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SubjectRe: [PATCH 05/12] mm: support async buffered reads in generic_file_buffered_read()
Date
On Sat, 2020-05-23 at 12:57 -0600, Jens Axboe wrote:
> Use the async page locking infrastructure, if IOCB_WAITQ is set in
> the
> passed in iocb. The caller must expect an -EIOCBQUEUED return value,
> which means that IO is started but not done yet. This is similar to
> how
> O_DIRECT signals the same operation. Once the callback is received by
> the caller for IO completion, the caller must retry the operation.
>
> Signed-off-by: Jens Axboe <axboe@kernel.dk>
> ---
> mm/filemap.c | 33 ++++++++++++++++++++++++++-------
> 1 file changed, 26 insertions(+), 7 deletions(-)
>
> diff --git a/mm/filemap.c b/mm/filemap.c
> index c746541b1d49..a3b86c9acdc8 100644
> --- a/mm/filemap.c
> +++ b/mm/filemap.c
> @@ -1219,6 +1219,14 @@ static int __wait_on_page_locked_async(struct
> page *page,
> return ret;
> }
>
> +static int wait_on_page_locked_async(struct page *page,
> + struct wait_page_queue *wait)
> +{
> + if (!PageLocked(page))
> + return 0;
> + return __wait_on_page_locked_async(compound_head(page), wait,
> false);
> +}
> +
> /**
> * put_and_wait_on_page_locked - Drop a reference and wait for it to
> be unlocked
> * @page: The page to wait for.
> @@ -2058,17 +2066,25 @@ static ssize_t
> generic_file_buffered_read(struct kiocb *iocb,
> index, last_index - index);
> }
> if (!PageUptodate(page)) {
> - if (iocb->ki_flags & IOCB_NOWAIT) {
> - put_page(page);
> - goto would_block;
> - }
> -
> /*
> * See comment in do_read_cache_page on why
> * wait_on_page_locked is used to avoid
> unnecessarily
> * serialisations and why it's safe.
> */
> - error = wait_on_page_locked_killable(page);
> + if (iocb->ki_flags & IOCB_WAITQ) {
> + if (written) {
> + put_page(page);
> + goto out;
> + }
> + error = wait_on_page_locked_async(page,
> + iocb-
> >private);

If it is being used in 'generic_file_buffered_read()' as storage for a
wait queue, then it is hard to consider this a 'private' field.

Perhaps either rename and add type checking, or else add a separate
field altogether to struct kiocb?

> + } else {
> + if (iocb->ki_flags & IOCB_NOWAIT) {
> + put_page(page);
> + goto would_block;
> + }
> + error =
> wait_on_page_locked_killable(page);
> + }
> if (unlikely(error))
> goto readpage_error;
> if (PageUptodate(page))
> @@ -2156,7 +2172,10 @@ static ssize_t
> generic_file_buffered_read(struct kiocb *iocb,
>
> page_not_up_to_date:
> /* Get exclusive access to the page ... */
> - error = lock_page_killable(page);
> + if (iocb->ki_flags & IOCB_WAITQ)
> + error = lock_page_async(page, iocb->private);
> + else
> + error = lock_page_killable(page);
> if (unlikely(error))
> goto readpage_error;
>
--
Trond Myklebust
Linux NFS client maintainer, Hammerspace
trond.myklebust@hammerspace.com


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