ext4: optimize ext4_writepage() for attempted 4k delalloc writes

In cases where the file system block size is the same as the page
size, and ext4_writepage() is asked to write out a page which is
either has the unwritten bit set in the extent tree, or which does not
yet have a block assigned due to delayed allocation, we can bail out
early and, unlocking the page earlier and avoiding a round trip
through ext4_bio_write_page() with the attendant calls to
set_page_writeback() and redirty_page_for_writeback().

Signed-off-by: Theodore Ts'o <tytso@mit.edu>

1 files changed, 12 insertions, 1 deletions

diff --git a/fs/ext4/inode.c b/fs/ext4/inode.c
index 612fbcf76b5c..c95d40658979 100644
--- a/fs/ext4/inode.c
+++ b/fs/ext4/inode.c
@@ -1815,11 +1815,22 @@ static int ext4_writepage(struct page *page,
          * the page. But we may reach here when we do a journal commit via
          * journal_submit_inode_data_buffers() and in that case we must write
          * allocated buffers to achieve data=ordered mode guarantees.
+         *
+         * Also, if there is only one buffer per page (the fs block
+         * size == the page size), if one buffer needs block
+         * allocation or needs to modify the extent tree to clear the
+         * unwritten flag, we know that the page can't be written at
+         * all, so we might as well refuse the write immediately.
+         * Unfortunately if the block size != page size, we can't as
+         * easily detect this case using ext4_walk_page_buffers(), but
+         * for the extremely common case, this is an optimization that
+         * skips a useless round trip through ext4_bio_write_page().
          */
         if (ext4_walk_page_buffers(NULL, page_bufs, 0, len, NULL,
                                    ext4_bh_delay_or_unwritten)) {
                 redirty_page_for_writepage(wbc, page);
-                if (current->flags & PF_MEMALLOC) {
+                if ((current->flags & PF_MEMALLOC) ||
+                    (inode->i_sb->s_blocksize == PAGE_CACHE_SIZE)) {
                         /*
                          * For memory cleaning there's no point in writing only
                          * some buffers. So just bail out. Warn if we came here