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    Subject[PATCH 13/24] CacheFiles: Permit the page lock state to be monitored
    Date
    Add a function to install a monitor on the page lock waitqueue for a particular
    page, thus allowing the page being unlocked to be detected.

    This is used by CacheFiles to detect read completion on a page in the backing
    filesystem so that it can then copy the data to the waiting netfs page.

    Signed-Off-By: David Howells <dhowells@redhat.com>
    ---

    include/linux/pagemap.h | 5 +++++
    mm/filemap.c | 19 +++++++++++++++++++
    2 files changed, 24 insertions(+), 0 deletions(-)

    diff --git a/include/linux/pagemap.h b/include/linux/pagemap.h
    index d1049b6..452fdcf 100644
    --- a/include/linux/pagemap.h
    +++ b/include/linux/pagemap.h
    @@ -220,6 +220,11 @@ static inline void wait_on_page_fscache_write(struct page *page)
    extern void end_page_fscache_write(struct page *page);

    /*
    + * Add an arbitrary waiter to a page's wait queue
    + */
    +extern void add_page_wait_queue(struct page *page, wait_queue_t *waiter);
    +
    +/*
    * Fault a userspace page into pagetables. Return non-zero on a fault.
    *
    * This assumes that two userspace pages are always sufficient. That's
    diff --git a/mm/filemap.c b/mm/filemap.c
    index 21aeee9..e48e862 100644
    --- a/mm/filemap.c
    +++ b/mm/filemap.c
    @@ -518,6 +518,25 @@ void fastcall wait_on_page_bit(struct page *page, int bit_nr)
    EXPORT_SYMBOL(wait_on_page_bit);

    /**
    + * add_page_wait_queue - Add an arbitrary waiter to a page's wait queue
    + * @page - Page defining the wait queue of interest
    + * @waiter - Waiter to add to the queue
    + *
    + * Add an arbitrary @waiter to the wait queue for the nominated @page.
    + */
    +void add_page_wait_queue(struct page *page, wait_queue_t *waiter)
    +{
    + wait_queue_head_t *q = page_waitqueue(page);
    + unsigned long flags;
    +
    + spin_lock_irqsave(&q->lock, flags);
    + __add_wait_queue(q, waiter);
    + spin_unlock_irqrestore(&q->lock, flags);
    +}
    +
    +EXPORT_SYMBOL_GPL(add_page_wait_queue);
    +
    +/**
    * unlock_page - unlock a locked page
    * @page: the page
    *
    -
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