[RFC] Documentation for folio_lock() and friends

[Matthew Wilcox <willy@infradead.org>]

It's a shame to not have these functions documented.  I'm sure I've
missed a few things that would be useful to document here.

diff --git a/include/linux/pagemap.h b/include/linux/pagemap.h
index ab47579af434..47b7851f1b64 100644
--- a/include/linux/pagemap.h
+++ b/include/linux/pagemap.h
@@ -888,6 +888,18 @@ bool __folio_lock_or_retry(struct folio *folio, struct mm_struct *mm,
 void unlock_page(struct page *page);
 void folio_unlock(struct folio *folio);
 
+/**
+ * folio_trylock() - Attempt to lock a folio.
+ * @folio: The folio to attempt to lock.
+ *
+ * Sometimes it is undesirable to wait for a folio to be unlocked (eg
+ * when the locks are being taken in the wrong order, or if making
+ * progress through a batch of folios is more important than processing
+ * them in order).  Usually folio_lock() is the correct function to call.
+ *
+ * Context: Any context.
+ * Return: Whether the lock was successfully acquired.
+ */
 static inline bool folio_trylock(struct folio *folio)
 {
 	return likely(!test_and_set_bit_lock(PG_locked, folio_flags(folio, 0)));
@@ -901,6 +913,26 @@ static inline int trylock_page(struct page *page)
 	return folio_trylock(page_folio(page));
 }
 
+/**
+ * folio_lock() - Lock this folio.
+ * @folio: The folio to lock.
+ *
+ * The folio lock protects against many things, probably more than it
+ * should.  It is primarily held while a folio is being read from storage,
+ * either from its backing file or from swap.  It is also held while a
+ * folio is being truncated from its address_space.
+ *
+ * Holding the lock usually prevents the contents of the folio from being
+ * modified by other kernel users, although it does not prevent userspace
+ * from modifying data if it's mapped.  The lock is not consistently held
+ * while a folio is being modified by DMA.
+ *
+ * Context: May sleep.  If you need to acquire the locks of two or
+ * more folios, they must be in order of ascending index, if they are
+ * in the same address_space.  If they are in different address_spaces,
+ * acquire the lock of the folio which belongs to the address_space which
+ * has the lowest address in memory first.
+ */
 static inline void folio_lock(struct folio *folio)
 {
 	might_sleep();
@@ -908,6 +940,17 @@ static inline void folio_lock(struct folio *folio)
 		__folio_lock(folio);
 }
 
+/**
+ * lock_page() - Lock the folio containing this page.
+ * @page: The page to lock.
+ *
+ * See folio_lock() for a description of what the lock protects.
+ * This is a legacy function and new code should probably use folio_lock()
+ * instead.
+ *
+ * Context: May sleep.  Pages in the same folio share a lock, so do not
+ * attempt to lock two pages which share a folio.
+ */
 static inline void lock_page(struct page *page)
 {
 	struct folio *folio;
@@ -918,6 +961,16 @@ static inline void lock_page(struct page *page)
 		__folio_lock(folio);
 }
 
+/**
+ * folio_lock_killable() - Lock this folio, interruptible by a fatal signal.
+ * @folio: The folio to lock.
+ *
+ * Attempts to lock the folio, like folio_lock(), except that the sleep
+ * to acquire the lock is interruptible by a fatal signal.
+ *
+ * Context: May sleep; see folio_lock().
+ * Return: 0 if the lock was acquired; -EINTR if a fatal signal was received.
+ */
 static inline int folio_lock_killable(struct folio *folio)
 {
 	might_sleep();
@@ -964,8 +1017,8 @@ int folio_wait_bit_killable(struct folio *folio, int bit_nr);
  * Wait for a folio to be unlocked.
  *
  * This must be called with the caller "holding" the folio,
- * ie with increased "page->count" so that the folio won't
- * go away during the wait..
+ * ie with increased folio reference count so that the folio won't
+ * go away during the wait.
  */
 static inline void folio_wait_locked(struct folio *folio)
 {