cachefiles: Mark a backing file in use with an inode flag

Use an inode flag, S_KERNEL_FILE, to mark that a backing file is in use by
the kernel to prevent cachefiles or other kernel services from interfering
with that file.

Using S_SWAPFILE instead isn't really viable as that has other effects in
the I/O paths.
Signed-off-by: David Howells <dhowells@redhat.com>
Reviewed-by: Jeff Layton <jlayton@kernel.org>
cc: linux-cachefs@redhat.com
Link: https://lore.kernel.org/r/163819642273.215744.6414248677118690672.stgit@warthog.procyon.org.uk/ # v1
Link: https://lore.kernel.org/r/163906943215.143852.16972351425323967014.stgit@warthog.procyon.org.uk/ # v2
Link: https://lore.kernel.org/r/163967154118.1823006.13227551961786743991.stgit@warthog.procyon.org.uk/ # v3
Link: https://lore.kernel.org/r/164021541207.640689.564689725898537127.stgit@warthog.procyon.org.uk/ # v4
Link: https://lore.kernel.org/r/164021552299.640689.10578652796777392062.stgit@warthog.procyon.org.uk/ # v4

fs/cachefiles/internal.h

@@ -187,6 +187,8 @@ extern struct kmem_cache *cachefiles_object_jar;
 /*
  * namei.c
  */
+extern void cachefiles_unmark_inode_in_use(struct cachefiles_object *object,
+					   struct file *file);
 extern struct dentry *cachefiles_get_directory(struct cachefiles_cache *cache,
 					       struct dentry *dir,
 					       const char *name,

fs/cachefiles/namei.c

@@ -31,6 +31,18 @@ static bool __cachefiles_mark_inode_in_use(struct cachefiles_object *object,
 	return can_use;
 }
 
+static bool cachefiles_mark_inode_in_use(struct cachefiles_object *object,
+					 struct dentry *dentry)
+{
+	struct inode *inode = d_backing_inode(dentry);
+	bool can_use;
+
+	inode_lock(inode);
+	can_use = __cachefiles_mark_inode_in_use(object, dentry);
+	inode_unlock(inode);
+	return can_use;
+}
+
 /*
  * Unmark a backing inode.  The caller must hold the inode lock.
  */
@@ -43,6 +55,29 @@ static void __cachefiles_unmark_inode_in_use(struct cachefiles_object *object,
 	trace_cachefiles_mark_inactive(object, inode);
 }
 
+/*
+ * Unmark a backing inode and tell cachefilesd that there's something that can
+ * be culled.
+ */
+void cachefiles_unmark_inode_in_use(struct cachefiles_object *object,
+				    struct file *file)
+{
+	struct cachefiles_cache *cache = object->volume->cache;
+	struct inode *inode = file_inode(file);
+
+	if (inode) {
+		inode_lock(inode);
+		__cachefiles_unmark_inode_in_use(object, file->f_path.dentry);
+		inode_unlock(inode);
+
+		if (!test_bit(CACHEFILES_OBJECT_USING_TMPFILE, &object->flags)) {
+			atomic_long_add(inode->i_blocks, &cache->b_released);
+			if (atomic_inc_return(&cache->f_released))
+				cachefiles_state_changed(cache);
+		}
+	}
+}
+
 /*
  * get a subdirectory
  */