Re: [PATCH v6 05/16] iomap: Add async buffered write support

["Darrick J. Wong" <djwong@kernel.org>]

On Thu, May 26, 2022 at 10:38:29AM -0700, Stefan Roesch wrote:
> This adds async buffered write support to iomap.
> 
> This replaces the call to balance_dirty_pages_ratelimited() with the
> call to balance_dirty_pages_ratelimited_flags. This allows to specify if
> the write request is async or not.
> 
> In addition this also moves the above function call to the beginning of
> the function. If the function call is at the end of the function and the
> decision is made to throttle writes, then there is no request that
> io-uring can wait on. By moving it to the beginning of the function, the
> write request is not issued, but returns -EAGAIN instead. io-uring will
> punt the request and process it in the io-worker.
> 
> By moving the function call to the beginning of the function, the write
> throttling will happen one page later.

It does?  I would have thought that moving it before iomap_write_begin
call would make the throttling happen one page sooner?  Sorry if I'm
being dense here...

> Signed-off-by: Stefan Roesch <shr@fb.com>
> Reviewed-by: Jan Kara <jack@suse.cz>
> ---
>  fs/iomap/buffered-io.c | 31 +++++++++++++++++++++++++++----
>  1 file changed, 27 insertions(+), 4 deletions(-)
> 
> diff --git a/fs/iomap/buffered-io.c b/fs/iomap/buffered-io.c
> index d6ddc54e190e..2281667646d2 100644
> --- a/fs/iomap/buffered-io.c
> +++ b/fs/iomap/buffered-io.c
> @@ -559,6 +559,7 @@ static int __iomap_write_begin(const struct iomap_iter *iter, loff_t pos,
>  	loff_t block_size = i_blocksize(iter->inode);
>  	loff_t block_start = round_down(pos, block_size);
>  	loff_t block_end = round_up(pos + len, block_size);
> +	unsigned int nr_blocks = i_blocks_per_folio(iter->inode, folio);
>  	size_t from = offset_in_folio(folio, pos), to = from + len;
>  	size_t poff, plen;
>  
> @@ -567,6 +568,8 @@ static int __iomap_write_begin(const struct iomap_iter *iter, loff_t pos,
>  	folio_clear_error(folio);
>  
>  	iop = iomap_page_create(iter->inode, folio, iter->flags);
> +	if ((iter->flags & IOMAP_NOWAIT) && !iop && nr_blocks > 1)
> +		return -EAGAIN;
>  
>  	do {
>  		iomap_adjust_read_range(iter->inode, folio, &block_start,
> @@ -584,7 +587,12 @@ static int __iomap_write_begin(const struct iomap_iter *iter, loff_t pos,
>  				return -EIO;
>  			folio_zero_segments(folio, poff, from, to, poff + plen);
>  		} else {
> -			int status = iomap_read_folio_sync(block_start, folio,
> +			int status;
> +
> +			if (iter->flags & IOMAP_NOWAIT)
> +				return -EAGAIN;
> +
> +			status = iomap_read_folio_sync(block_start, folio,
>  					poff, plen, srcmap);
>  			if (status)
>  				return status;
> @@ -613,6 +621,9 @@ static int iomap_write_begin(const struct iomap_iter *iter, loff_t pos,
>  	unsigned fgp = FGP_LOCK | FGP_WRITE | FGP_CREAT | FGP_STABLE | FGP_NOFS;
>  	int status = 0;
>  
> +	if (iter->flags & IOMAP_NOWAIT)
> +		fgp |= FGP_NOWAIT;

FGP_NOWAIT can cause __filemap_get_folio to return a NULL folio, which
makes iomap_write_begin return -ENOMEM.  If nothing has been written
yet, won't that cause the ENOMEM to escape to userspace?  Why do we want
that instead of EAGAIN?

> +
>  	BUG_ON(pos + len > iter->iomap.offset + iter->iomap.length);
>  	if (srcmap != &iter->iomap)
>  		BUG_ON(pos + len > srcmap->offset + srcmap->length);
> @@ -750,6 +761,8 @@ static loff_t iomap_write_iter(struct iomap_iter *iter, struct iov_iter *i)
>  	loff_t pos = iter->pos;
>  	ssize_t written = 0;
>  	long status = 0;
> +	struct address_space *mapping = iter->inode->i_mapping;
> +	unsigned int bdp_flags = (iter->flags & IOMAP_NOWAIT) ? BDP_ASYNC : 0;
>  
>  	do {
>  		struct folio *folio;
> @@ -762,6 +775,11 @@ static loff_t iomap_write_iter(struct iomap_iter *iter, struct iov_iter *i)
>  		bytes = min_t(unsigned long, PAGE_SIZE - offset,
>  						iov_iter_count(i));
>  again:
> +		status = balance_dirty_pages_ratelimited_flags(mapping,
> +							       bdp_flags);
> +		if (unlikely(status))
> +			break;
> +
>  		if (bytes > length)
>  			bytes = length;
>  
> @@ -770,6 +788,10 @@ static loff_t iomap_write_iter(struct iomap_iter *iter, struct iov_iter *i)
>  		 * Otherwise there's a nasty deadlock on copying from the
>  		 * same page as we're writing to, without it being marked
>  		 * up-to-date.
> +		 *
> +		 * For async buffered writes the assumption is that the user
> +		 * page has already been faulted in. This can be optimized by
> +		 * faulting the user page in the prepare phase of io-uring.

I don't think this pattern is unique to async writes with io_uring --
gfs2 also wanted this "try as many pages as you can until you hit a page
fault and then return a short write to caller so it can fault in the
rest" behavior.

--D

>  		 */
>  		if (unlikely(fault_in_iov_iter_readable(i, bytes) == bytes)) {
>  			status = -EFAULT;
> @@ -781,7 +803,7 @@ static loff_t iomap_write_iter(struct iomap_iter *iter, struct iov_iter *i)
>  			break;
>  
>  		page = folio_file_page(folio, pos >> PAGE_SHIFT);
> -		if (mapping_writably_mapped(iter->inode->i_mapping))
> +		if (mapping_writably_mapped(mapping))
>  			flush_dcache_page(page);
>  
>  		copied = copy_page_from_iter_atomic(page, offset, bytes, i);
> @@ -806,8 +828,6 @@ static loff_t iomap_write_iter(struct iomap_iter *iter, struct iov_iter *i)
>  		pos += status;
>  		written += status;
>  		length -= status;
> -
> -		balance_dirty_pages_ratelimited(iter->inode->i_mapping);
>  	} while (iov_iter_count(i) && length);
>  
>  	return written ? written : status;
> @@ -825,6 +845,9 @@ iomap_file_buffered_write(struct kiocb *iocb, struct iov_iter *i,
>  	};
>  	int ret;
>  
> +	if (iocb->ki_flags & IOCB_NOWAIT)
> +		iter.flags |= IOMAP_NOWAIT;
> +
>  	while ((ret = iomap_iter(&iter, ops)) > 0)
>  		iter.processed = iomap_write_iter(&iter, i);
>  	if (iter.pos == iocb->ki_pos)
> -- 
> 2.30.2
>