Commit cccd147a authored by Theodore Ts'o's avatar Theodore Ts'o
Browse files

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: default avatarTheodore Ts'o <tytso@mit.edu>
parent 937d7b84

fs/ext4/inode.c

+12 −1
Original line number Diff line number Diff line
@@ -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

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