[PATCH v4 02/28] mm: Add an unlock function for PG_private_2/PG_fscache

[David Howells <dhowells@redhat.com>]

Add a function, unlock_page_private_2(), to unlock PG_private_2 analogous
to that of PG_lock.  Add a kerneldoc banner to that indicating the example
usage case.

A wrapper will need to be placed in the netfs header in the patch that adds
that.

[This implements a suggestion by Linus[1] to not mix the terminology of
 PG_private_2 and PG_fscache in the mm core function]

Changes:
- Remove extern from the declaration[2].

Suggested-by: Linus Torvalds <torvalds@linux-foundation.org>
Signed-off-by: David Howells <dhowells@redhat.com>
Reviewed-by: Christoph Hellwig <hch@lst.de>
cc: Matthew Wilcox (Oracle) <willy@infradead.org>
cc: Alexander Viro <viro@zeniv.linux.org.uk>
cc: Christoph Hellwig <hch@lst.de>
cc: linux-mm@kvack.org
cc: linux-cachefs@redhat.com
cc: linux-afs@lists.infradead.org
cc: linux-nfs@vger.kernel.org
cc: linux-cifs@vger.kernel.org
cc: ceph-devel@vger.kernel.org
cc: v9fs-developer@lists.sourceforge.net
cc: linux-fsdevel@vger.kernel.org
Link: https://lore.kernel.org/r/1330473.1612974547@warthog.procyon.org.uk/ # v1
Link: https://lore.kernel.org/r/CAHk-=wjgA-74ddehziVk=XAEMTKswPu1Yw4uaro1R3ibs27ztw@mail.gmail.com/ [1]
Link: https://lore.kernel.org/r/20210216102659.GA27714@lst.de/ [2]
Link: https://lore.kernel.org/r/161340387944.1303470.7944159520278177652.stgit@warthog.procyon.org.uk/ # v3
---

 include/linux/pagemap.h |    1 +
 mm/filemap.c            |   20 ++++++++++++++++++++
 2 files changed, 21 insertions(+)

diff --git a/include/linux/pagemap.h b/include/linux/pagemap.h
index 20225b067583..ac91b33f3873 100644
--- a/include/linux/pagemap.h
+++ b/include/linux/pagemap.h
@@ -591,6 +591,7 @@ extern int __lock_page_async(struct page *page, struct wait_page_queue *wait);
 extern int __lock_page_or_retry(struct page *page, struct mm_struct *mm,
 				unsigned int flags);
 extern void unlock_page(struct page *page);
+void unlock_page_private_2(struct page *page);
 
 /*
  * Return true if the page was successfully locked
diff --git a/mm/filemap.c b/mm/filemap.c
index 43700480d897..925964b67583 100644
--- a/mm/filemap.c
+++ b/mm/filemap.c
@@ -1432,6 +1432,26 @@ void unlock_page(struct page *page)
 }
 EXPORT_SYMBOL(unlock_page);
 
+/**
+ * unlock_page_private_2 - Unlock a page that's locked with PG_private_2
+ * @page: The page
+ *
+ * Unlocks a page that's locked with PG_private_2 and wakes up sleepers in
+ * wait_on_page_private_2().
+ *
+ * This is, for example, used 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.
+ */
+void unlock_page_private_2(struct page *page)
+{
+	page = compound_head(page);
+	VM_BUG_ON_PAGE(!PagePrivate2(page), page);
+	clear_bit_unlock(PG_private_2, &page->flags);
+	wake_up_page_bit(page, PG_private_2);
+}
+EXPORT_SYMBOL(unlock_page_private_2);
+
 /**
  * end_page_writeback - end writeback against a page
  * @page: the page