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

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Date:   Mon, 28 Mar 2022 18:55:38 +0800
From:   Gao Xiang <hsiangkao@...ux.alibaba.com>
To:     Jeffle Xu <jefflexu@...ux.alibaba.com>
Cc:     dhowells@...hat.com, linux-cachefs@...hat.com, xiang@...nel.org,
        chao@...nel.org, linux-erofs@...ts.ozlabs.org,
        torvalds@...ux-foundation.org, gregkh@...uxfoundation.org,
        willy@...radead.org, linux-fsdevel@...r.kernel.org,
        joseph.qi@...ux.alibaba.com, bo.liu@...ux.alibaba.com,
        tao.peng@...ux.alibaba.com, gerry@...ux.alibaba.com,
        eguan@...ux.alibaba.com, linux-kernel@...r.kernel.org,
        luodaowen.backend@...edance.com, tianzichen@...ishou.com,
        fannaihao@...du.com
Subject: Re: [PATCH v6 21/22] erofs: implement fscache-based data readahead

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@...ux.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