When page cache sharing enabled, page caches are managed in the address
space of blobs rather than erofs inodes.  All erofs inodes sharing one
chunk will refer to and share the page cache in the blob's address
space.

Signed-off-by: Jingbo Xu <jefflexu@linux.alibaba.com>
---
 fs/erofs/fscache.c | 48 ++++++++++++++++++++++++++++++++++++++++++++++
 1 file changed, 48 insertions(+)

diff --git a/fs/erofs/fscache.c b/fs/erofs/fscache.c
index f715b3efcc77..4075d9519a7d 100644
--- a/fs/erofs/fscache.c
+++ b/fs/erofs/fscache.c
@@ -384,8 +384,56 @@ static int erofs_fscache_share_file_open(struct inode *inode, struct file *filp)
 	return 0;
 }
 
+static ssize_t erofs_fscache_share_file_read_iter(struct kiocb *iocb,
+						  struct iov_iter *to)
+{
+	struct file *filp = iocb->ki_filp;
+	struct inode *inode = file_inode(filp);
+	struct file *realfile = filp->private_data;
+	struct inode *realinode = file_inode(realfile);
+	struct erofs_fscache_share_file_info *finfo = realfile->private_data;
+	ssize_t already_read = 0;
+	int ret = 0;
+
+	/* no need taking (shared) inode lock since it's a ro filesystem */
+	if (!iov_iter_count(to))
+		return 0;
+
+	if (IS_DAX(inode) || iocb->ki_flags & IOCB_DIRECT)
+		return -EOPNOTSUPP;
+
+	do {
+		struct folio *folio;
+		size_t bytes, copied, offset, fsize;
+		pgoff_t index = (finfo->pa + iocb->ki_pos) >> PAGE_SHIFT;
+
+		folio = read_cache_folio(realinode->i_mapping, index, NULL, NULL);
+		if (IS_ERR(folio)) {
+			ret = PTR_ERR(folio);
+			break;
+		}
+
+		fsize = folio_size(folio);
+		offset = iocb->ki_pos & (fsize - 1);
+		bytes = min_t(size_t, inode->i_size - iocb->ki_pos, iov_iter_count(to));
+		bytes = min_t(size_t, bytes, fsize - offset);
+		copied = copy_folio_to_iter(folio, offset, bytes, to);
+		folio_put(folio);
+		iocb->ki_pos += copied;
+		already_read += copied;
+		if (copied < bytes) {
+			ret = -EFAULT;
+			break;
+		}
+	} while (iov_iter_count(to) && iocb->ki_pos < inode->i_size);
+
+	file_accessed(filp);
+	return already_read ? already_read : ret;
+}
+
 const struct file_operations erofs_fscache_share_file_fops = {
 	.llseek		= generic_file_llseek,
+	.read_iter	= erofs_fscache_share_file_read_iter,
 	.open		= erofs_fscache_share_file_open,
 	.release	= erofs_fscache_share_file_release,
 };
-- 
2.19.1.6.gb485710b