linux-kernel - Re: [PATCH v1 13/23] erofs: implement fscache-based data read

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Date:   Tue, 4 Jan 2022 22:40:54 +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,
        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
Subject: Re: [PATCH v1 13/23] erofs: implement fscache-based data read

On Mon, Dec 27, 2021 at 08:54:34PM +0800, Jeffle Xu wrote:
> This patch implements the data plane of reading data from bootstrap blob
> file over fscache.
> 
> Be noted that currently compressed layout is not supported yet.
> 
> Signed-off-by: Jeffle Xu <jefflexu@...ux.alibaba.com>
> ---
>  fs/erofs/fscache.c  | 91 +++++++++++++++++++++++++++++++++++++++++++++
>  fs/erofs/inode.c    |  6 ++-
>  fs/erofs/internal.h |  1 +
>  3 files changed, 97 insertions(+), 1 deletion(-)
> 
> diff --git a/fs/erofs/fscache.c b/fs/erofs/fscache.c
> index 325f5663836b..bfcec831d58a 100644
> --- a/fs/erofs/fscache.c
> +++ b/fs/erofs/fscache.c
> @@ -65,6 +65,97 @@ struct page *erofs_readpage_from_fscache(struct erofs_cookie_ctx *ctx,
>  	return page;
>  }
>  
> +static inline void do_copy_page(struct page *from, struct page *to,
> +				size_t offset, size_t len)
> +{
> +	char *vfrom, *vto;
> +
> +	vfrom = kmap_atomic(from);
> +	vto = kmap_atomic(to);
> +	memcpy(vto, vfrom + offset, len);
> +	kunmap_atomic(vto);
> +	kunmap_atomic(vfrom);
> +}
> +
> +static int erofs_fscache_do_readpage(struct file *file, struct page *page)
> +{
> +	struct inode *inode = page->mapping->host;
> +	struct erofs_inode *vi = EROFS_I(inode);
> +	struct super_block *sb = inode->i_sb;
> +	struct erofs_map_blocks map;
> +	erofs_off_t o_la, pa;
> +	size_t offset, len;
> +	struct page *ipage;
> +	int ret;
> +
> +	if (erofs_inode_is_data_compressed(vi->datalayout)) {
> +		erofs_info(sb, "compressed layout not supported yet");
> +		return -EOPNOTSUPP;
> +	}
> +
> +	o_la = page_offset(page);
> +	map.m_la = o_la;
> +
> +	ret = erofs_map_blocks(inode, &map, EROFS_GET_BLOCKS_RAW);
> +	if (ret)
> +		return ret;
> +
> +	if (!(map.m_flags & EROFS_MAP_MAPPED)) {
> +		zero_user(page, 0, PAGE_SIZE);
> +		return 0;
> +	}
> +
> +	/*
> +	 * 1) For FLAT_PLAIN/FLAT_INLINE layout, the output map.m_la shall be
> +	 * equal to o_la, and the output map.m_pa is exactly the physical
> +	 * address of o_la.
> +	 * 2) For CHUNK_BASED layout, the output map.m_la is rounded down to the
> +	 * nearest chunk boundary, and the output map.m_pa is actually the
> +	 * physical address of this chunk boundary. So we need to recalculate
> +	 * the actual physical address of o_la.
> +	 */
> +	pa = map.m_pa + o_la - map.m_la;
> +
> +	ipage = erofs_get_meta_page(sb, erofs_blknr(pa));
> +	if (IS_ERR(ipage))
> +		return PTR_ERR(ipage);
> +
> +	/*
> +	 * @offset refers to the page offset inside @ipage.
> +	 * 1) Except for the inline layout, the offset shall all be 0, and @pa
> +	 * shall be aligned with EROFS_BLKSIZ in this case. Thus we can
> +	 * conveniently get the offset from @pa.
> +	 * 2) While for the inline layout, the offset may be non-zero. Since
> +	 * currently only flat layout supports inline, we can calculate the
> +	 * offset from the corresponding physical address.
> +	 */
> +	offset = erofs_blkoff(pa);
> +	len = min_t(u64, map.m_llen, PAGE_SIZE);
> +
> +	do_copy_page(ipage, page, offset, len);

If my understanding is correct, I still have no idea why we need to
copy data here even if fscache can do direct I/O for us without extra
efforts.

I think the only case would be tail-packing inline (which should go
through metadata path), otherwise, all data is block-aligned. So
fscache can handle it directly.

Thanks,
Gao Xiang