[PATCH v4 07/28] netfs, mm: Add unlock_page_fscache() and wait_on_page_fscache()

[David Howells]

Add unlock_page_fscache() as an alias of unlock_page_private_2().  This
allows a page 'locked' with PG_fscache to be unlocked.

Add wait_on_page_fscache() to wait for PG_fscache to be unlocked.

[Linus suggested putting the fscache-themed functions into the
 caching-specific headers rather than pagemap.h[1]]

Signed-off-by: David Howells <dhowells at redhat.com>
cc: Linus Torvalds <torvalds at linux-foundation.org>
cc: Matthew Wilcox <willy at infradead.org>
cc: linux-mm at kvack.org
cc: linux-cachefs at redhat.com
cc: linux-afs at lists.infradead.org
cc: linux-nfs at vger.kernel.org
cc: linux-cifs at vger.kernel.org
cc: ceph-devel at vger.kernel.org
cc: v9fs-developer at lists.sourceforge.net
cc: linux-fsdevel at vger.kernel.org
Link: https://lore.kernel.org/r/1330473.1612974547@warthog.procyon.org.uk/
Link: https://lore.kernel.org/r/CAHk-=wjgA-74ddehziVk=XAEMTKswPu1Yw4uaro1R3ibs27ztw@mail.gmail.com/ [1]
Link: https://lore.kernel.org/r/161340393568.1303470.4997526899111310530.stgit@warthog.procyon.org.uk/ # v3
---

 include/linux/netfs.h |   29 +++++++++++++++++++++++++++++
 1 file changed, 29 insertions(+)

diff --git a/include/linux/netfs.h b/include/linux/netfs.h
index cc1102040488..5ed8c6dc7453 100644
--- a/include/linux/netfs.h
+++ b/include/linux/netfs.h
@@ -26,4 +26,33 @@
 #define TestSetPageFsCache(page)	TestSetPagePrivate2((page))
 #define TestClearPageFsCache(page)	TestClearPagePrivate2((page))
 
+/**
+ * unlock_page_fscache - Unlock a page that's locked with PG_fscache
+ * @page: The page
+ *
+ * Unlocks a page that's locked with PG_fscache and wakes up sleepers in
+ * wait_on_page_fscache().  This page bit is used by the netfs helpers when a
+ * netfs page is being written to a local disk cache, thereby allowing writes
+ * to the cache for the same page to be serialised.
+ */
+static inline void unlock_page_fscache(struct page *page)
+{
+	unlock_page_private_2(page);
+}
+
+/**
+ * wait_on_page_fscache - Wait for PG_fscache to be cleared on a page
+ * @page: The page
+ *
+ * Wait for the PG_fscache (PG_private_2) page bit to be removed from a page.
+ * This is, for example, used to handle a netfs page being written to a local
+ * disk cache, thereby allowing writes to the cache for the same page to be
+ * serialised.
+ */
+static inline void wait_on_page_fscache(struct page *page)
+{
+	if (PageFsCache(page))
+		wait_on_page_bit(compound_head(page), PG_fscache);
+}
+
 #endif /* _LINUX_NETFS_H */