mm: pagecache add lock

Add a per address space lock around adding pages to the pagecache - making it
possible for fallocate INSERT_RANGE/COLLAPSE_RANGE to work correctly, and also
hopefully making truncate and dio a bit saner.

2 files changed, 28 insertions, 0 deletions

diff --git a/include/linux/fs.h b/include/linux/fs.h
index 897eae8faee1..439818420857 100644
--- a/include/linux/fs.h
+++ b/include/linux/fs.h
@@ -403,6 +403,28 @@ int pagecache_write_end(struct file *, struct address_space *mapping,
 				loff_t pos, unsigned len, unsigned copied,
 				struct page *page, void *fsdata);
 
+/*
+ * Two-state lock - can be taken for add or block - both states are shared,
+ * like read side of rwsem, but conflict with other state:
+ */
+struct pagecache_lock {
+	atomic_long_t		v;
+	wait_queue_head_t	wait;
+};
+
+static inline void pagecache_lock_init(struct pagecache_lock *lock)
+{
+	atomic_long_set(&lock->v, 0);
+	init_waitqueue_head(&lock->wait);
+}
+
+void pagecache_add_put(struct pagecache_lock *);
+void pagecache_add_get(struct pagecache_lock *);
+void __pagecache_block_put(struct pagecache_lock *);
+void __pagecache_block_get(struct pagecache_lock *);
+void pagecache_block_put(struct pagecache_lock *);
+void pagecache_block_get(struct pagecache_lock *);
+
 struct address_space {
 	struct inode		*host;		/* owner: inode, block_device */
 	struct radix_tree_root	i_pages;	/* cached pages */
@@ -421,6 +443,8 @@ struct address_space {
 	struct list_head	private_list;	/* for use by the address_space */
 	void			*private_data;	/* ditto */
 	errseq_t		wb_err;
+	struct pagecache_lock	add_lock
+		____cacheline_aligned_in_smp;	/* protects adding new pages */
 } __attribute__((aligned(sizeof(long)))) __randomize_layout;
 	/*
 	 * On most architectures that alignment is already the case; but
diff --git a/include/linux/sched.h b/include/linux/sched.h
index 977cb57d7bc9..3a5382a321bd 100644
--- a/include/linux/sched.h
+++ b/include/linux/sched.h
@@ -41,6 +41,7 @@ struct io_context;
 struct mempolicy;
 struct nameidata;
 struct nsproxy;
+struct pagecache_lock;
 struct perf_event_context;
 struct pid_namespace;
 struct pipe_inode_info;
@@ -940,6 +941,9 @@ struct task_struct {
 	unsigned int			in_ubsan;
 #endif
 
+	/* currently held lock, for avoiding recursing in fault path: */
+	struct pagecache_lock *pagecache_lock;
+
 	/* Journalling filesystem info: */
 	void				*journal_info;