erofs: Support sharing cookies in the same domain

Several erofs filesystems can belong to one domain, and data blobs can
be shared among these erofs filesystems of same domain.

Users could specify domain_id mount option to create or join into a
domain.

Signed-off-by: Jia Zhu <zhujia.zj@bytedance.com>
Reviewed-by: Jingbo Xu <jefflexu@linux.alibaba.com>
Link: https://lore.kernel.org/r/20220918110150.6338-1-zhujia.zj@bytedance.com
Signed-off-by: Gao Xiang <hsiangkao@linux.alibaba.com>

fs/erofs/fscache.c

@@ -7,6 +7,7 @@
 #include "internal.h"
 
 static DEFINE_MUTEX(erofs_domain_list_lock);
+static DEFINE_MUTEX(erofs_domain_cookies_lock);
 static LIST_HEAD(erofs_domain_list);
 static struct vfsmount *erofs_pseudo_mnt;
 
@@ -531,8 +532,9 @@ static int erofs_fscache_register_domain(struct super_block *sb)
 	return err;
 }
 
-struct erofs_fscache *erofs_fscache_register_cookie(struct super_block *sb,
-						     char *name, bool need_inode)
+static
+struct erofs_fscache *erofs_fscache_acquire_cookie(struct super_block *sb,
+						    char *name, bool need_inode)
 {
 	struct fscache_volume *volume = EROFS_SB(sb)->volume;
 	struct erofs_fscache *ctx;
@@ -581,17 +583,102 @@ struct erofs_fscache *erofs_fscache_register_cookie(struct super_block *sb,
 	return ERR_PTR(ret);
 }
 
-void erofs_fscache_unregister_cookie(struct erofs_fscache *ctx)
+static void erofs_fscache_relinquish_cookie(struct erofs_fscache *ctx)
 {
-	if (!ctx)
-		return;
-
 	fscache_unuse_cookie(ctx->cookie, NULL, NULL);
 	fscache_relinquish_cookie(ctx->cookie, false);
 	iput(ctx->inode);
+	kfree(ctx->name);
 	kfree(ctx);
 }
 
+static
+struct erofs_fscache *erofs_fscache_domain_init_cookie(struct super_block *sb,
+		char *name, bool need_inode)
+{
+	int err;
+	struct inode *inode;
+	struct erofs_fscache *ctx;
+	struct erofs_domain *domain = EROFS_SB(sb)->domain;
+
+	ctx = erofs_fscache_acquire_cookie(sb, name, need_inode);
+	if (IS_ERR(ctx))
+		return ctx;
+
+	ctx->name = kstrdup(name, GFP_KERNEL);
+	if (!ctx->name) {
+		err = -ENOMEM;
+		goto out;
+	}
+
+	inode = new_inode(erofs_pseudo_mnt->mnt_sb);
+	if (!inode) {
+		err = -ENOMEM;
+		goto out;
+	}
+
+	ctx->domain = domain;
+	ctx->anon_inode = inode;
+	inode->i_private = ctx;
+	refcount_inc(&domain->ref);
+	return ctx;
+out:
+	erofs_fscache_relinquish_cookie(ctx);
+	return ERR_PTR(err);
+}
+
+static
+struct erofs_fscache *erofs_domain_register_cookie(struct super_block *sb,
+						   char *name, bool need_inode)
+{
+	struct inode *inode;
+	struct erofs_fscache *ctx;
+	struct erofs_domain *domain = EROFS_SB(sb)->domain;
+	struct super_block *psb = erofs_pseudo_mnt->mnt_sb;
+
+	mutex_lock(&erofs_domain_cookies_lock);
+	list_for_each_entry(inode, &psb->s_inodes, i_sb_list) {
+		ctx = inode->i_private;
+		if (!ctx || ctx->domain != domain || strcmp(ctx->name, name))
+			continue;
+		igrab(inode);
+		mutex_unlock(&erofs_domain_cookies_lock);
+		return ctx;
+	}
+	ctx = erofs_fscache_domain_init_cookie(sb, name, need_inode);
+	mutex_unlock(&erofs_domain_cookies_lock);
+	return ctx;
+}
+
+struct erofs_fscache *erofs_fscache_register_cookie(struct super_block *sb,
+						    char *name, bool need_inode)
+{
+	if (EROFS_SB(sb)->opt.domain_id)
+		return erofs_domain_register_cookie(sb, name, need_inode);
+	return erofs_fscache_acquire_cookie(sb, name, need_inode);
+}
+
+void erofs_fscache_unregister_cookie(struct erofs_fscache *ctx)
+{
+	bool drop;
+	struct erofs_domain *domain;
+
+	if (!ctx)
+		return;
+	domain = ctx->domain;
+	if (domain) {
+		mutex_lock(&erofs_domain_cookies_lock);
+		drop = atomic_read(&ctx->anon_inode->i_count) == 1;
+		iput(ctx->anon_inode);
+		mutex_unlock(&erofs_domain_cookies_lock);
+		if (!drop)
+			return;
+	}
+
+	erofs_fscache_relinquish_cookie(ctx);
+	erofs_fscache_domain_put(domain);
+}
+
 int erofs_fscache_register_fs(struct super_block *sb)
 {
 	int ret;

fs/erofs/internal.h

@@ -109,6 +109,9 @@ struct erofs_domain {
 struct erofs_fscache {
 	struct fscache_cookie *cookie;
 	struct inode *inode;
+	struct inode *anon_inode;
+	struct erofs_domain *domain;
+	char *name;
 };
 
 struct erofs_sb_info {