erofs: add erofs_fscache_read_folios() helper

Add erofs_fscache_read_folios() 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 read from the cache.

This helper can then be used to implement .readpage()/.readahead() of
on-demand read semantics.
Signed-off-by: Jeffle Xu <jefflexu@linux.alibaba.com>
Reviewed-by: Gao Xiang <hsiangkao@linux.alibaba.com>
Link: https://lore.kernel.org/r/20220425122143.56815-15-jefflexu@linux.alibaba.comAcked-by: Chao Yu <chao@kernel.org>
Signed-off-by: Gao Xiang <hsiangkao@linux.alibaba.com>

fs/erofs/fscache.c

@@ -5,6 +5,60 @@
 #include <linux/fscache.h>
 #include "internal.h"
 
+/*
+ * Read data from fscache and fill the read data into page cache described by
+ * @start/len, which shall be both aligned with PAGE_SIZE. @pstart describes
+ * the start physical address in the cache file.
+ */
+static int erofs_fscache_read_folios(struct fscache_cookie *cookie,
+				     struct address_space *mapping,
+				     loff_t start, size_t len,
+				     loff_t pstart)
+{
+	enum netfs_io_source source;
+	struct netfs_io_request rreq = {};
+	struct netfs_io_subrequest subreq = { .rreq = &rreq, };
+	struct netfs_cache_resources *cres = &rreq.cache_resources;
+	struct super_block *sb = mapping->host->i_sb;
+	struct iov_iter iter;
+	size_t done = 0;
+	int ret;
+
+	ret = fscache_begin_read_operation(cres, cookie);
+	if (ret)
+		return ret;
+
+	while (done < len) {
+		subreq.start = pstart + done;
+		subreq.len = len - done;
+		subreq.flags = 1 << NETFS_SREQ_ONDEMAND;
+
+		source = cres->ops->prepare_read(&subreq, LLONG_MAX);
+		if (WARN_ON(subreq.len == 0))
+			source = NETFS_INVALID_READ;
+		if (source != NETFS_READ_FROM_CACHE) {
+			erofs_err(sb, "failed to fscache prepare_read (source %d)",
+				  source);
+			ret = -EIO;
+			goto out;
+		}
+
+		iov_iter_xarray(&iter, READ, &mapping->i_pages,
+				start + done, subreq.len);
+		ret = fscache_read(cres, subreq.start, &iter,
+				   NETFS_READ_HOLE_FAIL, NULL, NULL);
+		if (ret) {
+			erofs_err(sb, "failed to fscache_read (ret %d)", ret);
+			goto out;
+		}
+
+		done += subreq.len;
+	}
+out:
+	fscache_end_operation(cres);
+	return ret;
+}
+
 static const struct address_space_operations erofs_fscache_meta_aops = {
 };