(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?
