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

[Andrew Morton <akpm@linux-foundation.org>]

(cc linux-fsdevel and Jan)

On Mon, 22 Jul 2019 12:36:08 +0300 Konstantin Khlebnikov <khlebnikov@yandex-team.ru> 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?