[PATCH v4 13/21] erofs: add erofs_fscache_read_pages() helper

[Jeffle Xu]

Add erofs_fscache_read_pages() helper reading from fscache. It supports
on-demand read semantics. That is, it will make the backend prepare for
the data when cache miss. Once data ready, it will reinitiate a read
from the cache.

This helper can then be used to implement .readpage()/.readahead() of
on-demand read semantics.

Besides also add erofs_fscache_read_page() wrapper helper.

Signed-off-by: Jeffle Xu <jefflexu@xxxxxxxxxxxxxxxxx>
---
 fs/erofs/fscache.c | 38 ++++++++++++++++++++++++++++++++++++++
 1 file changed, 38 insertions(+)

diff --git a/fs/erofs/fscache.c b/fs/erofs/fscache.c
index 4d83125ff896..3f9f7d6ede57 100644
--- a/fs/erofs/fscache.c
+++ b/fs/erofs/fscache.c
@@ -6,6 +6,44 @@
 
 static struct fscache_volume *volume;
 
+/*
+ * erofs_fscache_read_pages - Read data from fscache.
+ *
+ * Fill the readed data into page cache described by @start/len, which
+ * shall be both aligned with PAGE_SIZE. @pstart describes the corresponding
+ * physical start address in the cache file.
+ */
+static int erofs_fscache_read_pages(struct fscache_cookie *cookie,
+				    struct address_space *mapping,
+				    loff_t start, size_t len,
+				    loff_t pstart)
+{
+	struct netfs_cache_resources cres;
+	struct iov_iter iter;
+	int ret;
+
+	memset(&cres, 0, sizeof(cres));
+
+	ret = fscache_begin_read_operation(&cres, cookie);
+	if (ret)
+		return ret;
+
+	iov_iter_xarray(&iter, READ, &mapping->i_pages, start, len);
+
+	ret = fscache_read(&cres, pstart, &iter,
+			   NETFS_READ_HOLE_ONDEMAND, NULL, NULL);
+
+	fscache_end_operation(&cres);
+	return ret;
+}
+
+static inline int erofs_fscache_read_page(struct fscache_cookie *cookie,
+					  struct page *page, loff_t pstart)
+{
+	return erofs_fscache_read_pages(cookie, page_file_mapping(page),
+					page_offset(page), PAGE_SIZE, pstart);
+}
+
 static const struct address_space_operations erofs_fscache_blob_aops = {
 };
 
-- 
2.27.0