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

[Konstantin Khlebnikov]

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.