Re: [PATCH 1/2] filemap: Convert generic_perform_write() to support large folios

[Shaun Tancheff <shaun.tancheff@hpe.com>]

On 5/27/24 23:36, Christoph Hellwig wrote:

> From: "Matthew Wilcox (Oracle)" <willy@infradead.org>
>
> Modelled after the loop in iomap_write_iter(), copy larger chunks from
> userspace if the filesystem has created large folios.
>
> Signed-off-by: Matthew Wilcox (Oracle) <willy@infradead.org>
> [hch: use mapping_max_folio_size to keep supporting file systems that do
>   not support large folios]
> Signed-off-by: Christoph Hellwig <hch@lst.de>
> ---
>   mm/filemap.c | 40 +++++++++++++++++++++++++---------------
>   1 file changed, 25 insertions(+), 15 deletions(-)
>
> diff --git a/mm/filemap.c b/mm/filemap.c
> index 382c3d06bfb10c..860728e26ccf32 100644
> --- a/mm/filemap.c
> +++ b/mm/filemap.c
> @@ -3981,21 +3981,24 @@ ssize_t generic_perform_write(struct kiocb *iocb, struct iov_iter *i)
>   	loff_t pos = iocb->ki_pos;
>   	struct address_space *mapping = file->f_mapping;
>   	const struct address_space_operations *a_ops = mapping->a_ops;
> +	size_t chunk = mapping_max_folio_size(mapping);

Better to default chunk to PAGE_SIZE for backward compat
+       size_t chunk = PAGE_SIZE;

>   	long status = 0;
>   	ssize_t written = 0;
>   

Have fs opt in to large folio support:

+       if (mapping_large_folio_support(mapping))
+               chunk = PAGE_SIZE << MAX_PAGECACHE_ORDER;

>   	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));
> +		bytes = iov_iter_count(i);
> +retry:
> +		offset = pos & (chunk - 1);
> +		bytes = min(chunk - offset, bytes);
> +		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
> @@ -4017,11 +4020,16 @@ 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);
> @@ -4039,14 +4047,16 @@ ssize_t generic_perform_write(struct kiocb *iocb, struct iov_iter *i)
>   			 * halfway through, might be a race with munmap,
>   			 * might be severe memory pressure.
>   			 */
> -			if (copied)
> +			if (chunk > PAGE_SIZE)
> +				chunk /= 2;
> +			if (copied) {
>   				bytes = copied;
> -			goto again;
> +				goto retry;
> +			}
> +		} else {
> +			pos += status;
> +			written += status;
>   		}
> -		pos += status;
> -		written += status;
> -
> -		balance_dirty_pages_ratelimited(mapping);
>   	} while (iov_iter_count(i));
>   
>   	if (!written)

Tested with Lustre with large folios and kernel 6.6 with this patch (and suggested changes).

Tested-by: Shaun Tancheff <shaun.tancheff@hpe.com>