Re: [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 <lihongb...@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