mm: add support for async page locking

Normally waiting for a page to become unlocked, or locking the page,
requires waiting for IO to complete. Add support for lock_page_async()
and wait_on_page_locked_async(), which are callback based instead. This
allows a caller to get notified when a page becomes unlocked, rather
than wait for it.

We add a new iocb field, ki_waitq, to pass in the necessary data for this
to happen. We can unionize this with ki_cookie, since that is only used
for polled IO. Polled IO can never co-exist with async callbacks, as it is
(by definition) polled completions. struct wait_page_key is made public,
and we define struct wait_page_async as the interface between the caller
and the core.

Acked-by: Johannes Weiner <hannes@cmpxchg.org>
Signed-off-by: Jens Axboe <axboe@kernel.dk>

diff --git a/include/linux/fs.h b/include/linux/fs.h
index 3f881a8..2a5cf60 100644 (file)
--- a/include/linux/fs.h
+++ b/include/linux/fs.h
@@ -315,6 +315,8 @@ enum rw_hint {
 #define IOCB_SYNC              (1 << 5)
 #define IOCB_WRITE             (1 << 6)
 #define IOCB_NOWAIT            (1 << 7)
+/* iocb->ki_waitq is valid */
+#define IOCB_WAITQ             (1 << 8)
 
 struct kiocb {
        struct file             *ki_filp;
@@ -328,7 +330,10 @@ struct kiocb {
        int                     ki_flags;
        u16                     ki_hint;
        u16                     ki_ioprio; /* See linux/ioprio.h */
-       unsigned int            ki_cookie; /* for ->iopoll */
+       union {
+               unsigned int            ki_cookie; /* for ->iopoll */
+               struct wait_page_queue  *ki_waitq; /* for async buffered IO */
+       };
 
        randomized_struct_fields_end
 };

diff --git a/include/linux/pagemap.h b/include/linux/pagemap.h
index 2f18221..e053e1d 100644 (file)
--- a/include/linux/pagemap.h
+++ b/include/linux/pagemap.h
@@ -535,6 +535,7 @@ static inline int wake_page_match(struct wait_page_queue *wait_page,
 
 extern void __lock_page(struct page *page);
 extern int __lock_page_killable(struct page *page);
+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);
@@ -572,6 +573,22 @@ static inline int lock_page_killable(struct page *page)
 }
 
 /*
+ * lock_page_async - Lock the page, unless this would block. If the page
+ * is already locked, then queue a callback when the page becomes unlocked.
+ * This callback can then retry the operation.
+ *
+ * Returns 0 if the page is locked successfully, or -EIOCBQUEUED if the page
+ * was already locked and the callback defined in 'wait' was queued.
+ */
+static inline int lock_page_async(struct page *page,
+                                 struct wait_page_queue *wait)
+{
+       if (!trylock_page(page))
+               return __lock_page_async(page, wait);
+       return 0;
+}
+
+/*
  * lock_page_or_retry - Lock the page, unless this would block and the
  * caller indicated that it can handle a retry.
  *

diff --git a/mm/filemap.c b/mm/filemap.c
index c3175db..e8aaf43 100644 (file)
--- a/mm/filemap.c
+++ b/mm/filemap.c
@@ -1180,6 +1180,36 @@ int wait_on_page_bit_killable(struct page *page, int bit_nr)
 }
 EXPORT_SYMBOL(wait_on_page_bit_killable);
 
+static int __wait_on_page_locked_async(struct page *page,
+                                      struct wait_page_queue *wait, bool set)
+{
+       struct wait_queue_head *q = page_waitqueue(page);
+       int ret = 0;
+
+       wait->page = page;
+       wait->bit_nr = PG_locked;
+
+       spin_lock_irq(&q->lock);
+       __add_wait_queue_entry_tail(q, &wait->wait);
+       SetPageWaiters(page);
+       if (set)
+               ret = !trylock_page(page);
+       else
+               ret = PageLocked(page);
+       /*
+        * If we were succesful now, we know we're still on the
+        * waitqueue as we're still under the lock. This means it's
+        * safe to remove and return success, we know the callback
+        * isn't going to trigger.
+        */
+       if (!ret)
+               __remove_wait_queue(q, &wait->wait);
+       else
+               ret = -EIOCBQUEUED;
+       spin_unlock_irq(&q->lock);
+       return ret;
+}
+
 /**
  * put_and_wait_on_page_locked - Drop a reference and wait for it to be unlocked
  * @page: The page to wait for.
@@ -1342,6 +1372,11 @@ int __lock_page_killable(struct page *__page)
 }
 EXPORT_SYMBOL_GPL(__lock_page_killable);
 
+int __lock_page_async(struct page *page, struct wait_page_queue *wait)
+{
+       return __wait_on_page_locked_async(page, wait, true);
+}
+
 /*
  * Return values:
  * 1 - page is locked; mmap_lock is still held.
@@ -2131,6 +2166,11 @@ page_not_up_to_date_locked:
                }
 
 readpage:
+               if (iocb->ki_flags & IOCB_NOWAIT) {
+                       unlock_page(page);
+                       put_page(page);
+                       goto would_block;
+               }
                /*
                 * A previous I/O error may have been due to temporary
                 * failures, eg. multipath errors.
@@ -2150,7 +2190,10 @@ readpage:
                }
 
                if (!PageUptodate(page)) {
-                       error = lock_page_killable(page);
+                       if (iocb->ki_flags & IOCB_WAITQ)
+                               error = lock_page_async(page, iocb->ki_waitq);
+                       else
+                               error = lock_page_killable(page);
                        if (unlikely(error))
                                goto readpage_error;
                        if (!PageUptodate(page)) {