[PATCH v2 3/4] erofs: add erofs_fileio_direct_io helper to handle direct io

[Hongbo Li via Linux-erofs]

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.

To be mentioned, the direct io is atomic, if the part of the iov_iter
of direct io failed, the whole direct io also fails.

Signed-off-by: Hongbo Li <lihongb...@huawei.com>
---
 fs/erofs/fileio.c | 71 +++++++++++++++++++++++++++++++++++++++++++++++
 1 file changed, 71 insertions(+)

diff --git a/fs/erofs/fileio.c b/fs/erofs/fileio.c
index cdd432ec266c..b652e3df050c 100644
--- a/fs/erofs/fileio.c
+++ b/fs/erofs/fileio.c
@@ -12,6 +12,7 @@ struct erofs_fileio_rq {
        struct kiocb iocb;
        struct super_block *sb;
        ssize_t ret;
+       void *private;
 };
 
 typedef void (fileio_rq_split_t)(void *data);
@@ -24,6 +25,11 @@ struct erofs_fileio {
        fileio_rq_split_t *split;
        void *private;
        bio_end_io_t *end;
+       /* the following members control the sync call */
+       struct completion ctr;
+       refcount_t ref;
+       size_t total;
+       size_t done;
 };
 
 static void erofs_fileio_ki_complete(struct kiocb *iocb, long ret)
@@ -50,6 +56,13 @@ static void erofs_folio_split(void *data)
        erofs_onlinefolio_split((struct folio *)data);
 }
 
+static void erofs_iter_split(void *data)
+{
+       struct erofs_fileio *io = (struct erofs_fileio *)data;
+
+       refcount_inc(&io->ref);
+}
+
 static void erofs_fileio_end_folio(struct bio *bio)
 {
        struct erofs_fileio_rq *rq =
@@ -62,6 +75,25 @@ static void erofs_fileio_end_folio(struct bio *bio)
        }
 }
 
+static void erofs_fileio_iter_complete(struct erofs_fileio *io)
+{
+       if (!refcount_dec_and_test(&io->ref))
+               return;
+       complete(&io->ctr);
+}
+
+static void erofs_fileio_end_iter(struct bio *bio)
+{
+       struct erofs_fileio_rq *rq =
+                       container_of(bio, struct erofs_fileio_rq, bio);
+       struct erofs_fileio *io = (struct erofs_fileio *)rq->private;
+
+       if (rq->ret > 0)
+               io->done += rq->ret;
+
+       erofs_fileio_iter_complete(io);
+}
+
 static void erofs_fileio_rq_submit(struct erofs_fileio_rq *rq)
 {
        struct iov_iter iter;
@@ -158,6 +190,7 @@ static int erofs_fileio_scan(struct erofs_fileio *io,
                                if (err)
                                        break;
                                io->rq = erofs_fileio_rq_alloc(&io->dev);
+                               io->rq->private = io;
                                io->rq->bio.bi_iter.bi_sector = io->dev.m_pa >> 
9;
                                io->rq->bio.bi_end_io = io->end;
                                attached = 0;
@@ -230,7 +263,45 @@ static void erofs_fileio_readahead(struct 
readahead_control *rac)
        erofs_fileio_rq_submit(io.rq);
 }
 
+static ssize_t erofs_fileio_direct_io(struct kiocb *iocb, struct iov_iter 
*iter)
+{
+       struct file *file = iocb->ki_filp;
+       struct inode *inode = file_inode(file);
+       size_t i_size = i_size_read(inode);
+       struct erofs_fileio io = {};
+       int err;
+
+       if (unlikely(iocb->ki_pos >= i_size))
+               return 0;
+
+       iter->count = min_t(size_t, iter->count,
+                           max_t(size_t, 0, i_size - iocb->ki_pos));
+       io.total = iter->count;
+       if (!io.total)
+               return 0;
+
+       io.inode = inode;
+       io.done = 0;
+       io.split = erofs_iter_split;
+       io.private = &io;
+       io.end = erofs_fileio_end_iter;
+       init_completion(&io.ctr);
+       refcount_set(&io.ref, 1);
+       err = erofs_fileio_scan(&io, iocb->ki_pos, iter);
+       erofs_fileio_rq_submit(io.rq);
+
+       erofs_fileio_iter_complete(&io);
+       wait_for_completion(&io.ctr);
+       if (io.total != io.done) {
+               iov_iter_revert(iter, io.done);
+               return err ?: -EIO;
+       }
+
+       return io.done;
+}
+
 const struct address_space_operations erofs_fileio_aops = {
        .read_folio = erofs_fileio_read_folio,
        .readahead = erofs_fileio_readahead,
+       .direct_IO = erofs_fileio_direct_io,
 };
-- 
2.34.1