Re: [RFC 2/4] buffer: add alloc_folio_buffers() helper

[Matthew Wilcox]

On Fri, Apr 14, 2023 at 01:08:19PM +0200, Pankaj Raghav wrote:
> Folio version of alloc_page_buffers() helper. This is required to convert
> create_page_buffers() to create_folio_buffers() later in the series.
> 
> It removes one call to compound_head() compared to alloc_page_buffers().

I would convert alloc_page_buffers() to folio_alloc_buffers() and
add

static struct buffer_head *alloc_page_buffers(struct page *page,
		unsigned long size, bool retry)
{
	return folio_alloc_buffers(page_folio(page), size, retry);
}

in buffer_head.h

(there are only five callers, so this feels like a better tradeoff
than creating a new function)

> Signed-off-by: Pankaj Raghav <p.raghav@xxxxxxxxxxx>
> ---
>  fs/buffer.c | 59 +++++++++++++++++++++++++++++++++++++++++++++++++++++
>  1 file changed, 59 insertions(+)
> 
> diff --git a/fs/buffer.c b/fs/buffer.c
> index 44380ff3a31f..0f9c2127543d 100644
> --- a/fs/buffer.c
> +++ b/fs/buffer.c
> @@ -900,6 +900,65 @@ struct buffer_head *alloc_page_buffers(struct page *page, unsigned long size,
>  }
>  EXPORT_SYMBOL_GPL(alloc_page_buffers);
>  
> +/*
> + * Create the appropriate buffers when given a folio for data area and
> + * the size of each buffer.. Use the bh->b_this_page linked list to
> + * follow the buffers created.  Return NULL if unable to create more
> + * buffers.
> + *
> + * The retry flag is used to differentiate async IO (paging, swapping)
> + * which may not fail from ordinary buffer allocations.
> + */
> +struct buffer_head *alloc_folio_buffers(struct folio *folio, unsigned long size,
> +					bool retry)
> +{
> +	struct buffer_head *bh, *head;
> +	gfp_t gfp = GFP_NOFS | __GFP_ACCOUNT;
> +	long offset;
> +	struct mem_cgroup *memcg, *old_memcg;
> +
> +	if (retry)
> +		gfp |= __GFP_NOFAIL;
> +
> +	/* The folio lock pins the memcg */
> +	memcg = folio_memcg(folio);
> +	old_memcg = set_active_memcg(memcg);
> +
> +	head = NULL;
> +	offset = folio_size(folio);
> +	while ((offset -= size) >= 0) {
> +		bh = alloc_buffer_head(gfp);
> +		if (!bh)
> +			goto no_grow;
> +
> +		bh->b_this_page = head;
> +		bh->b_blocknr = -1;
> +		head = bh;
> +
> +		bh->b_size = size;
> +
> +		/* Link the buffer to its folio */
> +		set_bh_folio(bh, folio, offset);
> +	}
> +out:
> +	set_active_memcg(old_memcg);
> +	return head;
> +/*
> + * In case anything failed, we just free everything we got.
> + */
> +no_grow:
> +	if (head) {
> +		do {
> +			bh = head;
> +			head = head->b_this_page;
> +			free_buffer_head(bh);
> +		} while (head);
> +	}
> +
> +	goto out;
> +}
> +EXPORT_SYMBOL_GPL(alloc_folio_buffers);
> +
>  static inline void
>  link_dev_buffers(struct page *page, struct buffer_head *head)
>  {
> -- 
> 2.34.1
>