Re: [PATCH 07/15] mm: add support for async page locking

Andreas Grünbacher
2020-07-07T11:32:59Z

Jens,

Am Do., 18. Juni 2020 um 16:47 Uhr schrieb Jens Axboe <axboe@kernel.dk>:
> 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>
> ---
> include/linux/fs.h | 7 ++++++-
> include/linux/pagemap.h | 17 ++++++++++++++++
> mm/filemap.c | 45 ++++++++++++++++++++++++++++++++++++++++-
> 3 files changed, 67 insertions(+), 2 deletions(-)
>
> diff --git a/include/linux/fs.h b/include/linux/fs.h
> index 6c4ab4dc1cd7..6ac919b40596 100644
> --- 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 2f18221bb5c8..e053e1d9a4d7 100644
> --- 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);
> @@ -571,6 +572,22 @@ static inline int lock_page_killable(struct page *page)
> return 0;
> }
>
> +/*
> + * 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 c3175dbd8fba..e8aaf43bee9f 100644
> --- 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 @@ ssize_t generic_file_buffered_read(struct kiocb *iocb,
> }
>
> readpage:
> + if (iocb->ki_flags & IOCB_NOWAIT) {
> + unlock_page(page);
> + put_page(page);
> + goto would_block;
> + }

This hunk should have been part of "mm: allow read-ahead with
IOCB_NOWAIT set" ...

> /*
> * A previous I/O error may have been due to temporary
> * failures, eg. multipath errors.
> @@ -2150,7 +2190,10 @@ ssize_t generic_file_buffered_read(struct kiocb *iocb,
> }
>
> 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)) {
> --
> 2.27.0
>

Andreas