[PATCH RFC] erofs: file-backed mount supports direct io

[Gao Xiang]

Hi Hongbo,

On 2025/1/15 15:09, Hongbo Li wrote:
> erofs has add file-backed mount support. In this scenario, only buffer
> io is allowed. So we enhance the io mode by implementing the direct
> io. Also, this can make the iov_iter (user buffer) interact with the
> backed file's page cache directly.
> 
> Signed-off-by: Hongbo Li <lihongbo22 at huawei.com>
> ---
>   fs/erofs/data.c   |  11 +++--
>   fs/erofs/fileio.c | 122 ++++++++++++++++++++++++++++++++++++++++++++++
>   2 files changed, 130 insertions(+), 3 deletions(-)
> 
> diff --git a/fs/erofs/data.c b/fs/erofs/data.c
> index 0cd6b5c4df98..b5baff61be16 100644
> --- a/fs/erofs/data.c
> +++ b/fs/erofs/data.c
> @@ -395,9 +395,14 @@ static ssize_t erofs_file_read_iter(struct kiocb *iocb, struct iov_iter *to)
>   	if (IS_DAX(inode))
>   		return dax_iomap_rw(iocb, to, &erofs_iomap_ops);
>   #endif
> -	if ((iocb->ki_flags & IOCB_DIRECT) && inode->i_sb->s_bdev)
> -		return iomap_dio_rw(iocb, to, &erofs_iomap_ops,
> -				    NULL, 0, NULL, 0);
> +	if (iocb->ki_flags & IOCB_DIRECT) {
> +		if (inode->i_sb->s_bdev)
> +			return iomap_dio_rw(iocb, to, &erofs_iomap_ops,
> +						NULL, 0, NULL, 0);
> +		if (erofs_is_fileio_mode(EROFS_SB(inode->i_sb)))
> +			return generic_file_read_iter(iocb, to);
> +	}
> +
>   	return filemap_read(iocb, to, 0);
>   }
>   
> diff --git a/fs/erofs/fileio.c b/fs/erofs/fileio.c
> index 33f8539dda4a..76ed16a8ee75 100644
> --- a/fs/erofs/fileio.c
> +++ b/fs/erofs/fileio.c
> @@ -10,12 +10,17 @@ struct erofs_fileio_rq {
>   	struct bio bio;
>   	struct kiocb iocb;
>   	struct super_block *sb;
> +	ssize_t ret;
> +	void *private;
>   };
>   
>   struct erofs_fileio {
> +	struct file *file;
>   	struct erofs_map_blocks map;
>   	struct erofs_map_dev dev;
>   	struct erofs_fileio_rq *rq;
> +	size_t total;
> +	size_t done;
>   };
>   
>   static void erofs_fileio_ki_complete(struct kiocb *iocb, long ret)
> @@ -24,6 +29,7 @@ static void erofs_fileio_ki_complete(struct kiocb *iocb, long ret)
>   			container_of(iocb, struct erofs_fileio_rq, iocb);
>   	struct folio_iter fi;
>   
> +	rq->ret = ret;
>   	if (ret > 0) {
>   		if (ret != rq->bio.bi_iter.bi_size) {
>   			bio_advance(&rq->bio, ret);
> @@ -43,6 +49,17 @@ static void erofs_fileio_ki_complete(struct kiocb *iocb, long ret)
>   	kfree(rq);
>   }
>   
> +static void erofs_fileio_end_io(struct bio *bio)
> +{
> +	struct erofs_fileio_rq *rq =
> +			container_of(bio, struct erofs_fileio_rq, bio);
> +	struct erofs_fileio *io = rq->private;
> +
> +	if (rq->ret > 0) {
> +		io->done += rq->ret;
> +	}
> +}
> +
>   static void erofs_fileio_rq_submit(struct erofs_fileio_rq *rq)
>   {
>   	struct iov_iter iter;
> @@ -189,7 +206,112 @@ static void erofs_fileio_readahead(struct readahead_control *rac)
>   	erofs_fileio_rq_submit(io.rq);
>   }
>   
> +static int erofs_fileio_scan_iter(struct erofs_fileio *io, struct kiocb *iocb,
> +				  struct iov_iter *iter)

I wonder if it's possible to just extract a folio from
`struct iov_iter` and reuse erofs_fileio_scan_folio() logic.

It simplifies the codebase a lot, and I think the performance
is almost the same.

Otherwise currently it looks good to me.

Thanks,
Gao Xiang