Re: [PATCH v6 21/22] erofs: implement fscache-based data readahead

Gao Xiang
2022-03-28T10:55:51Z

On Fri, Mar 25, 2022 at 08:22:22PM +0800, Jeffle Xu wrote:
> Implements fscache-based data readahead. Also registers an individual
> bdi for each erofs instance to enable readahead.
>
> Signed-off-by: Jeffle Xu <jefflexu@linux.alibaba.com>
> ---
> fs/erofs/fscache.c | 114 +++++++++++++++++++++++++++++++++++++++++++++
> fs/erofs/super.c | 4 ++
> 2 files changed, 118 insertions(+)
>
> diff --git a/fs/erofs/fscache.c b/fs/erofs/fscache.c
> index cbb39657615e..589d1e7c2b1b 100644
> --- a/fs/erofs/fscache.c
> +++ b/fs/erofs/fscache.c
> @@ -191,12 +191,126 @@ static int erofs_fscache_readpage(struct file *file, struct page *page)
> return ret;
> }
>
> +static inline size_t erofs_fscache_calc_len(struct erofs_map_blocks *map,
> + size_t len, size_t done)
> +{
> + /*
> + * 1) For CHUNK_BASED layout, the output m_la is rounded down to the
> + * nearest chunk boundary, and the output m_llen actually starts from
> + * the start of the containing chunk.
> + * 2) For other cases, the output m_la is equal to o_la.
> + */
> + size_t length = map->m_llen - (map->o_la - map->m_la);
> +
> + return min_t(size_t, length, len - done);

This helper can be folded too.

> +}
> +
> +static inline void erofs_fscache_unlock_folios(struct readahead_control *rac,
> + size_t len)
> +{
> + while (len) {
> + struct folio *folio = readahead_folio(rac);
> +
> + len -= folio_size(folio);
> + folio_mark_uptodate(folio);
> + folio_unlock(folio);
> + }
> +}
> +
> +static void erofs_fscache_readahead(struct readahead_control *rac)
> +{
> + struct inode *inode = rac->mapping->host;
> + struct erofs_inode *vi = EROFS_I(inode);
> + struct super_block *sb = inode->i_sb;
> + size_t len, done = 0;
> + loff_t start;
> + int ret;
> +
> + if (erofs_inode_is_data_compressed(vi->datalayout)) {
> + erofs_info(sb, "compressed layout not supported yet");
> + return;
> + }

Redundant check.

> +
> + if (!readahead_count(rac))
> + return;
> +
> + start = readahead_pos(rac);
> + len = readahead_length(rac);
> +
> + do {
> + struct erofs_map_blocks map;
> +
> + map.m_la = map.o_la = start + done;
> +
> + ret = erofs_map_blocks(inode, &map, EROFS_GET_BLOCKS_RAW);
> + if (ret)
> + return;
> +
> + /* Read-ahead Hole
> + * Two cases will hit this:
> + * 1) EOF. Imposibble in readahead routine;
> + * 2) hole. Only CHUNK_BASED layout supports hole.
> + */

/*
*
*/

and typo `Imposibble'. Also I think this comment may not be useful
though.

> + if (!(map.m_flags & EROFS_MAP_MAPPED)) {
> + struct iov_iter iter;
> + loff_t offset = start + done;
> + size_t count = erofs_fscache_calc_len(&map, len, done);
> +
> + iov_iter_xarray(&iter, READ, &rac->mapping->i_pages, offset, count);
> + iov_iter_zero(count, &iter);
> +
> + erofs_fscache_unlock_folios(rac, count);
> + ret = count;
> + continue;
> + }
> +
> + ret = erofs_fscache_get_map(&map, sb);
> + if (ret)
> + return;
> +
> + /* Read-ahead Inline */
> + if (map.m_flags & EROFS_MAP_META) {
> + struct folio *folio = readahead_folio(rac);
> +
> + ret = erofs_fscache_readpage_inline(folio, &map);
> + if (!ret) {
> + folio_mark_uptodate(folio);
> + ret = folio_size(folio);
> + }
> +
> + folio_unlock(folio);
> + continue;
> + }
> +
> + /* Read-ahead No-inline */
> + if (vi->datalayout == EROFS_INODE_FLAT_PLAIN ||
> + vi->datalayout == EROFS_INODE_FLAT_INLINE ||
> + vi->datalayout == EROFS_INODE_CHUNK_BASED) {
> + struct fscache_cookie *cookie = map.m_fscache->cookie;
> + loff_t offset = start + done;
> + size_t count = erofs_fscache_calc_len(&map, len, done);

You could promote `offset' and `count' to the outer block. So another
`offset' and `count' in !EROFS_MAP_MAPPED can be dropped then.

Thanks,
Gao Xiang