Re: [PATCH 05/12] mm: support async buffered reads in generic_file_buffered_read()

[Trond Myklebust <trondmy@hammerspace.com>]

On Sun, 2020-05-24 at 10:40 -0600, Jens Axboe wrote:
> On 5/24/20 10:30 AM, Jens Axboe wrote:
> > On 5/24/20 8:05 AM, Trond Myklebust wrote:
> > > 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.
> > 
> > private isn't the prettiest, and in fact this one in particular is
> > a bit
> > of a mess. It's not clear if it's caller or callee owned. It's
> > generally
> > not used, outside of the old usb gadget code, iomap O_DIRECT, and
> > ocfs2.
> > With FMODE_BUF_RASYNC, the fs obviously can't set it if it uses
> > ->private for buffered IO.
> > 
> > > Perhaps either rename and add type checking, or else add a
> > > separate
> > > field altogether to struct kiocb?
> > 
> > I'd hate to add a new field and increase the size of the kiocb...
> > One
> > alternative is to do:
> > 
> > 	union {
> > 		void *private;
> > 		struct wait_page_queue *ki_waitq;
> > 	};
> > 
> > and still use IOCB_WAITQ to say that ->ki_waitq is valid.
> > 
> > There's also 4 bytes of padding in the kiocb struct. And some
> > fields are
> > only used for O_DIRECT as well, eg ->ki_cookie which is just used
> > for
> > polled O_DIRECT. So we could also do:
> > 
> > 	union {
> > 		unsigned int ki_cookie;
> > 		struct wait_page_queue *ki_waitq;
> > 	};
> > 
> > and still not grow the kiocb. How about we go with this approach,
> > and
> > also add:
> > 
> > 	if (kiocb->ki_flags & IOCB_HIPRI)
> > 		return -EOPNOTSUPP;
> > 
> > to kiocb_wait_page_queue_init() to make sure that this combination
> > isn't
> > valid?
> 
> Here's the incremental, which is spread over 3 patches. I think this
> one
> makes sense, as polled IO doesn't support buffered IO. And because
> doing
> an async callback for completion is not how polled IO operates
> anyway,
> so even if buffered IO supported it, we'd not use the callback for
> polled IO anyway. kiocb_wait_page_queue_init() checks and backs this
> up.
> 
> 
> diff --git a/include/linux/fs.h b/include/linux/fs.h
> index 0ef5f5973b1c..f7b1eb765c6e 100644
> --- a/include/linux/fs.h
> +++ b/include/linux/fs.h
> @@ -317,7 +317,7 @@ 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 */
> +/* iocb->ki_waitq is valid */
>  #define IOCB_WAITQ		(1 << 8)
>  
>  struct kiocb {
> @@ -332,7 +332,10 @@ 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 def58de92053..8b65420410ee 100644
> --- a/include/linux/pagemap.h
> +++ b/include/linux/pagemap.h
> @@ -498,13 +498,16 @@ static inline int
> kiocb_wait_page_queue_init(struct kiocb *kiocb,
>  					     wait_queue_func_t func,
>  					     void *data)
>  {
> +	/* Can't support async wakeup with polled IO */
> +	if (kiocb->ki_flags & IOCB_HIPRI)
> +		return -EINVAL;
>  	if (kiocb->ki_filp->f_mode & FMODE_BUF_RASYNC) {
>  		wait->wait.func = func;
>  		wait->wait.private = data;
>  		wait->wait.flags = 0;
>  		INIT_LIST_HEAD(&wait->wait.entry);
>  		kiocb->ki_flags |= IOCB_WAITQ;
> -		kiocb->private = wait;
> +		kiocb->ki_waitq = wait;
>  		return 0;
>  	}
>  
> diff --git a/mm/filemap.c b/mm/filemap.c
> index a3b86c9acdc8..18022de7dc33 100644
> --- a/mm/filemap.c
> +++ b/mm/filemap.c
> @@ -2077,7 +2077,7 @@ static ssize_t
> generic_file_buffered_read(struct kiocb *iocb,
>  					goto out;
>  				}
>  				error = wait_on_page_locked_async(page,
> -								iocb-
> >private);
> +								iocb-
> >ki_waitq);
>  			} else {
>  				if (iocb->ki_flags & IOCB_NOWAIT) {
>  					put_page(page);
> @@ -2173,7 +2173,7 @@ static ssize_t
> generic_file_buffered_read(struct kiocb *iocb,
>  page_not_up_to_date:
>  		/* Get exclusive access to the page ... */
>  		if (iocb->ki_flags & IOCB_WAITQ)
> -			error = lock_page_async(page, iocb->private);
> +			error = lock_page_async(page, iocb->ki_waitq);
>  		else
>  			error = lock_page_killable(page);
>  		if (unlikely(error))

Ack. That seems cleaner to me.

-- 
Trond Myklebust
Linux NFS client maintainer, Hammerspace
trond.myklebust@hammerspace.com