erofs: add anonymous inode caching metadata for data blobs

Introduce one anonymous inode for data blobs so that erofs can cache
metadata directly within such anonymous inode.

Signed-off-by: Jeffle Xu <jefflexu@linux.alibaba.com>
Reviewed-by: Gao Xiang <hsiangkao@linux.alibaba.com>
Link: https://lore.kernel.org/r/20220425122143.56815-14-jefflexu@linux.alibaba.com
Acked-by: Chao Yu <chao@kernel.org>
Signed-off-by: Gao Xiang <hsiangkao@linux.alibaba.com>

fs/erofs/fscache.c

@@ -5,12 +5,17 @@
 #include <linux/fscache.h>
 #include "internal.h"
 
+static const struct address_space_operations erofs_fscache_meta_aops = {
+};
+
 int erofs_fscache_register_cookie(struct super_block *sb,
-				  struct erofs_fscache **fscache, char *name)
+				  struct erofs_fscache **fscache,
+				  char *name, bool need_inode)
 {
 	struct fscache_volume *volume = EROFS_SB(sb)->volume;
 	struct erofs_fscache *ctx;
 	struct fscache_cookie *cookie;
+	int ret;
 
 	ctx = kzalloc(sizeof(*ctx), GFP_KERNEL);
 	if (!ctx)
@@ -20,15 +25,40 @@ int erofs_fscache_register_cookie(struct super_block *sb,
 					name, strlen(name), NULL, 0, 0);
 	if (!cookie) {
 		erofs_err(sb, "failed to get cookie for %s", name);
-		kfree(name);
-		return -EINVAL;
+		ret = -EINVAL;
+		goto err;
 	}
 
 	fscache_use_cookie(cookie, false);
 	ctx->cookie = cookie;
 
+	if (need_inode) {
+		struct inode *const inode = new_inode(sb);
+
+		if (!inode) {
+			erofs_err(sb, "failed to get anon inode for %s", name);
+			ret = -ENOMEM;
+			goto err_cookie;
+		}
+
+		set_nlink(inode, 1);
+		inode->i_size = OFFSET_MAX;
+		inode->i_mapping->a_ops = &erofs_fscache_meta_aops;
+		mapping_set_gfp_mask(inode->i_mapping, GFP_NOFS);
+
+		ctx->inode = inode;
+	}
+
 	*fscache = ctx;
 	return 0;
+
+err_cookie:
+	fscache_unuse_cookie(ctx->cookie, NULL, NULL);
+	fscache_relinquish_cookie(ctx->cookie, false);
+	ctx->cookie = NULL;
+err:
+	kfree(ctx);
+	return ret;
 }
 
 void erofs_fscache_unregister_cookie(struct erofs_fscache **fscache)
@@ -42,6 +72,9 @@ void erofs_fscache_unregister_cookie(struct erofs_fscache **fscache)
 	fscache_relinquish_cookie(ctx->cookie, false);
 	ctx->cookie = NULL;
 
+	iput(ctx->inode);
+	ctx->inode = NULL;
+
 	kfree(ctx);
 	*fscache = NULL;
 }

fs/erofs/internal.h

@@ -99,6 +99,7 @@ struct erofs_sb_lz4_info {
 
 struct erofs_fscache {
 	struct fscache_cookie *cookie;
+	struct inode *inode;
 };
 
 struct erofs_sb_info {
@@ -607,7 +608,8 @@ int erofs_fscache_register_fs(struct super_block *sb);
 void erofs_fscache_unregister_fs(struct super_block *sb);
 
 int erofs_fscache_register_cookie(struct super_block *sb,
-				  struct erofs_fscache **fscache, char *name);
+				  struct erofs_fscache **fscache,
+				  char *name, bool need_inode);
 void erofs_fscache_unregister_cookie(struct erofs_fscache **fscache);
 #else
 static inline int erofs_fscache_register_fs(struct super_block *sb)
@@ -618,7 +620,7 @@ static inline void erofs_fscache_unregister_fs(struct super_block *sb) {}
 
 static inline int erofs_fscache_register_cookie(struct super_block *sb,
 						struct erofs_fscache **fscache,
-						char *name)
+						char *name, bool need_inode)
 {
 	return -EOPNOTSUPP;
 }