[PATCH 04/12] mm: add support for async page locking

Jens Axboe
2020-05-23T18:58:12Z

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 use the iocb->private field to pass in this necessary data for this
to happen. struct wait_page_key is made public, and we define struct
wait_page_async as the interface between the caller and the core.

Signed-off-by: Jens Axboe <axboe@kernel.dk>
---
include/linux/fs.h | 2 ++
include/linux/pagemap.h | 9 +++++++++
mm/filemap.c | 41 +++++++++++++++++++++++++++++++++++++++++
3 files changed, 52 insertions(+)

diff --git a/include/linux/fs.h b/include/linux/fs.h
index 7e84d823c6a8..82b989695ab9 100644
--- a/include/linux/fs.h
+++ b/include/linux/fs.h
@@ -314,6 +314,8 @@ enum rw_hint {
#define IOCB_SYNC (1 << 5)
#define IOCB_WRITE (1 << 6)
#define IOCB_NOWAIT (1 << 7)
+/* iocb->private holds wait_page_async struct */
+#define IOCB_WAITQ (1 << 8)

struct kiocb {
struct file *ki_filp;
diff --git a/include/linux/pagemap.h b/include/linux/pagemap.h
index 53d980f2208d..d3e63c9c61ae 100644
--- a/include/linux/pagemap.h
+++ b/include/linux/pagemap.h
@@ -495,6 +495,7 @@ 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);
@@ -531,6 +532,14 @@ static inline int lock_page_killable(struct page *page)
return 0;
}

+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 e891b5bee8fd..c746541b1d49 100644
--- a/mm/filemap.c
+++ b/mm/filemap.c
@@ -1183,6 +1183,42 @@ 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);
+ if (set)
+ ret = !trylock_page(page);
+ else
+ ret = PageLocked(page);
+ if (ret) {
+ __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.
@@ -1345,6 +1381,11 @@ 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_sem is still held.
--
2.26.2