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@xxxxxxxxxxxxxxxxx>
> ---
> 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
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