erofs: implement fscache-based metadata read

Implement the data plane of reading metadata from primary data blob
over fscache.
Signed-off-by: Jeffle Xu <jefflexu@linux.alibaba.com>
Reviewed-by: Gao Xiang <hsiangkao@linux.alibaba.com>
Link: https://lore.kernel.org/r/20220425122143.56815-18-jefflexu@linux.alibaba.comAcked-by: Chao Yu <chao@kernel.org>
Signed-off-by: Gao Xiang <hsiangkao@linux.alibaba.com>

fs/erofs/data.c

@@ -6,6 +6,7 @@
  */
 #include "internal.h"
 #include <linux/prefetch.h>
+#include <linux/sched/mm.h>
 #include <linux/dax.h>
 #include <trace/events/erofs.h>
 
@@ -35,14 +36,20 @@ void *erofs_bread(struct erofs_buf *buf, struct inode *inode,
 	erofs_off_t offset = blknr_to_addr(blkaddr);
 	pgoff_t index = offset >> PAGE_SHIFT;
 	struct page *page = buf->page;
+	struct folio *folio;
+	unsigned int nofs_flag;
 
 	if (!page || page->index != index) {
 		erofs_put_metabuf(buf);
-		page = read_cache_page_gfp(mapping, index,
-				mapping_gfp_constraint(mapping, ~__GFP_FS));
-		if (IS_ERR(page))
-			return page;
+
+		nofs_flag = memalloc_nofs_save();
+		folio = read_cache_folio(mapping, index, NULL, NULL);
+		memalloc_nofs_restore(nofs_flag);
+		if (IS_ERR(folio))
+			return folio;
+
 		/* should already be PageUptodate, no need to lock page */
+		page = folio_file_page(folio, index);
 		buf->page = page;
 	}
 	if (buf->kmap_type == EROFS_NO_KMAP) {
@@ -63,6 +70,10 @@ void *erofs_bread(struct erofs_buf *buf, struct inode *inode,
 void *erofs_read_metabuf(struct erofs_buf *buf, struct super_block *sb,
 			 erofs_blk_t blkaddr, enum erofs_kmap_type type)
 {
+	if (erofs_is_fscache_mode(sb))
+		return erofs_bread(buf, EROFS_SB(sb)->s_fscache->inode,
+				   blkaddr, type);
+
 	return erofs_bread(buf, sb->s_bdev->bd_inode, blkaddr, type);
 }
 

fs/erofs/fscache.c

@@ -59,7 +59,32 @@ static int erofs_fscache_read_folios(struct fscache_cookie *cookie,
 	return ret;
 }
 
+static int erofs_fscache_meta_readpage(struct file *data, struct page *page)
+{
+	int ret;
+	struct folio *folio = page_folio(page);
+	struct super_block *sb = folio_mapping(folio)->host->i_sb;
+	struct erofs_map_dev mdev = {
+		.m_deviceid = 0,
+		.m_pa = folio_pos(folio),
+	};
+
+	ret = erofs_map_dev(sb, &mdev);
+	if (ret)
+		goto out;
+
+	ret = erofs_fscache_read_folios(mdev.m_fscache->cookie,
+			folio_mapping(folio), folio_pos(folio),
+			folio_size(folio), mdev.m_pa);
+	if (!ret)
+		folio_mark_uptodate(folio);
+out:
+	folio_unlock(folio);
+	return ret;
+}
+
 static const struct address_space_operations erofs_fscache_meta_aops = {
+	.readpage = erofs_fscache_meta_readpage,
 };
 
 int erofs_fscache_register_cookie(struct super_block *sb,