Re: [PATCH 1/2] mm/filemap: don't initiate writeback if mapping has no dirty pages

From: Konstantin Khlebnikov
Date: Tue Jul 23 2019 - 04:16:57 EST

On 23.07.2019 3:52, Andrew Morton wrote:

(cc linux-fsdevel and Jan)

On Mon, 22 Jul 2019 12:36:08 +0300 Konstantin Khlebnikov <khlebnikov@xxxxxxxxxxxxxx> wrote:

Functions like filemap_write_and_wait_range() should do nothing if inode
has no dirty pages or pages currently under writeback. But they anyway
construct struct writeback_control and this does some atomic operations
if CONFIG_CGROUP_WRITEBACK=y - on fast path it locks inode->i_lock and
updates state of writeback ownership, on slow path might be more work.
Current this path is safely avoided only when inode mapping has no pages.

For example generic_file_read_iter() calls filemap_write_and_wait_range()
at each O_DIRECT read - pretty hot path.

This patch skips starting new writeback if mapping has no dirty tags set.
If writeback is already in progress filemap_write_and_wait_range() will
wait for it.


--- a/mm/filemap.c
+++ b/mm/filemap.c
@@ -408,7 +408,8 @@ int __filemap_fdatawrite_range(struct address_space *mapping, loff_t start,
.range_end = end,
- if (!mapping_cap_writeback_dirty(mapping))
+ if (!mapping_cap_writeback_dirty(mapping) ||
+ !mapping_tagged(mapping, PAGECACHE_TAG_DIRTY))
return 0;
wbc_attach_fdatawrite_inode(&wbc, mapping->host);

How does this play with tagged_writepages? We assume that no tagging
has been performed by any __filemap_fdatawrite_range() caller?

Checking also PAGECACHE_TAG_TOWRITE is cheap but seems redundant.

To-write tags are supposed to be a subset of dirty tags:
to-write is set only when dirty is set and cleared after starting writeback.

Special case set_page_writeback_keepwrite() which does not clear to-write
should be for dirty page thus dirty tag is not going to be cleared either.
Ext4 calls it after redirty_page_for_writepage()
XFS even without clear_page_dirty_for_io()

Anyway to-write tag without dirty tag or at clear page is confusing.