Re: [RFC PATCH] filemap: Convert generic_perform_write() to support large folios

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

On Tue, Aug 22, 2023 at 09:09:37PM +0100, Matthew Wilcox (Oracle) wrote:
> Modelled after the loop in iomap_write_iter(), copy larger chunks from
> userspace if the filesystem has created large folios.

Hum.  Which filesystems are those?  Is this for the in-memory ones like
tmpfs?

--D

> Signed-off-by: Matthew Wilcox (Oracle) <willy@infradead.org>
> ---
> This patch dependss on patches currently in the iomap tree.  Sending it
> out now for feedback, but I'll resend it after rc1.
> 
>  mm/filemap.c | 34 ++++++++++++++++++++--------------
>  1 file changed, 20 insertions(+), 14 deletions(-)
> 
> diff --git a/mm/filemap.c b/mm/filemap.c
> index bf6219d9aaac..fd28767c760a 100644
> --- a/mm/filemap.c
> +++ b/mm/filemap.c
> @@ -3908,6 +3908,7 @@ EXPORT_SYMBOL(generic_file_direct_write);
>  ssize_t generic_perform_write(struct kiocb *iocb, struct iov_iter *i)
>  {
>  	struct file *file = iocb->ki_filp;
> +	size_t chunk = PAGE_SIZE << MAX_PAGECACHE_ORDER;
>  	loff_t pos = iocb->ki_pos;
>  	struct address_space *mapping = file->f_mapping;
>  	const struct address_space_operations *a_ops = mapping->a_ops;
> @@ -3916,16 +3917,16 @@ ssize_t generic_perform_write(struct kiocb *iocb, struct iov_iter *i)
>  
>  	do {
>  		struct page *page;
> -		unsigned long offset;	/* Offset into pagecache page */
> -		unsigned long bytes;	/* Bytes to write to page */
> +		struct folio *folio;
> +		size_t offset;		/* Offset into folio */
> +		size_t bytes;		/* Bytes to write to folio */
>  		size_t copied;		/* Bytes copied from user */
>  		void *fsdata = NULL;
>  
> -		offset = (pos & (PAGE_SIZE - 1));
> -		bytes = min_t(unsigned long, PAGE_SIZE - offset,
> -						iov_iter_count(i));
> +		offset = pos & (chunk - 1);
> +		bytes = min(chunk - offset, iov_iter_count(i));
> +		balance_dirty_pages_ratelimited(mapping);
>  
> -again:
>  		/*
>  		 * Bring in the user page that we will copy from _first_.
>  		 * Otherwise there's a nasty deadlock on copying from the
> @@ -3947,11 +3948,16 @@ ssize_t generic_perform_write(struct kiocb *iocb, struct iov_iter *i)
>  		if (unlikely(status < 0))
>  			break;
>  
> +		folio = page_folio(page);
> +		offset = offset_in_folio(folio, pos);
> +		if (bytes > folio_size(folio) - offset)
> +			bytes = folio_size(folio) - offset;
> +
>  		if (mapping_writably_mapped(mapping))
> -			flush_dcache_page(page);
> +			flush_dcache_folio(folio);
>  
> -		copied = copy_page_from_iter_atomic(page, offset, bytes, i);
> -		flush_dcache_page(page);
> +		copied = copy_folio_from_iter_atomic(folio, offset, bytes, i);
> +		flush_dcache_folio(folio);
>  
>  		status = a_ops->write_end(file, mapping, pos, bytes, copied,
>  						page, fsdata);
> @@ -3971,12 +3977,12 @@ ssize_t generic_perform_write(struct kiocb *iocb, struct iov_iter *i)
>  			 */
>  			if (copied)
>  				bytes = copied;
> -			goto again;
> +			if (chunk > PAGE_SIZE)
> +				chunk /= 2;
> +		} else {
> +			pos += status;
> +			written += status;
>  		}
> -		pos += status;
> -		written += status;
> -
> -		balance_dirty_pages_ratelimited(mapping);
>  	} while (iov_iter_count(i));
>  
>  	if (!written)
> -- 
> 2.40.1
>