[RFC PATCH 30/76] vfs, fscache: Force ->write_inode() to occur if cookie pinned for writeback

[David Howells]

Use an inode flag, I_PINNING_FSCACHE_WB, to indicate that a cookie is
pinned in use by that inode for the purposes of writeback.

Pinning is necessary because the in-use pin from the open file is released
before the writeback takes place, but if the resources aren't pinned, the
dirty data can't be written to the cache.

Signed-off-by: David Howells <dhowells@xxxxxxxxxx>
---

 fs/fs-writeback.c         |    8 ++++++++
 fs/fscache/io.c           |   38 ++++++++++++++++++++++++++++++++++++++
 include/linux/fs.h        |    3 +++
 include/linux/fscache.h   |   22 ++++++++++++++++++++++
 include/linux/writeback.h |    1 +
 5 files changed, 72 insertions(+)

diff --git a/fs/fs-writeback.c b/fs/fs-writeback.c
index e6005c78bfa9..f7fb7fb37337 100644
--- a/fs/fs-writeback.c
+++ b/fs/fs-writeback.c
@@ -1506,6 +1506,13 @@ __writeback_single_inode(struct inode *inode, struct writeback_control *wbc)
 
 	if (mapping_tagged(mapping, PAGECACHE_TAG_DIRTY))
 		inode->i_state |= I_DIRTY_PAGES;
+	else if (unlikely(inode->i_state & I_PINNING_FSCACHE_WB)) {
+		if (!(inode->i_state & I_DIRTY_PAGES)) {
+			inode->i_state &= ~I_PINNING_FSCACHE_WB;
+			wbc->unpinned_fscache_wb = true;
+			dirty |= I_PINNING_FSCACHE_WB; /* Cause write_inode */
+		}
+	}
 
 	spin_unlock(&inode->i_lock);
 
@@ -1517,6 +1524,7 @@ __writeback_single_inode(struct inode *inode, struct writeback_control *wbc)
 		if (ret == 0)
 			ret = err;
 	}
+	wbc->unpinned_fscache_wb = false;
 	trace_writeback_single_inode(inode, wbc, nr_to_write);
 	return ret;
 }
diff --git a/fs/fscache/io.c b/fs/fscache/io.c
index 7ad13900b281..87ffe84c9f27 100644
--- a/fs/fscache/io.c
+++ b/fs/fscache/io.c
@@ -142,3 +142,41 @@ int __fscache_begin_operation(struct fscache_cookie *cookie,
 	return -ENOBUFS;
 }
 EXPORT_SYMBOL(__fscache_begin_operation);
+
+/**
+ * fscache_set_page_dirty - Mark page dirty and pin a cache object for writeback
+ * @page: The page being dirtied
+ * @cookie: The cookie referring to the cache object
+ *
+ * Set the dirty flag on a page and pin an in-use cache object in memory when
+ * dirtying a page so that writeback can later write to it.  This is intended
+ * to be called from the filesystem's ->set_page_dirty() method.
+ *
+ *  Returns 1 if PG_dirty was set on the page, 0 otherwise.
+ */
+int fscache_set_page_dirty(struct page *page, struct fscache_cookie *cookie)
+{
+	struct inode *inode = page->mapping->host;
+	bool need_use = false;
+
+	_enter("");
+
+	if (!__set_page_dirty_nobuffers(page))
+		return 0;
+	if (!fscache_cookie_valid(cookie))
+		return 1;
+
+	if (!(inode->i_state & I_PINNING_FSCACHE_WB)) {
+		spin_lock(&inode->i_lock);
+		if (!(inode->i_state & I_PINNING_FSCACHE_WB)) {
+			inode->i_state |= I_PINNING_FSCACHE_WB;
+			need_use = true;
+		}
+		spin_unlock(&inode->i_lock);
+
+		if (need_use)
+			fscache_use_cookie(cookie, true);
+	}
+	return 1;
+}
+EXPORT_SYMBOL(fscache_set_page_dirty);
diff --git a/include/linux/fs.h b/include/linux/fs.h
index f6dad577ea07..fdbac1252381 100644
--- a/include/linux/fs.h
+++ b/include/linux/fs.h
@@ -2151,6 +2151,8 @@ static inline void kiocb_clone(struct kiocb *kiocb, struct kiocb *kiocb_src,
  *			Used to detect that mark_inode_dirty() should not move
  * 			inode between dirty lists.
  *
+ * I_PINNING_FSCACHE_WB	Inode is pinning an fscache object for writeback.
+ *
  * Q: What is the difference between I_WILL_FREE and I_FREEING?
  */
 #define I_DIRTY_SYNC		(1 << 0)
@@ -2173,6 +2175,7 @@ static inline void kiocb_clone(struct kiocb *kiocb, struct kiocb *kiocb_src,
 #define I_CREATING		(1 << 15)
 #define I_DONTCACHE		(1 << 16)
 #define I_SYNC_QUEUED		(1 << 17)
+#define I_PINNING_FSCACHE_WB	(1 << 18)
 
 #define I_DIRTY_INODE (I_DIRTY_SYNC | I_DIRTY_DATASYNC)
 #define I_DIRTY (I_DIRTY_INODE | I_DIRTY_PAGES)
diff --git a/include/linux/fscache.h b/include/linux/fscache.h
index 8d47c7a08456..d2fc98a5755a 100644
--- a/include/linux/fscache.h
+++ b/include/linux/fscache.h
@@ -19,6 +19,7 @@
 #include <linux/pagemap.h>
 #include <linux/pagevec.h>
 #include <linux/list_bl.h>
+#include <linux/writeback.h>
 
 #if defined(CONFIG_FSCACHE) || defined(CONFIG_FSCACHE_MODULE)
 #define __fscache_available (1)
@@ -580,4 +581,25 @@ int fscache_write(struct fscache_op_resources *opr,
 	return ops->write(opr, start_pos, iter, term_func, term_func_priv);
 }
 
+#if __fscache_available
+extern int fscache_set_page_dirty(struct page *page, struct fscache_cookie *cookie);
+#else
+#define fscache_set_page_dirty(PAGE, COOKIE) (__set_page_dirty_nobuffers((PAGE)))
+#endif
+
+/**
+ * fscache_unpin_writeback - Unpin writeback resources
+ * @wbc: The writeback control
+ * @cookie: The cookie referring to the cache object
+ *
+ * Unpin the writeback resources pinned by fscache_set_page_dirty().  This is
+ * intended to be called by the netfs's ->write_inode() method.
+ */
+static inline void fscache_unpin_writeback(struct writeback_control *wbc,
+					   struct fscache_cookie *cookie)
+{
+	if (wbc->unpinned_fscache_wb)
+		fscache_unuse_cookie(cookie, NULL, NULL);
+}
+
 #endif /* _LINUX_FSCACHE_H */
diff --git a/include/linux/writeback.h b/include/linux/writeback.h
index 8e5c5bb16e2d..5e7d1d38f261 100644
--- a/include/linux/writeback.h
+++ b/include/linux/writeback.h
@@ -69,6 +69,7 @@ struct writeback_control {
 	unsigned for_reclaim:1;		/* Invoked from the page allocator */
 	unsigned range_cyclic:1;	/* range_start is cyclic */
 	unsigned for_sync:1;		/* sync(2) WB_SYNC_ALL writeback */
+	unsigned unpinned_fscache_wb:1;	/* Cleared I_PINNING_FSCACHE_WB */
 
 	/*
 	 * When writeback IOs are bounced through async layers, only the