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.
