[RFC PATCH v3 08/18] fs/dcache.c: add shrink_dcache_inode()

[Eric Biggers <ebiggers@kernel.org>]

From: Eric Biggers <ebiggers@google.com>

When a filesystem encryption key is removed, we need all files which had
been "unlocked" (had ->i_crypt_info set up) with it to appear "locked"
again.  This is most easily done by evicting the inodes.  This can
currently be done using 'echo 2 > /proc/sys/vm/drop_caches'; however,
that is overkill and not usable by non-root users.

To evict just the needed inodes we also need the ability to evict those
inodes' dentries, since an inode is pinned by its dentries.  Therefore,
add a function shrink_dcache_inode() which iterates through an inode's
dentries and evicts any unused ones as well as any unused descendants
(since there may be negative dentries pinning the inode's dentries).

Signed-off-by: Eric Biggers <ebiggers@google.com>
---
 fs/dcache.c            | 32 ++++++++++++++++++++++++++++++++
 include/linux/dcache.h |  1 +
 2 files changed, 33 insertions(+)

diff --git a/fs/dcache.c b/fs/dcache.c
index 2593153471cf..6a7db8797b2d 100644
--- a/fs/dcache.c
+++ b/fs/dcache.c
@@ -1484,6 +1484,38 @@ void shrink_dcache_parent(struct dentry *parent)
 }
 EXPORT_SYMBOL(shrink_dcache_parent);
 
+/**
+ * shrink_dcache_inode - prune dcache for inode
+ * @inode: inode to prune
+ *
+ * Evict all unused aliases of the specified inode from the dcache.  This is
+ * intended to be used when trying to evict a specific inode, since inodes are
+ * pinned by their dentries.  We also have to descend to ->d_subdirs for each
+ * alias, since aliases may be pinned by negative child dentries.
+ */
+void shrink_dcache_inode(struct inode *inode)
+{
+	for (;;) {
+		struct select_data data;
+		struct dentry *dentry;
+
+		INIT_LIST_HEAD(&data.dispose);
+		data.start = NULL;
+		data.found = 0;
+
+		spin_lock(&inode->i_lock);
+		hlist_for_each_entry(dentry, &inode->i_dentry, d_u.d_alias)
+			d_walk(dentry, &data, select_collect);
+		spin_unlock(&inode->i_lock);
+
+		if (!data.found)
+			break;
+
+		shrink_dentry_list(&data.dispose);
+		cond_resched();
+	}
+}
+
 static enum d_walk_ret umount_check(void *_data, struct dentry *dentry)
 {
 	/* it has busy descendents; complain about those instead */
diff --git a/include/linux/dcache.h b/include/linux/dcache.h
index ef4b70f64f33..c02dab98bc97 100644
--- a/include/linux/dcache.h
+++ b/include/linux/dcache.h
@@ -245,6 +245,7 @@ extern struct dentry * d_obtain_alias(struct inode *);
 extern struct dentry * d_obtain_root(struct inode *);
 extern void shrink_dcache_sb(struct super_block *);
 extern void shrink_dcache_parent(struct dentry *);
+extern void shrink_dcache_inode(struct inode *);
 extern void shrink_dcache_for_umount(struct super_block *);
 extern void d_invalidate(struct dentry *);
 
-- 
2.20.1