_start = page_offset(page);
+       const u64 page_start = page_offset(page);
+       const u64 page_end = page_start + PAGE_SIZE - 1;
+       u64 delalloc_start = page_start;
+       u64 delalloc_end = page_end;
        u64 delalloc_to_write = 0;
        /* How many pages are started by btrfs_run_delalloc_range() */
        unsigned long nr_written = 0;
@@ -1194,13 +1196,9 @@ static noinline_for_stack int writepage_delalloc(struct btrfs_inode *inode,
        int page_started = 0;
 
        while (delalloc_start < page_end) {
-               u64 delalloc_end = page_end;
-               bool found;
-
-               found = find_lock_delalloc_range(&inode->vfs_inode, page,
-                                              &delalloc_start,
-                                              &delalloc_end);
-               if (!found) {
+               delalloc_end = page_end;
+               if (!find_lock_delalloc_range(&inode->vfs_inode, page,
+                                             &delalloc_start, &delalloc_end)) {
                        delalloc_start = delalloc_end + 1;
                        continue;
                }
@@ -1209,14 +1207,15 @@ static noinline_for_stack int writepage_delalloc(struct btrfs_inode *inode,
                if (ret)
                        return ret;
 
-               /*
-                * delalloc_end is already one less than the total length, so
-                * we don't subtract one from PAGE_SIZE
-                */
-               delalloc_to_write += (delalloc_end - delalloc_start +
-                                     PAGE_SIZE) >> PAGE_SHIFT;
                delalloc_start = delalloc_end + 1;
        }
+
+       /*
+        * delalloc_end is already one less than the total length, so
+        * we don't subtract one from PAGE_SIZE
+        */
+       delalloc_to_write +=
+               DIV_ROUND_UP(delalloc_end + 1 - page_start, PAGE_SIZE);
        if (wbc->nr_to_write < delalloc_to_write) {
                int thresh = 8192;