linux-kernel - [RFC PATCH 31/76] fscache: Allow ->put_super() to be used to wait for cache operations

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Date:   Fri, 20 Nov 2020 15:09:10 +0000
From:   David Howells <dhowells@...hat.com>
To:     Trond Myklebust <trondmy@...merspace.com>,
        Anna Schumaker <anna.schumaker@...app.com>,
        Steve French <sfrench@...ba.org>,
        Dominique Martinet <asmadeus@...ewreck.org>
Cc:     dhowells@...hat.com, Jeff Layton <jlayton@...hat.com>,
        Matthew Wilcox <willy@...radead.org>,
        Alexander Viro <viro@...iv.linux.org.uk>,
        linux-cachefs@...hat.com, linux-afs@...ts.infradead.org,
        linux-nfs@...r.kernel.org, linux-cifs@...r.kernel.org,
        ceph-devel@...r.kernel.org, v9fs-developer@...ts.sourceforge.net,
        linux-fsdevel@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: [RFC PATCH 31/76] fscache: Allow ->put_super() to be used to wait for
 cache operations

Provide a helper to allow ->put_super() to be used to wait for outstanding
cache operations that are pinning inodes.  The helper has a loop that waits
for the first inode that has a non-zero usage and a cookie.  It then calls
evict_inodes() to reduce the list and loops round again until it finds no
more candidate inodes.

Without this, evict_inodes() won't get rid of such operations, and the
"VFS: Busy inodes ..." message will be displayed and the inode abandoned.

Signed-off-by: David Howells <dhowells@...hat.com>
---

 fs/fscache/io.c         |   50 +++++++++++++++++++++++++++++++++++++++++++++++
 include/linux/fscache.h |    2 ++
 2 files changed, 52 insertions(+)

diff --git a/fs/fscache/io.c b/fs/fscache/io.c
index 87ffe84c9f27..de9ffc16eb4f 100644
--- a/fs/fscache/io.c
+++ b/fs/fscache/io.c
@@ -180,3 +180,53 @@ int fscache_set_page_dirty(struct page *page, struct fscache_cookie *cookie)
 	return 1;
 }
 EXPORT_SYMBOL(fscache_set_page_dirty);
+
+/**
+ * fscache_put_super - Wait for outstanding ops to complete
+ * @sb: The superblock to wait on
+ * @get_cookie: Function to get the cookie on an inode
+ *
+ * Wait for outstanding cache operations on the inodes of a superblock to
+ * complete as they might be pinning an inode.  This is designed to be called
+ * from ->put_super(), right before the "VFS: Busy inodes" check.
+ */
+void fscache_put_super(struct super_block *sb,
+		       struct fscache_cookie *(*get_cookie)(struct inode *inode))
+{
+	struct fscache_cookie *cookie;
+	struct inode *inode, *p;
+
+	while (!list_empty(&sb->s_inodes)) {
+		/* Find the first inode that we need to wait on */
+		inode = NULL;
+		cookie = NULL;
+		spin_lock(&sb->s_inode_list_lock);
+		list_for_each_entry(p, &sb->s_inodes, i_sb_list) {
+			if (atomic_inc_not_zero(&p->i_count)) {
+				inode = p;
+				cookie = get_cookie(inode);
+				if (!cookie) {
+					iput(inode);
+					inode = NULL;
+					cookie = NULL;
+					continue;
+				}
+				break;
+			}
+		}
+		spin_unlock(&sb->s_inode_list_lock);
+
+		if (inode) {
+			/* n_ops is kept artificially raised to stop wakeups */
+			atomic_dec(&cookie->n_ops);
+			wait_var_event(&cookie->n_ops, atomic_read(&cookie->n_ops) == 0);
+			atomic_inc(&cookie->n_ops);
+			iput(inode);
+		}
+
+		evict_inodes(sb);
+		if (!inode)
+			break;
+	}
+}
+EXPORT_SYMBOL(fscache_put_super);
diff --git a/include/linux/fscache.h b/include/linux/fscache.h
index d2fc98a5755a..38a252b06b54 100644
--- a/include/linux/fscache.h
+++ b/include/linux/fscache.h
@@ -204,6 +204,8 @@ extern int __fscache_begin_operation(struct fscache_cookie *, struct fscache_op_
 extern void __fscache_relinquish_cookie(struct fscache_cookie *, bool);
 extern void __fscache_update_cookie(struct fscache_cookie *, const void *, const loff_t *);
 extern void __fscache_invalidate(struct fscache_cookie *, loff_t);
+extern void fscache_put_super(struct super_block *,
+			      struct fscache_cookie *(*get_cookie)(struct inode *));
 
 /**
  * fscache_register_netfs - Register a filesystem as desiring caching services